v/doc/docs.md

V Documentation

Introduction

V is a statically typed compiled programming language designed for building maintainable software.

It's similar to Go and its design has also been influenced by Oberon, Rust, Swift, Kotlin, and Python.

V is a very simple language. Going through this documentation will take you about an hour, and by the end of it you will have pretty much learned the entire language.

The language promotes writing simple and clear code with minimal abstraction.

Despite being simple, V gives the developer a lot of power. Anything you can do in other languages, you can do in V.

Install from source

The major way to get the latest and greatest V, is to install it from source. It is easy, and it usually takes only a few seconds.

Linux, macOS, FreeBSD, etc:

You need git, and a C compiler like tcc, gcc or clang, and make:

git clone https://github.com/vlang/v
cd v
make

Windows:

You need git, and a C compiler like tcc, gcc, clang or msvc:

git clone https://github.com/vlang/v
cd v
make.bat -tcc

NB: You can also pass one of -gcc, -msvc, -clang to make.bat instead, if you do prefer to use a different C compiler, but -tcc is small, fast, and easy to install (V will download a prebuilt binary automatically).

It is recommended to add this folder to the PATH of your environment variables. This can be done with the command v.exe symlink.

Android

Running V graphical apps on Android is also possible via vab.

V Android dependencies: V, Java JDK >= 8, Android SDK + NDK.

1. Install dependencies (see vab)
2. Connect your Android device
3. Run:

git clone https://github.com/vlang/vab && cd vab && v vab.v
./vab --device auto run /path/to/v/examples/sokol/particles

For more details and troubleshooting, please visit the vab GitHub repository.

Table of Contents

Hello world Running a project folder Comments Functions Returning multiple values Hoistings Symbol visibility Variables V types Strings Numbers Arrays Fixed size arrays Maps Module imports Statements & expressions If In operator For loop Match Defer Structs Embedded structs Default field values Short struct literal syntax Access modifiers Methods Unions

Functions 2 Pure functions by default Mutable arguments Variable number of arguments Anonymous & higher-order functions Closures References Constants Builtin functions Printing custom types Modules Type Declarations Interfaces Enums Sum types Type aliases Option/Result types & error handling Generics Concurrency Spawning Concurrent Tasks Channels Shared Objects Decoding JSON Testing Memory management Stack and Heap ORM

Writing documentation Tools v fmt v shader Profiling Package Management Publish package Advanced Topics Dumping expressions at runtime Memory-unsafe code Structs with reference fields sizeof and __offsetof Calling C from V Calling V from C Atomics Global Variables Debugging Conditional compilation Compile time pseudo variables Compile-time reflection Limited operator overloading Inline assembly Translating C to V Hot code reloading Cross compilation Cross-platform shell scripts in V Attributes Goto Appendices Keywords Operators

Hello World

fn main() {
	println('hello world')
}

Save this snippet into a file named hello.v. Now do: v run hello.v.

That is assuming you have symlinked your V with v symlink, as described here. If you haven't yet, you have to type the path to V manually.

Congratulations - you just wrote and executed your first V program!

You can compile a program without execution with v hello.v. See v help for all supported commands.

From the example above, you can see that functions are declared with the fn keyword. The return type is specified after the function name. In this case main doesn't return anything, so there is no return type.

As in many other languages (such as C, Go, and Rust), main is the entry point of your program.

println is one of the few built-in functions. It prints the value passed to it to standard output.

fn main() declaration can be skipped in one file programs. This is useful when writing small programs, "scripts", or just learning the language. For brevity, fn main() will be skipped in this tutorial.

This means that a "hello world" program in V is as simple as

println('hello world')

Running a project folder with several files

Suppose you have a folder with several .v files in it, where one of them contains your main() function, and the other files have other helper functions. They may be organized by topic, but still not yet structured enough to be their own separate reusable modules, and you want to compile them all into one program.

In other languages, you would have to use includes or a build system to enumerate all files, compile them separately to object files, then link them into one final executable.

In V however, you can compile and run the whole folder of .v files together, using just v run .. Passing parameters also works, so you can do: v run . --yourparam some_other_stuff

The above will first compile your files into a single program (named after your folder/project), and then it will execute the program with --yourparam some_other_stuff passed to it as CLI parameters.

Your program can then use the CLI parameters like this:

import os

println(os.args)

NB: after a successful run, V will delete the generated executable. If you want to keep it, use v -keepc run . instead, or just compile manually with v . .

NB: any V compiler flags should be passed before the run command. Everything after the source file/folder, will be passed to the program as is - it will not be processed by V.

Comments

// This is a single line comment.
/*
This is a multiline comment.
   /* It can be nested. */
*/

Functions

fn main() {
	println(add(77, 33))
	println(sub(100, 50))
}

fn add(x int, y int) int {
	return x + y
}

fn sub(x int, y int) int {
	return x - y
}

Again, the type comes after the argument's name.

Just like in Go and C, functions cannot be overloaded. This simplifies the code and improves maintainability and readability.

Hoistings

Functions can be used before their declaration: add and sub are declared after main, but can still be called from main. This is true for all declarations in V and eliminates the need for header files or thinking about the order of files and declarations.

Returning multiple values

fn foo() (int, int) {
	return 2, 3
}

a, b := foo()
println(a) // 2
println(b) // 3
c, _ := foo() // ignore values using `_`

Symbol visibility

pub fn public_function() {
}

fn private_function() {
}

Functions are private (not exported) by default. To allow other modules to use them, prepend pub. The same applies to constants and types.

Note: pub can only be used from a named module. For information about creating a module, see Modules.

Variables

name := 'Bob'
age := 20
large_number := i64(9999999999)
println(name)
println(age)
println(large_number)

Variables are declared and initialized with :=. This is the only way to declare variables in V. This means that variables always have an initial value.

The variable's type is inferred from the value on the right hand side. To choose a different type, use type conversion: the expression T(v) converts the value v to the type T.

Unlike most other languages, V only allows defining variables in functions. Global (module level) variables are not allowed. There's no global state in V (see Pure functions by default for details).

For consistency across different code bases, all variable and function names must use the snake_case style, as opposed to type names, which must use PascalCase.

Mutable variables

mut age := 20
println(age)
age = 21
println(age)

To change the value of the variable use =. In V, variables are immutable by default. To be able to change the value of the variable, you have to declare it with mut.

Try compiling the program above after removing mut from the first line.

Initialization vs assignment

Note the (important) difference between := and =. := is used for declaring and initializing, = is used for assigning.

fn main() {
	age = 21
}

This code will not compile, because the variable age is not declared. All variables need to be declared in V.

fn main() {
	age := 21
}

The values of multiple variables can be changed in one line. In this way, their values can be swapped without an intermediary variable.

mut a := 0
mut b := 1
println('$a, $b') // 0, 1
a, b = b, a
println('$a, $b') // 1, 0

Declaration errors

In development mode the compiler will warn you that you haven't used the variable (you'll get an "unused variable" warning). In production mode (enabled by passing the -prod flag to v – v -prod foo.v) it will not compile at all (like in Go).

fn main() {
	a := 10
	if true {
		a := 20 // error: redefinition of `a`
	}
	// warning: unused variable `a`
}

Unlike most languages, variable shadowing is not allowed. Declaring a variable with a name that is already used in a parent scope will cause a compilation error.

You can shadow imported modules though, as it is very useful in some situations:

import ui
import gg

fn draw(ctx &gg.Context) {
	gg := ctx.parent.get_ui().gg
	gg.draw_rect(10, 10, 100, 50)
}

V Types

Primitive types

bool

string

i8    i16  int  i64      i128 (soon)
byte  u16  u32  u64      u128 (soon)

rune // represents a Unicode code point

f32 f64

isize, usize // platform-dependent, the size is how many bytes it takes to reference any location in memory

voidptr // this one is mostly used for C interoperability

any // similar to C's void* and Go's interface{}

Please note that unlike C and Go, int is always a 32 bit integer.

There is an exception to the rule that all operators in V must have values of the same type on both sides. A small primitive type on one side can be automatically promoted if it fits completely into the data range of the type on the other side. These are the allowed possibilities:

   i8 → i16 → int → i64
                  ↘     ↘
                    f32 → f64
                  ↗     ↗
 byte → u16 → u32 → u64 ⬎
      ↘     ↘     ↘      ptr
   i8 → i16 → int → i64 ⬏

An int value for example can be automatically promoted to f64 or i64 but not to u32. (u32 would mean loss of the sign for negative values). Promotion from int to f32, however, is currently done automatically (but can lead to precision loss for large values).

Literals like 123 or 4.56 are treated in a special way. They do not lead to type promotions, however they default to int and f64 respectively, when their type has to be decided:

u := u16(12)
v := 13 + u    // v is of type `u16` - no promotion
x := f32(45.6)
y := x + 3.14  // x is of type `f32` - no promotion
a := 75        // a is of type `int` - default for int literal
b := 14.7      // b is of type `f64` - default for float literal
c := u + a     // c is of type `int` - automatic promotion of `u`'s value
d := b + x     // d is of type `f64` - automatic promotion of `x`'s value

Strings

name := 'Bob'
println(name.len)
println(name[0]) // indexing gives a byte B
println(name[1..3]) // slicing gives a string 'ob'
windows_newline := '\r\n' // escape special characters like in C
assert windows_newline.len == 2

In V, a string is a read-only array of bytes. String data is encoded using UTF-8:

s := 'hello 🌎' // emoji takes 4 bytes
assert s.len == 10

arr := s.bytes() // convert `string` to `[]byte`
assert arr.len == 10

s2 := arr.bytestr() // convert `[]byte` to `string`
assert s2 == s

String values are immutable. You cannot mutate elements:

mut s := 'hello 🌎'
s[0] = `H` // not allowed

error: cannot assign to s[i] since V strings are immutable

Note that indexing a string will produce a byte, not a rune nor another string. Indexes correspond to bytes in the string, not Unicode code points. If you want to convert the byte to a string, use the ascii_str() method:

country := 'Netherlands'
println(country[0]) // Output: 78
println(country[0].ascii_str()) // Output: N

Character literals have type rune. To denote them, use `

rocket := `🚀`
assert 'aloha!'[0] == `a`

Both single and double quotes can be used to denote strings. For consistency, vfmt converts double quotes to single quotes unless the string contains a single quote character.

For raw strings, prepend r. Raw strings are not escaped:

s := r'hello\nworld'
println(s) // "hello\nworld"

Strings can be easily converted to integers:

s := '42'
n := s.int() // 42

Runes

A rune represents a unicode character and is an alias for u32. Runes can be created like this:

x := `🚀`

A string can be converted to runes by the .runes() method.

hello := 'Hello World 👋'
hello_runes := hello.runes() // [`H`, `e`, `l`, `l`, `o`, ` `, `W`, `o`, `r`, `l`, `d`, ` `, `👋`]

String interpolation

Basic interpolation syntax is pretty simple - use $ before a variable name. The variable will be converted to a string and embedded into the literal:

name := 'Bob'
println('Hello, $name!') // Hello, Bob!

It also works with fields: 'age = $user.age'. If you need more complex expressions, use ${}: 'can register = ${user.age > 13}'.

Format specifiers similar to those in C's printf() are also supported. f, g, x, etc. are optional and specify the output format. The compiler takes care of the storage size, so there is no hd or llu.

x := 123.4567
println('x = ${x:4.2f}')
println('[${x:10}]') // pad with spaces on the left => [   123.457]
println('[${int(x):-10}]') // pad with spaces on the right => [123       ]
println('[${int(x):010}]') // pad with zeros on the left => [0000000123]

String operators

name := 'Bob'
bobby := name + 'by' // + is used to concatenate strings
println(bobby) // "Bobby"
mut s := 'hello '
s += 'world' // `+=` is used to append to a string
println(s) // "hello world"

All operators in V must have values of the same type on both sides. You cannot concatenate an integer to a string:

age := 10
println('age = ' + age) // not allowed

error: infix expr: cannot use int (right expression) as string

We have to either convert age to a string:

age := 11
println('age = ' + age.str())

or use string interpolation (preferred):

age := 12
println('age = $age')

Numbers

a := 123

This will assign the value of 123 to a. By default a will have the type int.

You can also use hexadecimal, binary or octal notation for integer literals:

a := 0x7B
b := 0b01111011
c := 0o173

All of these will be assigned the same value, 123. They will all have type int, no matter what notation you used.

V also supports writing numbers with _ as separator:

num := 1_000_000 // same as 1000000
three := 0b0_11 // same as 0b11
float_num := 3_122.55 // same as 3122.55
hexa := 0xF_F // same as 255
oct := 0o17_3 // same as 0o173

If you want a different type of integer, you can use casting:

a := i64(123)
b := byte(42)
c := i16(12345)

Assigning floating point numbers works the same way:

f := 1.0
f1 := f64(3.14)
f2 := f32(3.14)

If you do not specify the type explicitly, by default float literals will have the type of f64.

Float literals can also be declared as a power of ten:

f0 := 42e1 // 420
f1 := 123e-2 // 1.23
f2 := 456e+2 // 45600

Arrays

Basic Array Concepts

Arrays are collections of data elements of the same type. They can be represented by a list of elements surrounded by brackets. The elements can be accessed by appending an index (starting with 0) in brackets to the array variable:

mut nums := [1, 2, 3]
println(nums) // `[1, 2, 3]`
println(nums[0]) // `1`
println(nums[1]) // `2`
nums[1] = 5
println(nums) // `[1, 5, 3]`

Array Properties

There are two properties that control the "size" of an array:

• len: length - the number of pre-allocated and initialized elements in the array
• cap: capacity - the amount of memory space which has been reserved for elements, but not initialized or counted as elements. The array can grow up to this size without being reallocated. Usually, V takes care of this property automatically but there are cases where the user may want to do manual optimizations (see below).

mut nums := [1, 2, 3]
println(nums.len) // "3"
println(nums.cap) // "3" or greater
nums = [] // The array is now empty
println(nums.len) // "0"

Note that the properties are read-only fields and can't be modified by the user.

Array Initialization

The basic initialization syntax is as described above. The type of an array is determined by the first element:

• [1, 2, 3] is an array of ints ([]int).
• ['a', 'b'] is an array of strings ([]string).

The user can explicitly specify the type for the first element: [byte(16), 32, 64, 128]. V arrays are homogeneous (all elements must have the same type). This means that code like [1, 'a'] will not compile.

The above syntax is fine for a small number of known elements but for very large or empty arrays there is a second initialization syntax:

mut a := []int{len: 10000, cap: 30000, init: 3}

This creates an array of 10000 int elements that are all initialized with 3. Memory space is reserved for 30000 elements. The parameters len, cap and init are optional; len defaults to 0 and init to the default initialization of the element type (0 for numerical type, '' for string, etc). The run time system makes sure that the capacity is not smaller than len (even if a smaller value is specified explicitly):

arr := []int{len: 5, init: -1}
// `arr == [-1, -1, -1, -1, -1]`, arr.cap == 5

// Declare an empty array:
users := []int{}

Setting the capacity improves performance of pushing elements to the array as reallocations can be avoided:

mut numbers := []int{cap: 1000}
println(numbers.len) // 0
// Now appending elements won't reallocate
for i in 0 .. 1000 {
	numbers << i
}

Note: The above code uses a range for statement and a push operator (<<).

You can initialize the array by accessing the it variable as shown here:

mut square := []int{len: 6, init: it * it}
// square == [0, 1, 4, 9, 16, 25]

Array Types

An array can be of these types:

Types	Example Definition
Number	[]int,[]i64
String	[]string
Rune	[]rune
Boolean	[]bool
Array	[][]int
Struct	[]MyStructName
Channel	[]chan f64
Function	[]MyFunctionType []fn (int) bool
Interface	[]MyInterfaceName
Sum Type	[]MySumTypeName
Generic Type	[]T
Map	[]map[string]f64
Enum	[]MyEnumType
Alias	[]MyAliasTypeName
Thread	[]thread int
Reference	[]&f64
Shared	[]shared MyStructType

Example Code:

This example uses Structs and Sum Types to create an array which can handle different types (e.g. Points, Lines) of data elements.

struct Point {
	x int
	y int
}

struct Line {
	p1 Point
	p2 Point
}

type ObjectSumType = Line | Point

mut object_list := []ObjectSumType{}
object_list << Point{1, 1}
object_list << Line{
	p1: Point{3, 3}
	p2: Point{4, 4}
}
dump(object_list)
/*
object_list: [ObjectSumType(Point{
    x: 1
    y: 1
}), ObjectSumType(Line{
    p1: Point{
        x: 3
        y: 3
    }
    p2: Point{
        x: 4
        y: 4
    }
})]
*/

Multidimensional Arrays

Arrays can have more than one dimension.

2d array example:

mut a := [][]int{len: 2, init: []int{len: 3}}
a[0][1] = 2
println(a) // [[0, 2, 0], [0, 0, 0]]

3d array example:

mut a := [][][]int{len: 2, init: [][]int{len: 3, init: []int{len: 2}}}
a[0][1][1] = 2
println(a) // [[[0, 0], [0, 2], [0, 0]], [[0, 0], [0, 0], [0, 0]]]

Array Operations

Elements can be appended to the end of an array using the push operator <<. It can also append an entire array.

mut nums := [1, 2, 3]
nums << 4
println(nums) // "[1, 2, 3, 4]"
// append array
nums << [5, 6, 7]
println(nums) // "[1, 2, 3, 4, 5, 6, 7]"
mut names := ['John']
names << 'Peter'
names << 'Sam'
// names << 10  <-- This will not compile. `names` is an array of strings.

val in array returns true if the array contains val. See in operator.

names := ['John', 'Peter', 'Sam']
println(names.len) // "3"
println('Alex' in names) // "false"

Array methods

All arrays can be easily printed with println(arr) and converted to a string with s := arr.str().

Copying the data from the array is done with .clone():

nums := [1, 2, 3]
nums_copy := nums.clone()

Arrays can be efficiently filtered and mapped with the .filter() and .map() methods:

nums := [1, 2, 3, 4, 5, 6]
even := nums.filter(it % 2 == 0)
println(even) // [2, 4, 6]
// filter can accept anonymous functions
even_fn := nums.filter(fn (x int) bool {
	return x % 2 == 0
})
println(even_fn)
words := ['hello', 'world']
upper := words.map(it.to_upper())
println(upper) // ['HELLO', 'WORLD']
// map can also accept anonymous functions
upper_fn := words.map(fn (w string) string {
	return w.to_upper()
})
println(upper_fn) // ['HELLO', 'WORLD']

it is a builtin variable which refers to element currently being processed in filter/map methods.

Additionally, .any() and .all() can be used to conveniently test for elements that satisfy a condition.

nums := [1, 2, 3]
println(nums.any(it == 2)) // true
println(nums.all(it >= 2)) // false

There are further built in methods for arrays:

• b := a.repeat(n) concatenate n times the elements of a
• a.insert(i, val) insert new element val at index i and move all following elements upwards
• a.insert(i, [3, 4, 5]) insert several elements
• a.prepend(val) insert value at beginning, equivalent to a.insert(0, val)
• a.prepend(arr) insert elements of array arr at beginning
• a.trim(new_len) truncate the length (if new_length < a.len, otherwise do nothing)
• a.clear() empty the array (without changing cap, equivalent to a.trim(0))
• a.delete_many(start, size) removes size consecutive elements beginning with index start – triggers reallocation
• a.delete(index) equivalent to a.delete_many(index, 1)
• v := a.first() equivalent to v := a[0]
• v := a.last() equivalent to v := a[a.len - 1]
• v := a.pop() get last element and remove it from array
• a.delete_last() remove last element from array
• b := a.reverse() make b contain the elements of a in reversed order
• a.reverse_in_place() reverse the order of elements in a
• a.join(joiner) concatenate array of strings into a string using joiner string as a separator

Sorting Arrays

Sorting arrays of all kinds is very simple and intuitive. Special variables a and b are used when providing a custom sorting condition.

mut numbers := [1, 3, 2]
numbers.sort() // 1, 2, 3
numbers.sort(a > b) // 3, 2, 1

struct User {
	age  int
	name string
}

mut users := [User{21, 'Bob'}, User{20, 'Zarkon'}, User{25, 'Alice'}]
users.sort(a.age < b.age) // sort by User.age int field
users.sort(a.name > b.name) // reverse sort by User.name string field

V also supports custom sorting, through the sort_with_compare array method. Which expects a comparing function which will define the sort order. Useful for sorting on multiple fields at the same time by custom sorting rules. The code below sorts the array ascending on name and descending age.

struct User {
	age  int
	name string
}

mut users := [User{21, 'Bob'}, User{65, 'Bob'}, User{25, 'Alice'}]

custom_sort_fn := fn (a &User, b &User) int {
	// return -1 when a comes before b
	// return 0, when both are in same order
	// return 1 when b comes before a
	if a.name == b.name {
		if a.age < b.age {
			return 1
		}
		if a.age > b.age {
			return -1
		}
		return 0
	}
	if a.name < b.name {
		return -1
	} else if a.name > b.name {
		return 1
	}
	return 0
}
users.sort_with_compare(custom_sort_fn)

Array Slices

A slice is a part of a parent array. Initially it refers to the elements between two indices separated by a .. operator. The right-side index must be greater than or equal to the left side index.

If a right-side index is absent, it is assumed to be the array length. If a left-side index is absent, it is assumed to be 0.

nums := [0, 10, 20, 30, 40]
println(nums[1..4]) // [10, 20, 30]
println(nums[..4]) // [0, 10, 20, 30]
println(nums[1..]) // [10, 20, 30, 40]

In V slices are arrays themselves (they are not distinct types). As a result all array operations may be performed on them. E.g. they can be pushed onto an array of the same type:

array_1 := [3, 5, 4, 7, 6]
mut array_2 := [0, 1]
array_2 << array_1[..3]
println(array_2) // `[0, 1, 3, 5, 4]`

A slice is always created with the smallest possible capacity cap == len (see cap above) no matter what the capacity or length of the parent array is. As a result it is immediately reallocated and copied to another memory location when the size increases thus becoming independent from the parent array (copy on grow). In particular pushing elements to a slice does not alter the parent:

mut a := [0, 1, 2, 3, 4, 5]
mut b := a[2..4]
b[0] = 7 // `b[0]` is referring to `a[2]`
println(a) // `[0, 1, 7, 3, 4, 5]`
b << 9
// `b` has been reallocated and is now independent from `a`
println(a) // `[0, 1, 7, 3, 4, 5]` - no change
println(b) // `[7, 3, 9]`

Appending to the parent array may or may not make it independent from its child slices. The behaviour depends on the parent's capacity and is predictable:

mut a := []int{len: 5, cap: 6, init: 2}
mut b := a[1..4]
a << 3
// no reallocation - fits in `cap`
b[2] = 13 // `a[3]` is modified
a << 4
// a has been reallocated and is now independent from `b` (`cap` was exceeded)
b[1] = 3 // no change in `a`
println(a) // `[2, 2, 2, 13, 2, 3, 4]`
println(b) // `[2, 3, 13]`

Fixed size arrays

V also supports arrays with fixed size. Unlike ordinary arrays, their length is constant. You cannot append elements to them, nor shrink them. You can only modify their elements in place.

However, access to the elements of fixed size arrays is more efficient, they need less memory than ordinary arrays, and unlike ordinary arrays, their data is on the stack, so you may want to use them as buffers if you do not want additional heap allocations.

Most methods are defined to work on ordinary arrays, not on fixed size arrays. You can convert a fixed size array to an ordinary array with slicing:

mut fnums := [3]int{} // fnums is a fixed size array with 3 elements.
fnums[0] = 1
fnums[1] = 10
fnums[2] = 100
println(fnums) // => [1, 10, 100]
println(typeof(fnums).name) // => [3]int

fnums2 := [1, 10, 100]! // short init syntax that does the same (the syntax will probably change)

anums := fnums[..] // same as `anums := fnums[0..fnums.len]`
println(anums) // => [1, 10, 100]
println(typeof(anums).name) // => []int

Note that slicing will cause the data of the fixed size array to be copied to the newly created ordinary array.

Maps

mut m := map[string]int{} // a map with `string` keys and `int` values
m['one'] = 1
m['two'] = 2
println(m['one']) // "1"
println(m['bad_key']) // "0"
println('bad_key' in m) // Use `in` to detect whether such key exists
println(m.keys()) // ['one', 'two']
m.delete('two')

Maps can have keys of type string, rune, integer, float or voidptr.

The whole map can be initialized using this short syntax:

numbers := {
	'one': 1
	'two': 2
}
println(numbers)

If a key is not found, a zero value is returned by default:

sm := {
	'abc': 'xyz'
}
val := sm['bad_key']
println(val) // ''

intm := {
	1: 1234
	2: 5678
}
s := intm[3]
println(s) // 0

It's also possible to use an or {} block to handle missing keys:

mm := map[string]int{}
val := mm['bad_key'] or { panic('key not found') }

The same optional check applies to arrays:

arr := [1, 2, 3]
large_index := 999
val := arr[large_index] or { panic('out of bounds') }
println(val)
// you can also do this, if you want to *propagate* the access error:
val2 := arr[333] ?
println(val2)

Module imports

For information about creating a module, see Modules.

Modules can be imported using the import keyword:

import os

fn main() {
	// read text from stdin
	name := os.input('Enter your name: ')
	println('Hello, $name!')
}

This program can use any public definitions from the os module, such as the input function. See the standard library documentation for a list of common modules and their public symbols.

By default, you have to specify the module prefix every time you call an external function. This may seem verbose at first, but it makes code much more readable and easier to understand - it's always clear which function from which module is being called. This is especially useful in large code bases.

Cyclic module imports are not allowed, like in Go.

Selective imports

You can also import specific functions and types from modules directly:

import os { input }

fn main() {
	// read text from stdin
	name := input('Enter your name: ')
	println('Hello, $name!')
}

Note: This will import the module as well. Also, this is not allowed for constants - they must always be prefixed.

You can import several specific symbols at once:

import os { input, user_os }

name := input('Enter your name: ')
println('Name: $name')
os := user_os()
println('Your OS is ${os}.')

Module import aliasing

Any imported module name can be aliased using the as keyword:

NOTE: this example will not compile unless you have created mymod/sha256.v

import crypto.sha256
import mymod.sha256 as mysha256

fn main() {
	v_hash := sha256.sum('hi'.bytes()).hex()
	my_hash := mysha256.sum('hi'.bytes()).hex()
	assert my_hash == v_hash
}

You cannot alias an imported function or type. However, you can redeclare a type.

import time
import math

type MyTime = time.Time

fn (mut t MyTime) century() int {
	return int(1.0 + math.trunc(f64(t.year) * 0.009999794661191))
}

fn main() {
	mut my_time := MyTime{
		year: 2020
		month: 12
		day: 25
	}
	println(time.new_time(my_time).utc_string())
	println('Century: $my_time.century()')
}

Statements & expressions

If

a := 10
b := 20
if a < b {
	println('$a < $b')
} else if a > b {
	println('$a > $b')
} else {
	println('$a == $b')
}

if statements are pretty straightforward and similar to most other languages. Unlike other C-like languages, there are no parentheses surrounding the condition and the braces are always required.

if can be used as an expression:

num := 777
s := if num % 2 == 0 { 'even' } else { 'odd' }
println(s)
// "odd"

Type checks and casts

You can check the current type of a sum type using is and its negated form !is.

You can do it either in an if:

struct Abc {
	val string
}

struct Xyz {
	foo string
}

type Alphabet = Abc | Xyz

x := Alphabet(Abc{'test'}) // sum type
if x is Abc {
	// x is automatically casted to Abc and can be used here
	println(x)
}
if x !is Abc {
	println('Not Abc')
}

or using match:

match x {
	Abc {
		// x is automatically casted to Abc and can be used here
		println(x)
	}
	Xyz {
		// x is automatically casted to Xyz and can be used here
		println(x)
	}
}

This works also with struct fields:

struct MyStruct {
	x int
}

struct MyStruct2 {
	y string
}

type MySumType = MyStruct | MyStruct2

struct Abc {
	bar MySumType
}

x := Abc{
	bar: MyStruct{123} // MyStruct will be converted to MySumType type automatically
}
if x.bar is MyStruct {
	// x.bar is automatically casted
	println(x.bar)
}
match x.bar {
	MyStruct {
		// x.bar is automatically casted
		println(x.bar)
	}
	else {}
}

Mutable variables can change, and doing a cast would be unsafe. However, sometimes it's useful to type cast despite mutability. In such cases the developer must mark the expression with the mut keyword to tell the compiler that they know what they're doing.

It works like this:

mut x := MySumType(MyStruct{123})
if mut x is MyStruct {
	// x is casted to MyStruct even if it's mutable
	// without the mut keyword that wouldn't work
	println(x)
}
// same with match
match mut x {
	MyStruct {
		// x is casted to MyStruct even if it's mutable
		// without the mut keyword that wouldn't work
		println(x)
	}
}

In operator

in allows to check whether an array or a map contains an element. To do the opposite, use !in.

nums := [1, 2, 3]
println(1 in nums) // true
println(4 !in nums) // true
m := {
	'one': 1
	'two': 2
}
println('one' in m) // true
println('three' !in m) // true

It's also useful for writing boolean expressions that are clearer and more compact:

enum Token {
	plus
	minus
	div
	mult
}

struct Parser {
	token Token
}

parser := Parser{}
if parser.token == .plus || parser.token == .minus || parser.token == .div || parser.token == .mult {
	// ...
}
if parser.token in [.plus, .minus, .div, .mult] {
	// ...
}

V optimizes such expressions, so both if statements above produce the same machine code and no arrays are created.

For loop

V has only one looping keyword: for, with several forms.

for/in

This is the most common form. You can use it with an array, map or numeric range.

Array for

numbers := [1, 2, 3, 4, 5]
for num in numbers {
	println(num)
}
names := ['Sam', 'Peter']
for i, name in names {
	println('$i) $name')
	// Output: 0) Sam
	//         1) Peter
}

The for value in arr form is used for going through elements of an array. If an index is required, an alternative form for index, value in arr can be used.

Note, that the value is read-only. If you need to modify the array while looping, you need to declare the element as mutable:

mut numbers := [0, 1, 2]
for mut num in numbers {
	num++
}
println(numbers) // [1, 2, 3]

When an identifier is just a single underscore, it is ignored.

Custom iterators

Types that implement a next method returning an Option can be iterated with a for loop.

struct SquareIterator {
	arr []int
mut:
	idx int
}

fn (mut iter SquareIterator) next() ?int {
	if iter.idx >= iter.arr.len {
		return error('')
	}
	defer {
		iter.idx++
	}
	return iter.arr[iter.idx] * iter.arr[iter.idx]
}

nums := [1, 2, 3, 4, 5]
iter := SquareIterator{
	arr: nums
}
for squared in iter {
	println(squared)
}

The code above prints:

1
4
9
16
25

Map for

m := {
	'one': 1
	'two': 2
}
for key, value in m {
	println('$key -> $value')
	// Output: one -> 1
	//         two -> 2
}

Either key or value can be ignored by using a single underscore as the identifier.

m := {
	'one': 1
	'two': 2
}
// iterate over keys
for key, _ in m {
	println(key)
	// Output: one
	//         two
}
// iterate over values
for _, value in m {
	println(value)
	// Output: 1
	//         2
}

Range for

// Prints '01234'
for i in 0 .. 5 {
	print(i)
}

low..high means an exclusive range, which represents all values from low up to but not including high.

Condition for

mut sum := 0
mut i := 0
for i <= 100 {
	sum += i
	i++
}
println(sum) // "5050"

This form of the loop is similar to while loops in other languages. The loop will stop iterating once the boolean condition evaluates to false. Again, there are no parentheses surrounding the condition, and the braces are always required.

Bare for

mut num := 0
for {
	num += 2
	if num >= 10 {
		break
	}
}
println(num) // "10"

The condition can be omitted, resulting in an infinite loop.

C for

for i := 0; i < 10; i += 2 {
	// Don't print 6
	if i == 6 {
		continue
	}
	println(i)
}

Finally, there's the traditional C style for loop. It's safer than the while form because with the latter it's easy to forget to update the counter and get stuck in an infinite loop.

Here i doesn't need to be declared with mut since it's always going to be mutable by definition.

Labelled break & continue

break and continue control the innermost for loop by default. You can also use break and continue followed by a label name to refer to an outer for loop:

outer: for i := 4; true; i++ {
	println(i)
	for {
		if i < 7 {
			continue outer
		} else {
			break outer
		}
	}
}

The label must immediately precede the outer loop. The above code prints:

4
5
6
7

Match

os := 'windows'
print('V is running on ')
match os {
	'darwin' { println('macOS.') }
	'linux' { println('Linux.') }
	else { println(os) }
}

A match statement is a shorter way to write a sequence of if - else statements. When a matching branch is found, the following statement block will be run. The else branch will be run when no other branches match.

number := 2
s := match number {
	1 { 'one' }
	2 { 'two' }
	else { 'many' }
}

A match statement can also to be used as an if - else if - else alternative:

match true {
	2 > 4 { println('if') }
	3 == 4 { println('else if') }
	2 == 2 { println('else if2') }
	else { println('else') }
}
// 'else if2' should be printed

or as an unless alternative: unless Ruby

match false {
	2 > 4 { println('if') }
	3 == 4 { println('else if') }
	2 == 2 { println('else if2') }
	else { println('else') }
}
// 'if' should be printed

A match expression returns the value of the final expression from the matching branch.

enum Color {
	red
	blue
	green
}

fn is_red_or_blue(c Color) bool {
	return match c {
		.red, .blue { true } // comma can be used to test multiple values
		.green { false }
	}
}

A match statement can also be used to branch on the variants of an enum by using the shorthand .variant_here syntax. An else branch is not allowed when all the branches are exhaustive.

c := `v`
typ := match c {
	`0`...`9` { 'digit' }
	`A`...`Z` { 'uppercase' }
	`a`...`z` { 'lowercase' }
	else { 'other' }
}
println(typ)
// 'lowercase'

You can also use ranges as match patterns. If the value falls within the range of a branch, that branch will be executed.

Note that the ranges use ... (three dots) rather than .. (two dots). This is because the range is inclusive of the last element, rather than exclusive (as .. ranges are). Using .. in a match branch will throw an error.

Note: match as an expression is not usable in for loop and if statements.

Defer

A defer statement defers the execution of a block of statements until the surrounding function returns.

import os

fn read_log() {
	mut ok := false
	mut f := os.open('log.txt') or { panic(err.msg) }
	defer {
		f.close()
	}
	// ...
	if !ok {
		// defer statement will be called here, the file will be closed
		return
	}
	// ...
	// defer statement will be called here, the file will be closed
}

If the function returns a value the defer block is executed after the return expression is evaluated:

import os

enum State {
	normal
	write_log
	return_error
}

// write log file and return number of bytes written
fn write_log(s State) ?int {
	mut f := os.create('log.txt') ?
	defer {
		f.close()
	}
	if s == .write_log {
		// `f.close()` will be called after `f.write()` has been
		// executed, but before `write_log()` finally returns the
		// number of bytes written to `main()`
		return f.writeln('This is a log file')
	} else if s == .return_error {
		// the file will be closed after the `error()` function
		// has returned - so the error message will still report
		// it as open
		return error('nothing written; file open: $f.is_opened')
	}
	// the file will be closed here, too
	return 0
}

fn main() {
	n := write_log(.return_error) or {
		println('Error: $err')
		0
	}
	println('$n bytes written')
}

Structs

struct Point {
	x int
	y int
}

mut p := Point{
	x: 10
	y: 20
}
println(p.x) // Struct fields are accessed using a dot
// Alternative literal syntax for structs with 3 fields or fewer
p = Point{10, 20}
assert p.x == 10

Heap structs

Structs are allocated on the stack. To allocate a struct on the heap and get a reference to it, use the & prefix:

struct Point {
	x int
	y int
}

p := &Point{10, 10}
// References have the same syntax for accessing fields
println(p.x)

The type of p is &Point. It's a reference to Point. References are similar to Go pointers and C++ references.

Embedded structs

V doesn't allow subclassing, but it supports embedded structs:

struct Widget {
mut:
	x int
	y int
}

struct Button {
	Widget
	title string
}

mut button := Button{
	title: 'Click me'
}
button.x = 3

Without embedding we'd have to name the Widget field and do:

button.widget.x = 3

Default field values

struct Foo {
	n   int    // n is 0 by default
	s   string // s is '' by default
	a   []int  // a is `[]int{}` by default
	pos int = -1 // custom default value
}

All struct fields are zeroed by default during the creation of the struct. Array and map fields are allocated.

It's also possible to define custom default values.

Required fields

struct Foo {
	n int [required]
}

You can mark a struct field with the [required] attribute, to tell V that that field must be initialized when creating an instance of that struct.

This example will not compile, since the field n isn't explicitly initialized:

_ = Foo{}

Short struct literal syntax

struct Point {
	x int
	y int
}

mut p := Point{
	x: 10
	y: 20
}
p = Point{
	x: 30
	y: 4
}
assert p.y == 4
//
// array: first element defines type of array
points := [Point{10, 20}, Point{20, 30}, Point{40, 50}]
println(points) // [Point{x: 10, y: 20}, Point{x: 20, y: 30}, Point{x: 40,y: 50}]

Omitting the struct name also works for returning a struct literal or passing one as a function argument.

Trailing struct literal arguments

V doesn't have default function arguments or named arguments, for that trailing struct literal syntax can be used instead:

[params]
struct ButtonConfig {
	text        string
	is_disabled bool
	width       int = 70
	height      int = 20
}

struct Button {
	text   string
	width  int
	height int
}

fn new_button(c ButtonConfig) &Button {
	return &Button{
		width: c.width
		height: c.height
		text: c.text
	}
}

button := new_button(text: 'Click me', width: 100)
// the height is unset, so it's the default value
assert button.height == 20

As you can see, both the struct name and braces can be omitted, instead of:

new_button(ButtonConfig{text:'Click me', width:100})

This only works for functions that take a struct for the last argument.

NB: the [params] tag is used to tell V, that the trailing struct parameter can be omitted entirely, so that you can write button := new_button(). Without it, you have to specify at least one of the field names, even if it has its default value, otherwise the compiler will produce this error message, when you call the function with no parameters: error: expected 1 arguments, but got 0.

Access modifiers

Struct fields are private and immutable by default (making structs immutable as well). Their access modifiers can be changed with pub and mut. In total, there are 5 possible options:

struct Foo {
	a int // private immutable (default)
mut:
	b int // private mutable
	c int // (you can list multiple fields with the same access modifier)
pub:
	d int // public immutable (readonly)
pub mut:
	e int // public, but mutable only in parent module
__global:
	// (not recommended to use, that's why the 'global' keyword starts with __)
	f int // public and mutable both inside and outside parent module
}

For example, here's the string type defined in the builtin module:

struct string {
    str &byte
pub:
    len int
}

It's easy to see from this definition that string is an immutable type. The byte pointer with the string data is not accessible outside builtin at all. The len field is public, but immutable:

fn main() {
	str := 'hello'
	len := str.len // OK
	str.len++ // Compilation error
}

This means that defining public readonly fields is very easy in V, no need in getters/setters or properties.

Methods

struct User {
	age int
}

fn (u User) can_register() bool {
	return u.age > 16
}

user := User{
	age: 10
}
println(user.can_register()) // "false"
user2 := User{
	age: 20
}
println(user2.can_register()) // "true"

V doesn't have classes, but you can define methods on types. A method is a function with a special receiver argument. The receiver appears in its own argument list between the fn keyword and the method name. Methods must be in the same module as the receiver type.

In this example, the can_register method has a receiver of type User named u. The convention is not to use receiver names like self or this, but a short, preferably one letter long, name.

Unions

Just like structs, unions support embedding.

struct Rgba32_Component {
	r byte
	g byte
	b byte
	a byte
}

union Rgba32 {
	Rgba32_Component
	value u32
}

clr1 := Rgba32{
	value: 0x008811FF
}

clr2 := Rgba32{
	Rgba32_Component: Rgba32_Component{
		a: 128
	}
}

sz := sizeof(Rgba32)
unsafe {
	println('Size: ${sz}B,clr1.b: $clr1.b,clr2.b: $clr2.b')
}

Output: Size: 4B, clr1.b: 136, clr2.b: 0

Union member access must be performed in an unsafe block.

Note that the embedded struct arguments are not necessarily stored in the order listed.

Functions 2

Pure functions by default

V functions are pure by default, meaning that their return values are a function of their arguments only, and their evaluation has no side effects (besides I/O).

This is achieved by a lack of global variables and all function arguments being immutable by default, even when references are passed.

V is not a purely functional language however.

There is a compiler flag to enable global variables (-enable-globals), but this is intended for low-level applications like kernels and drivers.

Mutable arguments

It is possible to modify function arguments by using the keyword mut:

struct User {
	name string
mut:
	is_registered bool
}

fn (mut u User) register() {
	u.is_registered = true
}

mut user := User{}
println(user.is_registered) // "false"
user.register()
println(user.is_registered) // "true"

In this example, the receiver (which is simply the first argument) is marked as mutable, so register() can change the user object. The same works with non-receiver arguments:

fn multiply_by_2(mut arr []int) {
	for i in 0 .. arr.len {
		arr[i] *= 2
	}
}

mut nums := [1, 2, 3]
multiply_by_2(mut nums)
println(nums)
// "[2, 4, 6]"

Note, that you have to add mut before nums when calling this function. This makes it clear that the function being called will modify the value.

It is preferable to return values instead of modifying arguments, e.g. user = register(user) (or user.register()) instead of register(mut user). Modifying arguments should only be done in performance-critical parts of your application to reduce allocations and copying.

For this reason V doesn't allow the modification of arguments with primitive types (e.g. integers). Only more complex types such as arrays and maps may be modified.

Struct update syntax

V makes it easy to return a modified version of an object:

struct User {
	name          string
	age           int
	is_registered bool
}

fn register(u User) User {
	return User{
		...u
		is_registered: true
	}
}

mut user := User{
	name: 'abc'
	age: 23
}
user = register(user)
println(user)

Variable number of arguments

fn sum(a ...int) int {
	mut total := 0
	for x in a {
		total += x
	}
	return total
}

println(sum()) // 0
println(sum(1)) // 1
println(sum(2, 3)) // 5
// using array decomposition
a := [2, 3, 4]
println(sum(...a)) // <-- using prefix ... here. output: 9
b := [5, 6, 7]
println(sum(...b)) // output: 18

Anonymous & higher order functions

fn sqr(n int) int {
	return n * n
}

fn cube(n int) int {
	return n * n * n
}

fn run(value int, op fn (int) int) int {
	return op(value)
}

fn main() {
	// Functions can be passed to other functions
	println(run(5, sqr)) // "25"
	// Anonymous functions can be declared inside other functions:
	double_fn := fn (n int) int {
		return n + n
	}
	println(run(5, double_fn)) // "10"
	// Functions can be passed around without assigning them to variables:
	res := run(5, fn (n int) int {
		return n + n
	})
	println(res) // "10"
	// You can even have an array/map of functions:
	fns := [sqr, cube]
	println(fns[0](10)) // "100"
	fns_map := {
		'sqr':  sqr
		'cube': cube
	}
	println(fns_map['cube'](2)) // "8"
}

Closures

V supports closures too. This means that anonymous functions can inherit variables from the scope they were created in. They must do so explicitly by listing all variables that are inherited.

Warning: currently works on Unix-based, x64 architectures only. Some work is in progress to make closures work on Windows, then other architectures.

my_int := 1
my_closure := fn [my_int] () {
	println(my_int)
}
my_closure() // prints 1

Inherited variables are copied when the anonymous function is created. This means that if the original variable is modified after the creation of the function, the modification won't be reflected in the function.

mut i := 1
func := fn [i] () int {
	return i
}
println(func() == 1) // true
i = 123
println(func() == 1) // still true

However, the variable can be modified inside the anonymous function. The change won't be reflected outside, but will be in the later function calls.

fn new_counter() fn () int {
	mut i := 0
	return fn [mut i] () int {
		i++
		return i
	}
}

c := new_counter()
println(c()) // 1
println(c()) // 2
println(c()) // 3

If you need the value to be modified outside the function, use a reference. Warning: you need to make sure the reference is always valid, otherwise this can result in undefined behavior.

mut i := 0
mut ref := &i
print_counter := fn [ref] () {
	println(*ref)
}

print_counter() // 0
i = 10
print_counter() // 10

References

struct Foo {}

fn (foo Foo) bar_method() {
	// ...
}

fn bar_function(foo Foo) {
	// ...
}

If a function argument is immutable (like foo in the examples above) V can pass it either by value or by reference. The compiler will decide, and the developer doesn't need to think about it.

You no longer need to remember whether you should pass the struct by value or by reference.

You can ensure that the struct is always passed by reference by adding &:

struct Foo {
	abc int
}

fn (foo &Foo) bar() {
	println(foo.abc)
}

foo is still immutable and can't be changed. For that, (mut foo Foo) must be used.

In general, V's references are similar to Go pointers and C++ references. For example, a generic tree structure definition would look like this:

struct Node<T> {
	val   T
	left  &Node<T>
	right &Node<T>
}

To dereference a reference, use the * operator, just like in C.

Constants

const (
	pi    = 3.14
	world = '世界'
)

println(pi)
println(world)

Constants are declared with const. They can only be defined at the module level (outside of functions). Constant values can never be changed. You can also declare a single constant separately:

const e = 2.71828

V constants are more flexible than in most languages. You can assign more complex values:

struct Color {
	r int
	g int
	b int
}

fn rgb(r int, g int, b int) Color {
	return Color{
		r: r
		g: g
		b: b
	}
}

const (
	numbers = [1, 2, 3]
	red     = Color{
		r: 255
		g: 0
		b: 0
	}
	// evaluate function call at compile-time*
	blue = rgb(0, 0, 255)
)

println(numbers)
println(red)
println(blue)

* WIP - for now function calls are evaluated at program start-up

Global variables are not normally allowed, so this can be really useful.

Modules

Constants can be made public with pub const:

module mymodule

pub const golden_ratio = 1.61803

fn calc() {
	println(mymodule.golden_ratio)
}

The pub keyword is only allowed before the const keyword and cannot be used inside a const ( ) block.

Outside from module main all constants need to be prefixed with the module name.

Required module prefix

When naming constants, snake_case must be used. In order to distinguish consts from local variables, the full path to consts must be specified. For example, to access the PI const, full math.pi name must be used both outside the math module, and inside it. That restriction is relaxed only for the main module (the one containing your fn main()), where you can use the unqualified name of constants defined there, i.e. numbers, rather than main.numbers.

vfmt takes care of this rule, so you can type println(pi) inside the math module, and vfmt will automatically update it to println(math.pi).

Builtin functions

Some functions are builtin like println. Here is the complete list:

fn print(s string) // print anything on sdtout
fn println(s string) // print anything and a newline on sdtout

fn eprint(s string) // same as print(), but use stderr
fn eprintln(s string) // same as println(), but use stderr

fn exit(code int) // terminate the program with a custom error code
fn panic(s string) // print a message and backtraces on stderr, and terminate the program with error code 1
fn print_backtrace() // print backtraces on stderr

println is a simple yet powerful builtin function, that can print anything: strings, numbers, arrays, maps, structs.

struct User {
	name string
	age  int
}

println(1) // "1"
println('hi') // "hi"
println([1, 2, 3]) // "[1, 2, 3]"
println(User{ name: 'Bob', age: 20 }) // "User{name:'Bob', age:20}"

Printing custom types

If you want to define a custom print value for your type, simply define a .str() string method:

struct Color {
	r int
	g int
	b int
}

pub fn (c Color) str() string {
	return '{$c.r, $c.g, $c.b}'
}

red := Color{
	r: 255
	g: 0
	b: 0
}
println(red)

Modules

Every file in the root of a folder is part of the same module. Simple programs don't need to specify module name, in which case it defaults to 'main'.

V is a very modular language. Creating reusable modules is encouraged and is quite easy to do. To create a new module, create a directory with your module's name containing .v files with code:

cd ~/code/modules
mkdir mymodule
vim mymodule/myfile.v

// myfile.v
module mymodule

// To export a function we have to use `pub`
pub fn say_hi() {
	println('hello from mymodule!')
}

You can now use mymodule in your code:

import mymodule

fn main() {
	mymodule.say_hi()
}

• Module names should be short, under 10 characters.
• Module names must use snake_case.
• Circular imports are not allowed.
• You can have as many .v files in a module as you want.
• You can create modules anywhere.
• All modules are compiled statically into a single executable.

init functions

If you want a module to automatically call some setup/initialization code when it is imported, you can use a module init function:

fn init() {
	// your setup code here ...
}

The init function cannot be public - it will be called automatically. This feature is particularly useful for initializing a C library.

Type Declarations

Interfaces

struct Dog {
	breed string
}

struct Cat {
	breed string
}

fn (d Dog) speak() string {
	return 'woof'
}

fn (c Cat) speak() string {
	return 'meow'
}

// unlike Go and like TypeScript, V's interfaces can define fields, not just methods.
interface Speaker {
	breed string
	speak() string
}

dog := Dog{'Leonberger'}
cat := Cat{'Siamese'}

mut arr := []Speaker{}
arr << dog
arr << cat
for item in arr {
	println('a $item.breed says: $item.speak()')
}

Implement an interface

A type implements an interface by implementing its methods and fields. There is no explicit declaration of intent, no "implements" keyword.

An interface can have a mut: section. Implementing types will need to have a mut receiver, for methods declared in the mut: section of an interface.

module main

pub interface Foo {
	write(string) string
}

// => the method signature of a type, implementing interface Foo should be:
// `pub fn (s Type) write(a string) string`

pub interface Bar {
mut:
	write(string) string
}

// => the method signature of a type, implementing interface Bar should be:
// `pub fn (mut s Type) write(a string) string`

struct MyStruct {}

// MyStruct implements the interface Foo, but *not* interface Bar
pub fn (s MyStruct) write(a string) string {
	return a
}

fn main() {
	s1 := MyStruct{}
	fn1(s1)
	// fn2(s1) -> compile error, since MyStruct does not implement Bar
}

fn fn1(s Foo) {
	println(s.write('Foo'))
}

// fn fn2(s Bar) { // does not match
//      println(s.write('Foo'))
// }

Casting an interface

We can test the underlying type of an interface using dynamic cast operators:

interface Something {}

fn announce(s Something) {
	if s is Dog {
		println('a $s.breed dog') // `s` is automatically cast to `Dog` (smart cast)
	} else if s is Cat {
		println('a $s.breed cat')
	} else {
		println('something else')
	}
}

For more information, see Dynamic casts.

Interface method definitions

Also unlike Go, an interface may implement a method. These methods are not implemented by structs which implement that interface.

When a struct is wrapped in an interface that has implemented a method with the same name as one implemented by this struct, only the method implemented on the interface is called.

struct Cat {}

fn (c Cat) speak() string {
	return 'meow!'
}

interface Adoptable {}

fn (a Adoptable) speak() string {
	return 'adopt me!'
}

fn new_adoptable() Adoptable {
	return Cat{}
}

fn main() {
	cat := Cat{}
	assert cat.speak() == 'meow!'
	a := new_adoptable()
	assert a.speak() == 'adopt me!'
	if a is Cat {
		println(a.speak()) // meow!
	}
}

Embedded interface

Interfaces support embedding, just like structs:

pub interface Reader {
mut:
	read(mut buf []byte) ?int
}

pub interface Writer {
mut:
	write(buf []byte) ?int
}

// ReaderWriter embeds both Reader and Writer.
// The effect is the same as copy/pasting all of the
// Reader and all of the Writer methods/fields into
// ReaderWriter.
pub interface ReaderWriter {
	Reader
	Writer
}

Function Types

You can use type aliases for naming specific function signatures - for example:

type Filter = fn (string) string

This works like any other type - for example, a function can accept an argument of a function type:

type Filter = fn (string) string

fn filter(s string, f Filter) string {
	return f(s)
}

V has duck-typing, so functions don't need to declare compatibility with a function type - they just have to be compatible:

fn uppercase(s string) string {
	return s.to_upper()
}

// now `uppercase` can be used everywhere where Filter is expected

Compatible functions can also be explicitly cast to a function type:

my_filter := Filter(uppercase)

The cast here is purely informational - again, duck-typing means that the resulting type is the same without an explicit cast:

my_filter := uppercase

You can pass the assigned function as an argument:

println(filter('Hello world', my_filter)) // prints `HELLO WORLD`

And you could of course have passed it directly as well, without using a local variable:

println(filter('Hello world', uppercase))

And this works with anonymous functions as well:

println(filter('Hello world', fn (s string) string {
	return s.to_upper()
}))

You can see the complete example here.

Enums

enum Color {
	red
	green
	blue
}

mut color := Color.red
// V knows that `color` is a `Color`. No need to use `color = Color.green` here.
color = .green
println(color) // "green"
match color {
	.red { println('the color was red') }
	.green { println('the color was green') }
	.blue { println('the color was blue') }
}

Enum match must be exhaustive or have an else branch. This ensures that if a new enum field is added, it's handled everywhere in the code.

Enum fields cannot re-use reserved keywords. However, reserved keywords may be escaped with an @.

enum Color {
	@none
	red
	green
	blue
}

color := Color.@none
println(color)

Integers may be assigned to enum fields.

enum Grocery {
	apple
	orange = 5
	pear
}

g1 := int(Grocery.apple)
g2 := int(Grocery.orange)
g3 := int(Grocery.pear)
println('Grocery IDs: $g1, $g2, $g3')

Output: Grocery IDs: 0, 5, 6.

Operations are not allowed on enum variables; they must be explicitly cast to int.

Sum types

A sum type instance can hold a value of several different types. Use the type keyword to declare a sum type:

struct Moon {}

struct Mars {}

struct Venus {}

type World = Mars | Moon | Venus

sum := World(Moon{})
assert sum.type_name() == 'Moon'
println(sum)

The built-in method type_name returns the name of the currently held type.

With sum types you could build recursive structures and write concise but powerful code on them.

// V's binary tree
struct Empty {}

struct Node {
	value f64
	left  Tree
	right Tree
}

type Tree = Empty | Node

// sum up all node values
fn sum(tree Tree) f64 {
	return match tree {
		Empty { 0 }
		Node { tree.value + sum(tree.left) + sum(tree.right) }
	}
}

fn main() {
	left := Node{0.2, Empty{}, Empty{}}
	right := Node{0.3, Empty{}, Node{0.4, Empty{}, Empty{}}}
	tree := Node{0.5, left, right}
	println(sum(tree)) // 0.2 + 0.3 + 0.4 + 0.5 = 1.4
}

Enums can have methods, just like structs

enum Cycle {
	one
	two
	three
}

fn (c Cycle) next() Cycle {
	match c {
		.one {
			return .two
		}
		.two {
			return .three
		}
		.three {
			return .one
		}
	}
}

mut c := Cycle.one
for _ in 0 .. 10 {
	println(c)
	c = c.next()
}

Output:

one
two
three
one
two
three
one
two
three
one

Dynamic casts

To check whether a sum type instance holds a certain type, use sum is Type. To cast a sum type to one of its variants you can use sum as Type:

struct Moon {}

struct Mars {}

struct Venus {}

type World = Mars | Moon | Venus

fn (m Mars) dust_storm() bool {
	return true
}

fn main() {
	mut w := World(Moon{})
	assert w is Moon
	w = Mars{}
	// use `as` to access the Mars instance
	mars := w as Mars
	if mars.dust_storm() {
		println('bad weather!')
	}
}

as will panic if w doesn't hold a Mars instance. A safer way is to use a smart cast.

Smart casting

if w is Mars {
	assert typeof(w).name == 'Mars'
	if w.dust_storm() {
		println('bad weather!')
	}
}

w has type Mars inside the body of the if statement. This is known as flow-sensitive typing. If w is a mutable identifier, it would be unsafe if the compiler smart casts it without a warning. That's why you have to declare a mut before the is expression:

if mut w is Mars {
	assert typeof(w).name == 'Mars'
	if w.dust_storm() {
		println('bad weather!')
	}
}

Otherwise w would keep its original type.

This works for both, simple variables and complex expressions like user.name

Matching sum types

You can also use match to determine the variant:

struct Moon {}

struct Mars {}

struct Venus {}

type World = Mars | Moon | Venus

fn open_parachutes(n int) {
	println(n)
}

fn land(w World) {
	match w {
		Moon {} // no atmosphere
		Mars {
			// light atmosphere
			open_parachutes(3)
		}
		Venus {
			// heavy atmosphere
			open_parachutes(1)
		}
	}
}

match must have a pattern for each variant or have an else branch.

struct Moon {}
struct Mars {}
struct Venus {}

type World = Moon | Mars | Venus

fn (m Moon) moon_walk() {}
fn (m Mars) shiver() {}
fn (v Venus) sweat() {}

fn pass_time(w World) {
    match w {
        // using the shadowed match variable, in this case `w` (smart cast)
        Moon { w.moon_walk() }
        Mars { w.shiver() }
        else {}
    }
}

Type aliases

To define a new type NewType as an alias for ExistingType, do type NewType = ExistingType.
This is a special case of a sum type declaration.

Option/Result types and error handling

Option types are declared with ?Type:

struct User {
	id   int
	name string
}

struct Repo {
	users []User
}

fn (r Repo) find_user_by_id(id int) ?User {
	for user in r.users {
		if user.id == id {
			// V automatically wraps this into an option type
			return user
		}
	}
	return error('User $id not found')
}

fn main() {
	repo := Repo{
		users: [User{1, 'Andrew'}, User{2, 'Bob'}, User{10, 'Charles'}]
	}
	user := repo.find_user_by_id(10) or { // Option types must be handled by `or` blocks
		return
	}
	println(user.id) // "10"
	println(user.name) // "Charles"
}

V combines Option and Result into one type, so you don't need to decide which one to use.

The amount of work required to "upgrade" a function to an optional function is minimal; you have to add a ? to the return type and return an error when something goes wrong.

If you don't need to return an error message, you can simply return none (this is a more efficient equivalent of return error("")).

This is the primary mechanism for error handling in V. They are still values, like in Go, but the advantage is that errors can't be unhandled, and handling them is a lot less verbose. Unlike other languages, V does not handle exceptions with throw/try/catch blocks.

err is defined inside an or block and is set to the string message passed to the error() function. err is empty if none was returned.

user := repo.find_user_by_id(7) or {
	println(err) // "User 7 not found"
	return
}

Handling optionals

There are four ways of handling an optional. The first method is to propagate the error:

import net.http

fn f(url string) ?string {
	resp := http.get(url) ?
	return resp.text
}

http.get returns ?http.Response. Because ? follows the call, the error will be propagated to the caller of f. When using ? after a function call producing an optional, the enclosing function must return an optional as well. If error propagation is used in the main() function it will panic instead, since the error cannot be propagated any further.

The body of f is essentially a condensed version of:

    resp := http.get(url) or { return err }
    return resp.text

---

The second method is to break from execution early:

user := repo.find_user_by_id(7) or { return }

Here, you can either call panic() or exit(), which will stop the execution of the entire program, or use a control flow statement (return, break, continue, etc) to break from the current block. Note that break and continue can only be used inside a for loop.

V does not have a way to forcibly "unwrap" an optional (as other languages do, for instance Rust's unwrap() or Swift's !). To do this, use or { panic(err.msg) } instead.

---

The third method is to provide a default value at the end of the or block. In case of an error, that value would be assigned instead, so it must have the same type as the content of the Option being handled.

fn do_something(s string) ?string {
	if s == 'foo' {
		return 'foo'
	}
	return error('invalid string') // Could be `return none` as well
}

a := do_something('foo') or { 'default' } // a will be 'foo'
b := do_something('bar') or { 'default' } // b will be 'default'
println(a)
println(b)

---

The fourth method is to use if unwrapping:

import net.http

if resp := http.get('https://google.com') {
	println(resp.text) // resp is a http.Response, not an optional
} else {
	println(err)
}

Above, http.get returns a ?http.Response. resp is only in scope for the first if branch. err is only in scope for the else branch.

Generics

struct Repo<T> {
    db DB
}

struct User {
	id   int
	name string
}

struct Post {
	id   int
	user_id int
	title string
	body string
}

fn new_repo<T>(db DB) Repo<T> {
    return Repo<T>{db: db}
}

// This is a generic function. V will generate it for every type it's used with.
fn (r Repo<T>) find_by_id(id int) ?T {
    table_name := T.name // in this example getting the name of the type gives us the table name
    return r.db.query_one<T>('select * from $table_name where id = ?', id)
}

db := new_db()
users_repo := new_repo<User>(db) // returns Repo<User>
posts_repo := new_repo<Post>(db) // returns Repo<Post>
user := users_repo.find_by_id(1)? // find_by_id<User>
post := posts_repo.find_by_id(1)? // find_by_id<Post>

Currently generic function definitions must declare their type parameters, but in future V will infer generic type parameters from single-letter type names in runtime parameter types. This is why find_by_id can omit <T>, because the receiver argument r uses a generic type T.

Another example:

fn compare<T>(a T, b T) int {
	if a < b {
		return -1
	}
	if a > b {
		return 1
	}
	return 0
}

// compare<int>
println(compare(1, 0)) // Outputs: 1
println(compare(1, 1)) //          0
println(compare(1, 2)) //         -1
// compare<string>
println(compare('1', '0')) // Outputs: 1
println(compare('1', '1')) //          0
println(compare('1', '2')) //         -1
// compare<f64>
println(compare(1.1, 1.0)) // Outputs: 1
println(compare(1.1, 1.1)) //          0
println(compare(1.1, 1.2)) //         -1

Concurrency

Spawning Concurrent Tasks

V's model of concurrency is very similar to Go's. To run foo() concurrently in a different thread, just call it with go foo():

import math

fn p(a f64, b f64) { // ordinary function without return value
	c := math.sqrt(a * a + b * b)
	println(c)
}

fn main() {
	go p(3, 4)
	// p will be run in parallel thread
}

Sometimes it is necessary to wait until a parallel thread has finished. This can be done by assigning a handle to the started thread and calling the wait() method to this handle later:

import math

fn p(a f64, b f64) { // ordinary function without return value
	c := math.sqrt(a * a + b * b)
	println(c) // prints `5`
}

fn main() {
	h := go p(3, 4)
	// p() runs in parallel thread
	h.wait()
	// p() has definitely finished
}

This approach can also be used to get a return value from a function that is run in a parallel thread. There is no need to modify the function itself to be able to call it concurrently.

import math { sqrt }

fn get_hypot(a f64, b f64) f64 { //       ordinary function returning a value
	c := sqrt(a * a + b * b)
	return c
}

fn main() {
	g := go get_hypot(54.06, 2.08) // spawn thread and get handle to it
	h1 := get_hypot(2.32, 16.74) //   do some other calculation here
	h2 := g.wait() //                 get result from spawned thread
	println('Results: $h1, $h2') //   prints `Results: 16.9, 54.1`
}

If there is a large number of tasks, it might be easier to manage them using an array of threads.

import time

fn task(id int, duration int) {
	println('task $id begin')
	time.sleep(duration * time.millisecond)
	println('task $id end')
}

fn main() {
	mut threads := []thread{}
	threads << go task(1, 500)
	threads << go task(2, 900)
	threads << go task(3, 100)
	threads.wait()
	println('done')
}

// Output:
// task 1 begin
// task 2 begin
// task 3 begin
// task 3 end
// task 1 end
// task 2 end
// done

Additionally for threads that return the same type, calling wait() on the thread array will return all computed values.

fn expensive_computing(i int) int {
	return i * i
}

fn main() {
	mut threads := []thread int{}
	for i in 1 .. 10 {
		threads << go expensive_computing(i)
	}
	// Join all tasks
	r := threads.wait()
	println('All jobs finished: $r')
}

// Output: All jobs finished: [1, 4, 9, 16, 25, 36, 49, 64, 81]

Channels

Channels are the preferred way to communicate between coroutines. V's channels work basically like those in Go. You can push objects into a channel on one end and pop objects from the other end. Channels can be buffered or unbuffered and it is possible to select from multiple channels.

Syntax and Usage

Channels have the type chan objtype. An optional buffer length can specified as the cap property in the declaration:

ch := chan int{} // unbuffered - "synchronous"
ch2 := chan f64{cap: 100} // buffer length 100

Channels do not have to be declared as mut. The buffer length is not part of the type but a property of the individual channel object. Channels can be passed to coroutines like normal variables:

fn f(ch chan int) {
	// ...
}

fn main() {
	ch := chan int{}
	go f(ch)
	// ...
}

Objects can be pushed to channels using the arrow operator. The same operator can be used to pop objects from the other end:

// make buffered channels so pushing does not block (if there is room in the buffer)
ch := chan int{cap: 1}
ch2 := chan f64{cap: 1}
n := 5
// push
ch <- n
ch2 <- 7.3
mut y := f64(0.0)
m := <-ch // pop creating new variable
y = <-ch2 // pop into existing variable

A channel can be closed to indicate that no further objects can be pushed. Any attempt to do so will then result in a runtime panic (with the exception of select and try_push() - see below). Attempts to pop will return immediately if the associated channel has been closed and the buffer is empty. This situation can be handled using an or branch (see Handling Optionals).

ch := chan int{}
ch2 := chan f64{}
// ...
ch.close()
// ...
m := <-ch or {
    println('channel has been closed')
}

// propagate error
y := <-ch2 ?

Channel Select

The select command allows monitoring several channels at the same time without noticeable CPU load. It consists of a list of possible transfers and associated branches of statements - similar to the match command:

import time

fn main() {
	ch := chan f64{}
	ch2 := chan f64{}
	ch3 := chan f64{}
	mut b := 0.0
	c := 1.0
	// ... setup go threads that will send on ch/ch2
	go fn (the_channel chan f64) {
		time.sleep(5 * time.millisecond)
		the_channel <- 1.0
	}(ch)
	go fn (the_channel chan f64) {
		time.sleep(1 * time.millisecond)
		the_channel <- 1.0
	}(ch2)
	go fn (the_channel chan f64) {
		_ := <-the_channel
	}(ch3)

	select {
		a := <-ch {
			// do something with `a`
			eprintln('> a: $a')
		}
		b = <-ch2 {
			// do something with predeclared variable `b`
			eprintln('> b: $b')
		}
		ch3 <- c {
			// do something if `c` was sent
			time.sleep(5 * time.millisecond)
			eprintln('> c: $c was send on channel ch3')
		}
		500 * time.millisecond {
			// do something if no channel has become ready within 0.5s
			eprintln('> more than 0.5s passed without a channel being ready')
		}
	}
	eprintln('> done')
}

The timeout branch is optional. If it is absent select waits for an unlimited amount of time. It is also possible to proceed immediately if no channel is ready in the moment select is called by adding an else { ... } branch. else and <timeout> are mutually exclusive.

The select command can be used as an expression of type bool that becomes false if all channels are closed:

if select {
    ch <- a {
        // ...
    }
} {
    // channel was open
} else {
    // channel is closed
}

Special Channel Features

For special purposes there are some builtin properties and methods:

struct Abc {
	x int
}

a := 2.13
ch := chan f64{}
res := ch.try_push(a) // try to perform `ch <- a`
println(res)
l := ch.len // number of elements in queue
c := ch.cap // maximum queue length
is_closed := ch.closed // bool flag - has `ch` been closed
println(l)
println(c)
mut b := Abc{}
ch2 := chan Abc{}
res2 := ch2.try_pop(mut b) // try to perform `b = <-ch2`

The try_push/pop() methods will return immediately with one of the results .success, .not_ready or .closed - dependent on whether the object has been transferred or the reason why not. Usage of these methods and properties in production is not recommended - algorithms based on them are often subject to race conditions. Especially .len and .closed should not be used to make decisions. Use or branches, error propagation or select instead (see Syntax and Usage and Channel Select above).

Shared Objects

Data can be exchanged between a coroutine and the calling thread via a shared variable. Such variables should be created as shared and passed to the coroutine as such, too. The underlying struct contains a hidden mutex that allows locking concurrent access using rlock for read-only and lock for read/write access.

struct St {
mut:
	x int // data to be shared
}

fn (shared b St) g() {
	lock b {
		// read/modify/write b.x
	}
}

fn main() {
	shared a := St{
		x: 10
	}
	go a.g()
	// ...
	rlock a {
		// read a.x
	}
}

Shared variables must be structs, arrays or maps.

Decoding JSON

import json

struct Foo {
	x int
}

struct User {
	// Adding a [required] attribute will make decoding fail, if that
	// field is not present in the input.
	// If a field is not [required], but is missing, it will be assumed
	// to have its default value, like 0 for numbers, or '' for strings,
	// and decoding will not fail.
	name string [required]
	age  int
	// Use the `skip` attribute to skip certain fields
	foo Foo [skip]
	// If the field name is different in JSON, it can be specified
	last_name string [json: lastName]
}

data := '{ "name": "Frodo", "lastName": "Baggins", "age": 25 }'
user := json.decode(User, data) or {
	eprintln('Failed to decode json, error: $err')
	return
}
println(user.name)
println(user.last_name)
println(user.age)
// You can also decode JSON arrays:
sfoos := '[{"x":123},{"x":456}]'
foos := json.decode([]Foo, sfoos) ?
println(foos[0].x)
println(foos[1].x)

Because of the ubiquitous nature of JSON, support for it is built directly into V.

The json.decode function takes two arguments: the first is the type into which the JSON value should be decoded and the second is a string containing the JSON data.

V generates code for JSON encoding and decoding. No runtime reflection is used. This results in much better performance.

Testing

Asserts

fn foo(mut v []int) {
	v[0] = 1
}

mut v := [20]
foo(mut v)
assert v[0] < 4

An assert statement checks that its expression evaluates to true. If an assert fails, the program will abort. Asserts should only be used to detect programming errors. When an assert fails it is reported to stderr, and the values on each side of a comparison operator (such as <, ==) will be printed when possible. This is useful to easily find an unexpected value. Assert statements can be used in any function.

Test files

// hello.v
module main

fn hello() string {
	return 'Hello world'
}

fn main() {
	println(hello())
}

// hello_test.v
module main

fn test_hello() {
	assert hello() == 'Hello world'
}

To run the test above, use v hello_test.v. This will check that the function hello is producing the correct output. V executes all test functions in the file.

• All test functions have to be inside a test file whose name ends in _test.v.
• Test function names must begin with test_ to mark them for execution.
• Normal functions can also be defined in test files, and should be called manually. Other symbols can also be defined in test files e.g. types.
• There are two kinds of tests: external and internal.
• Internal tests must declare their module, just like all other .v files from the same module. Internal tests can even call private functions in the same module.
• External tests must import the modules which they test. They do not have access to the private functions/types of the modules. They can test only the external/public API that a module provides.

In the example above, test_hello is an internal test, that can call the private function hello() because hello_test.v has module main, just like hello.v, i.e. both are part of the same module. Note also that since module main is a regular module like the others, internal tests can be used to test private functions in your main program .v files too.

You can also define these special test functions in a test file:

• testsuite_begin which will be run before all other test functions.
• testsuite_end which will be run after all other test functions.

If a test function has an error return type, any propagated errors will fail the test:

import strconv

fn test_atoi() ? {
	assert strconv.atoi('1') ? == 1
	assert strconv.atoi('one') ? == 1 // test will fail
}

Running tests

To run test functions in an individual test file, use v foo_test.v.

To test an entire module, use v test mymodule. You can also use v test . to test everything inside your current folder (and subfolders). You can pass the -stats option to see more details about the individual tests run.

You can put additional test data, including .v source files in a folder, named testdata, right next to your _test.v files. V's test framework will ignore such folders, while scanning for tests to run. This is useful, if you want to put .v files with invalid V source code, or other tests, including known failing ones, that should be run in a specific way/options by a parent _test.v file.

NB: the path to the V compiler, is available through @VEXE, so a _test.v file, can easily run other test files like this:

import os

fn test_subtest() {
	res := os.execute('${@VEXE} other_test.v')
	assert res.exit_code == 1
	assert res.output.contains('other_test.v does not exist')
}

Memory management

V avoids doing unnecessary allocations in the first place by using value types, string buffers, promoting a simple abstraction-free code style.

Most objects (~90-100%) are freed by V's autofree engine: the compiler inserts necessary free calls automatically during compilation. Remaining small percentage of objects is freed via reference counting.

The developer doesn't need to change anything in their code. "It just works", like in Python, Go, or Java, except there's no heavy GC tracing everything or expensive RC for each object.

Control

You can take advantage of V's autofree engine and define a free() method on custom data types:

struct MyType {}

[unsafe]
fn (data &MyType) free() {
	// ...
}

Just as the compiler frees C data types with C's free(), it will statically insert free() calls for your data type at the end of each variable's lifetime.

For developers willing to have more low level control, autofree can be disabled with -manualfree, or by adding a [manualfree] on each function that wants manage its memory manually. (See attributes).

Note: right now autofree is hidden behind the -autofree flag. It will be enabled by default in V 0.3. If autofree is not used, V programs will leak memory.

Examples

import strings

fn draw_text(s string, x int, y int) {
	// ...
}

fn draw_scene() {
	// ...
	name1 := 'abc'
	name2 := 'def ghi'
	draw_text('hello $name1', 10, 10)
	draw_text('hello $name2', 100, 10)
	draw_text(strings.repeat(`X`, 10000), 10, 50)
	// ...
}

The strings don't escape draw_text, so they are cleaned up when the function exits.

In fact, with the -prealloc flag, the first two calls won't result in any allocations at all. These two strings are small, so V will use a preallocated buffer for them.

struct User {
	name string
}

fn test() []int {
	number := 7 // stack variable
	user := User{} // struct allocated on stack
	numbers := [1, 2, 3] // array allocated on heap, will be freed as the function exits
	println(number)
	println(user)
	println(numbers)
	numbers2 := [4, 5, 6] // array that's being returned, won't be freed here
	return numbers2
}

Stack and Heap

Stack and Heap Basics

Like with most other programming languages there are two locations where data can be stored:

• The stack allows fast allocations with almost zero administrative overhead. The stack grows and shrinks with the function call depth – so every called function has its stack segment that remains valid until the function returns. No freeing is necessary, however, this also means that a reference to a stack object becomes invalid on function return. Furthermore stack space is limited (typically to a few Megabytes per thread).
• The heap is a large memory area (typically some Gigabytes) that is administrated by the operating system. Heap objects are allocated and freed by special function calls that delegate the administrative tasks to the OS. This means that they can remain valid across several function calls, however, the administration is expensive.

V's default approach

Due to performance considerations V tries to put objects on the stack if possible but allocates them on the heap when obviously necessary. Example:

struct MyStruct {
	n int
}

struct RefStruct {
	r &MyStruct
}

fn main() {
	q, w := f()
	println('q: $q.r.n, w: $w.n')
}

fn f() (RefStruct, &MyStruct) {
	a := MyStruct{
		n: 1
	}
	b := MyStruct{
		n: 2
	}
	c := MyStruct{
		n: 3
	}
	e := RefStruct{
		r: &b
	}
	x := a.n + c.n
	println('x: $x')
	return e, &c
}

Here a is stored on the stack since it's address never leaves the function f(). However a reference to b is part of e which is returned. Also a reference to c is returned. For this reason b and c will be heap allocated.

Things become less obvious when a reference to an object is passed as function argument:

struct MyStruct {
mut:
	n int
}

fn main() {
	mut q := MyStruct{
		n: 7
	}
	w := MyStruct{
		n: 13
	}
	x := q.f(&w) // references of `q` and `w` are passed
	println('q: $q\nx: $x')
}

fn (mut a MyStruct) f(b &MyStruct) int {
	a.n += b.n
	x := a.n * b.n
	return x
}

Here the call q.f(&w) passes references to q and w because a is mut and b is of type &MyStruct in f()'s declaration, so technically these references are leaving main(). However the lifetime of these references lies inside the scope of main() so q and w are allocated on the stack.

Manual Control for Stack and Heap

In the last example the V compiler could put q and w on the stack because it assumed that in the call q.f(&w) these references were only used for reading and modifying the referred values – and not to pass the references themselves somewhere else. This can be seen in a way that the references to q and w are only borrowed to f().

Things become different if f() is doing something with a reference itself:

struct RefStruct {
mut:
	r &MyStruct
}

// see discussion below
[heap]
struct MyStruct {
	n int
}

fn main() {
	m := MyStruct{}
	mut r := RefStruct{
		r: &m
	}
	r.g()
	println('r: $r')
}

fn (mut r RefStruct) g() {
	s := MyStruct{
		n: 7
	}
	r.f(&s) // reference to `s` inside `r` is passed back to `main() `
}

fn (mut r RefStruct) f(s &MyStruct) {
	r.r = s // would trigger error without `[heap]`
}

Here f() looks quite innocent but is doing nasty things – it inserts a reference to s into r. The problem with this is that s lives only as long as g() is running but r is used in main() after that. For this reason the compiler would complain about the assignment in f() because s "might refer to an object stored on stack". The assumption made in g() that the call r.f(&s) would only borrow the reference to s is wrong.

A solution to this dilemma is the [heap] attribute at the declaration of struct MyStruct. It instructs the compiler to always allocate MyStruct-objects on the heap. This way the reference to s remains valid even after g() returns. The compiler takes into consideration that MyStruct objects are always heap allocated when checking f() and allows assigning the reference to s to the r.r field.

There is a pattern often seen in other programming languages:

fn (mut a MyStruct) f() &MyStruct {
	// do something with a
	return &a // would return address of borrowed object
}

Here f() is passed a reference a as receiver that is passed back to the caller and returned as result at the same time. The intention behind such a declaration is method chaining like y = x.f().g(). However, the problem with this approach is that a second reference to a is created – so it is not only borrowed and MyStruct has to be declared as [heap].

In V the better approach is:

struct MyStruct {
mut:
	n int
}

fn (mut a MyStruct) f() {
	// do something with `a`
}

fn (mut a MyStruct) g() {
	// do something else with `a`
}

fn main() {
	x := MyStruct{} // stack allocated
	mut y := x
	y.f()
	y.g()
	// instead of `mut y := x.f().g()
}

This way the [heap] attribute can be avoided – resulting in better performance.

However, stack space is very limited as mentioned above. For this reason the [heap] attribute might be suitable for very large structures even if not required by use cases like those mentioned above.

There is an alternative way to manually control allocation on a case to case basis. This approach is not recommended but shown here for the sake of completeness:

struct MyStruct {
	n int
}

struct RefStruct {
mut:
	r &MyStruct
}

// simple function - just to overwrite stack segment previously used by `g()`
fn use_stack() {
	x := 7.5
	y := 3.25
	z := x + y
	println('$x $y $z')
}

fn main() {
	m := MyStruct{}
	mut r := RefStruct{
		r: &m
	}
	r.g()
	use_stack() // to erase invalid stack contents
	println('r: $r')
}

fn (mut r RefStruct) g() {
	s := &MyStruct{ // `s` explicitly refers to a heap object
		n: 7
	}
	// change `&MyStruct` -> `MyStruct` above and `r.f(s)` -> `r.f(&s)` below
	// to see data in stack segment being overwritten
	r.f(s)
}

fn (mut r RefStruct) f(s &MyStruct) {
	r.r = unsafe { s } // override compiler check
}

Here the compiler check is suppressed by the unsafe block. To make s be heap allocated even without [heap] attribute the struct literal is prefixed with an ampersand: &MyStruct{...}.

This last step would not be required by the compiler but without it the reference inside r becomes invalid (the memory area pointed to will be overwritten by use_stack()) and the program might crash (or at least produce an unpredictable final output). That's why this approach is unsafe and should be avoided!

ORM

(This is still in an alpha state)

V has a built-in ORM (object-relational mapping) which supports SQLite, MySQL and Postgres, but soon it will support MS SQL and Oracle.

V's ORM provides a number of benefits:

• One syntax for all SQL dialects. (Migrating between databases becomes much easier.)
• Queries are constructed using V's syntax. (There's no need to learn another syntax.)
• Safety. (All queries are automatically sanitised to prevent SQL injection.)
• Compile time checks. (This prevents typos which can only be caught during runtime.)
• Readability and simplicity. (You don't need to manually parse the results of a query and then manually construct objects from the parsed results.)

import sqlite

// sets a custom table name. Default is struct name (case-sensitive)
[table: 'customers']
struct Customer {
	id        int    [primary; sql: serial] // a field named `id` of integer type must be the first field
	name      string [nonull]
	nr_orders int
	country   string [nonull]
}

db := sqlite.connect('customers.db') ?

// you can create tables:
// CREATE TABLE IF NOT EXISTS `Customer` (
//      `id` INTEGER PRIMARY KEY,
//      `name` TEXT NOT NULL,
//      `nr_orders` INTEGER,
//      `country` TEXT NOT NULL
// )
sql db {
	create table Customer
}

// select count(*) from customers
nr_customers := sql db {
	select count from Customer
}
println('number of all customers: $nr_customers')
// V syntax can be used to build queries
uk_customers := sql db {
	select from Customer where country == 'uk' && nr_orders > 0
}
println(uk_customers.len)
for customer in uk_customers {
	println('$customer.id - $customer.name')
}
// by adding `limit 1` we tell V that there will be only one object
customer := sql db {
	select from Customer where id == 1 limit 1
}
println('$customer.id - $customer.name')
// insert a new customer
new_customer := Customer{
	name: 'Bob'
	nr_orders: 10
}
sql db {
	insert new_customer into Customer
}

For more examples and the docs, see vlib/orm.

Writing Documentation

The way it works is very similar to Go. It's very simple: there's no need to write documentation separately for your code, vdoc will generate it from docstrings in the source code.

Documentation for each function/type/const must be placed right before the declaration:

// clearall clears all bits in the array
fn clearall() {
}

The comment must start with the name of the definition.

Sometimes one line isn't enough to explain what a function does, in that case comments should span to the documented function using single line comments:

// copy_all recursively copies all elements of the array by their value,
// if `dupes` is false all duplicate values are eliminated in the process.
fn copy_all(dupes bool) {
	// ...
}

By convention it is preferred that comments are written in present tense.

An overview of the module must be placed in the first comment right after the module's name.

To generate documentation use vdoc, for example v doc net.http.

Newlines in Documentation Comments

Comments spanning multiple lines are merged together using spaces, unless

• the line is empty
• the line ends with a . (end of sentence)
• the line is purely of at least 3 of -, =, _, *, ~ (horizontal rule)
• the line starts with at least one # followed by a space (header)
• the line starts and ends with a | (table)
• the line starts with - (list)

Tools

v fmt

You don't need to worry about formatting your code or setting style guidelines. v fmt takes care of that:

v fmt file.v

It's recommended to set up your editor, so that v fmt -w runs on every save. A vfmt run is usually pretty cheap (takes <30ms).

Always run v fmt -w file.v before pushing your code.

v shader

You can use GPU shaders with V graphical apps. You write your shaders in an annotated GLSL dialect and use v shader to compile them for all supported target platforms.

v shader /path/to/project/dir/or/file.v

Currently you need to include a header and declare a glue function before using the shader in your code.

Profiling

V has good support for profiling your programs: v -profile profile.txt run file.v That will produce a profile.txt file, which you can then analyze.

The generated profile.txt file will have lines with 4 columns: a) how many times a function was called b) how much time in total a function took (in ms) c) how much time on average, a call to a function took (in ns) d) the name of the v function

You can sort on column 3 (average time per function) using: sort -n -k3 profile.txt|tail

You can also use stopwatches to measure just portions of your code explicitly:

import time

fn main() {
	sw := time.new_stopwatch()
	println('Hello world')
	println('Greeting the world took: ${sw.elapsed().nanoseconds()}ns')
}

Package management

v [module option] [param]

module options:

   install           Install a module from VPM.
   remove            Remove a module that was installed from VPM.
   search            Search for a module from VPM.
   update            Update an installed module from VPM.
   upgrade           Upgrade all the outdated modules.
   list              List all installed modules.
   outdated          Show installed modules that need updates.

You can install modules already created by someone else with VPM:

v install [module]

Example:

v install ui

Modules can be installed directly from git or mercurial repositories.

v install [--git|--hg] [url]

Example:

v install --git https://github.com/vlang/markdown

Removing a module with v:

v remove [module]

Example:

v remove ui

Updating an installed module from VPM:

v update [module]

Example:

v update ui

Or you can update all your modules:

v update

To see all the modules you have installed, you can use:

v list

Example:

> v list
Installed modules:
  markdown
  ui

To see all the modules that need updates:

v outdated

Example:

> v outdated
Modules are up to date.

Publish package

1. Put a v.mod file inside the toplevel folder of your module (if you created your module with the command v new mymodule or v init you already have a v.mod file).

   v new mymodule
   Input your project description: My nice module.
   Input your project version: (0.0.0) 0.0.1
   Input your project license: (MIT)
   Initialising ...
   Complete!
   

   Example v.mod:

   Module {
   	name: 'mymodule'
   	description: 'My nice module.'
   	version: '0.0.1'
   	license: 'MIT'
   	dependencies: []
   }
   

   Minimal file structure:

   v.mod
   mymodule.v
   

   The name of your module should be used with the module directive at the top of all files in your module. For mymodule.v:

   module mymodule
   
   pub fn hello_world() {
   	println('Hello World!')
   }
   

2. Create a git repository in the folder with the v.mod file (this is not required if you used v new or v init):

   git init
   git add .
   git commit -m "INIT"
   

3. Create a public repository on github.com.

4. Connect your local repository to the remote repository and push the changes.

5. Add your module to the public V module registry VPM: https://vpm.vlang.io/new

   You will have to login with your Github account to register the module. Warning: Currently it is not possible to edit your entry after submitting. Check your module name and github url twice as this cannot be changed by you later.

6. The final module name is a combination of your github account and the module name you provided e.g. mygithubname.mymodule.

Optional: tag your V module with vlang and vlang-module on github.com to allow for a better search experience.

Advanced Topics

Dumping expressions at runtime

You can dump/trace the value of any V expression using dump(expr). For example, save this code sample as factorial.v, then run it with v run factorial.v:

fn factorial(n u32) u32 {
	if dump(n <= 1) {
		return dump(1)
	}
	return dump(n * factorial(n - 1))
}

fn main() {
	println(factorial(5))
}

You will get:

[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: true
[factorial.v:3] 1: 1
[factorial.v:5] n * factorial(n - 1): 2
[factorial.v:5] n * factorial(n - 1): 6
[factorial.v:5] n * factorial(n - 1): 24
[factorial.v:5] n * factorial(n - 1): 120
120

Note that dump(expr) will trace both the source location, the expression itself, and the expression value.

Memory-unsafe code

Sometimes for efficiency you may want to write low-level code that can potentially corrupt memory or be vulnerable to security exploits. V supports writing such code, but not by default.

V requires that any potentially memory-unsafe operations are marked intentionally. Marking them also indicates to anyone reading the code that there could be memory-safety violations if there was a mistake.

Examples of potentially memory-unsafe operations are:

• Pointer arithmetic
• Pointer indexing
• Conversion to pointer from an incompatible type
• Calling certain C functions, e.g. free, strlen and strncmp.

To mark potentially memory-unsafe operations, enclose them in an unsafe block:

// allocate 2 uninitialized bytes & return a reference to them
mut p := unsafe { malloc(2) }
p[0] = `h` // Error: pointer indexing is only allowed in `unsafe` blocks
unsafe {
    p[0] = `h` // OK
    p[1] = `i`
}
p++ // Error: pointer arithmetic is only allowed in `unsafe` blocks
unsafe {
    p++ // OK
}
assert *p == `i`

Best practice is to avoid putting memory-safe expressions inside an unsafe block, so that the reason for using unsafe is as clear as possible. Generally any code you think is memory-safe should not be inside an unsafe block, so the compiler can verify it.

If you suspect your program does violate memory-safety, you have a head start on finding the cause: look at the unsafe blocks (and how they interact with surrounding code).

• Note: This is work in progress.

Structs with reference fields

Structs with references require explicitly setting the initial value to a reference value unless the struct already defines its own initial value.

Zero-value references, or nil pointers, will NOT be supported in the future, for now data structures such as Linked Lists or Binary Trees that rely on reference fields that can use the value 0, understanding that it is unsafe, and that it can cause a panic.

struct Node {
	a &Node
	b &Node = 0 // Auto-initialized to nil, use with caution!
}

// Reference fields must be initialized unless an initial value is declared.
// Zero (0) is OK but use with caution, it's a nil pointer.
foo := Node{
	a: 0
}
bar := Node{
	a: &foo
}
baz := Node{
	a: 0
	b: 0
}
qux := Node{
	a: &foo
	b: &bar
}
println(baz)
println(qux)

sizeof and __offsetof

• sizeof(Type) gives the size of a type in bytes.
• __offsetof(Struct, field_name) gives the offset in bytes of a struct field.

struct Foo {
	a int
	b int
}

assert sizeof(Foo) == 8
assert __offsetof(Foo, a) == 0
assert __offsetof(Foo, b) == 4

Calling C from V

Example

#flag -lsqlite3
#include "sqlite3.h"
// See also the example from https://www.sqlite.org/quickstart.html
struct C.sqlite3 {
}

struct C.sqlite3_stmt {
}

type FnSqlite3Callback = fn (voidptr, int, &&char, &&char) int

fn C.sqlite3_open(&char, &&C.sqlite3) int

fn C.sqlite3_close(&C.sqlite3) int

fn C.sqlite3_column_int(stmt &C.sqlite3_stmt, n int) int

// ... you can also just define the type of parameter and leave out the C. prefix
fn C.sqlite3_prepare_v2(&C.sqlite3, &char, int, &&C.sqlite3_stmt, &&char) int

fn C.sqlite3_step(&C.sqlite3_stmt)

fn C.sqlite3_finalize(&C.sqlite3_stmt)

fn C.sqlite3_exec(db &C.sqlite3, sql &char, cb FnSqlite3Callback, cb_arg voidptr, emsg &&char) int

fn C.sqlite3_free(voidptr)

fn my_callback(arg voidptr, howmany int, cvalues &&char, cnames &&char) int {
	unsafe {
		for i in 0 .. howmany {
			print('| ${cstring_to_vstring(cnames[i])}: ${cstring_to_vstring(cvalues[i]):20} ')
		}
	}
	println('|')
	return 0
}

fn main() {
	db := &C.sqlite3(0) // this means `sqlite3* db = 0`
	// passing a string literal to a C function call results in a C string, not a V string
	C.sqlite3_open(c'users.db', &db)
	// C.sqlite3_open(db_path.str, &db)
	query := 'select count(*) from users'
	stmt := &C.sqlite3_stmt(0)
	// NB: you can also use the `.str` field of a V string,
	// to get its C style zero terminated representation
	C.sqlite3_prepare_v2(db, &char(query.str), -1, &stmt, 0)
	C.sqlite3_step(stmt)
	nr_users := C.sqlite3_column_int(stmt, 0)
	C.sqlite3_finalize(stmt)
	println('There are $nr_users users in the database.')
	//
	error_msg := &char(0)
	query_all_users := 'select * from users'
	rc := C.sqlite3_exec(db, &char(query_all_users.str), my_callback, voidptr(7), &error_msg)
	if rc != C.SQLITE_OK {
		eprintln(unsafe { cstring_to_vstring(error_msg) })
		C.sqlite3_free(error_msg)
	}
	C.sqlite3_close(db)
}

Calling V from C

Since V can compile to C, calling V code from C is very easy.

By default all V functions have the following naming scheme in C: [module name]__[fn_name].

For example, fn foo() {} in module bar will result in bar__foo().

To use a custom export name, use the [export] attribute:

[export: 'my_custom_c_name']
fn foo() {
}

Atomics

V has no special support for atomics, yet, nevertheless it's possible to treat variables as atomics by calling C functions from V. The standard C11 atomic functions like atomic_store() are usually defined with the help of macros and C compiler magic to provide a kind of overloaded C functions. Since V does not support overloading functions by intention there are wrapper functions defined in C headers named atomic.h that are part of the V compiler infrastructure.

There are dedicated wrappers for all unsigned integer types and for pointers. (byte is not fully supported on Windows) – the function names include the type name as suffix. e.g. C.atomic_load_ptr() or C.atomic_fetch_add_u64().

To use these functions the C header for the used OS has to be included and the functions that are intended to be used have to be declared. Example:

$if windows {
	#include "@VEXEROOT/thirdparty/stdatomic/win/atomic.h"
} $else {
	#include "@VEXEROOT/thirdparty/stdatomic/nix/atomic.h"
}

// declare functions we want to use - V does not parse the C header
fn C.atomic_store_u32(&u32, u32)
fn C.atomic_load_u32(&u32) u32
fn C.atomic_compare_exchange_weak_u32(&u32, &u32, u32) bool
fn C.atomic_compare_exchange_strong_u32(&u32, &u32, u32) bool

const num_iterations = 10000000

// see section "Global Variables" below
__global (
	atom u32 // ordinary variable but used as atomic
)

fn change() int {
	mut races_won_by_change := 0
	for {
		mut cmp := u32(17) // addressable value to compare with and to store the found value
		// atomic version of `if atom == 17 { atom = 23 races_won_by_change++ } else { cmp = atom }`
		if C.atomic_compare_exchange_strong_u32(&atom, &cmp, 23) {
			races_won_by_change++
		} else {
			if cmp == 31 {
				break
			}
			cmp = 17 // re-assign because overwritten with value of atom
		}
	}
	return races_won_by_change
}

fn main() {
	C.atomic_store_u32(&atom, 17)
	t := go change()
	mut races_won_by_main := 0
	mut cmp17 := u32(17)
	mut cmp23 := u32(23)
	for i in 0 .. num_iterations {
		// atomic version of `if atom == 17 { atom = 23 races_won_by_main++ }`
		if C.atomic_compare_exchange_strong_u32(&atom, &cmp17, 23) {
			races_won_by_main++
		} else {
			cmp17 = 17
		}
		desir := if i == num_iterations - 1 { u32(31) } else { u32(17) }
		// atomic version of `for atom != 23 {} atom = desir`
		for !C.atomic_compare_exchange_weak_u32(&atom, &cmp23, desir) {
			cmp23 = 23
		}
	}
	races_won_by_change := t.wait()
	atom_new := C.atomic_load_u32(&atom)
	println('atom: $atom_new, #exchanges: ${races_won_by_main + races_won_by_change}')
	// prints `atom: 31, #exchanges: 10000000`)
	println('races won by\n- `main()`: $races_won_by_main\n- `change()`: $races_won_by_change')
}

In this example both main() and the spawned thread change() try to replace a value of 17 in the global atom with a value of 23. The replacement in the opposite direction is done exactly 10000000 times. The last replacement will be with 31 which makes the spawned thread finish.

It is not predictable how many replacements occur in which thread, but the sum will always be 10000000. (With the non-atomic commands from the comments the value will be higher or the program will hang – dependent on the compiler optimization used.)

Global Variables

By default V does not allow global variables. However, in low level applications they have their place so their usage can be enabled with the compiler flag -enable-globals. Declarations of global variables must be surrounded with a __global ( ... ) specification – as in the example above.

An initializer for global variables must be explicitly converted to the desired target type. If no initializer is given a default initialization is done. Some objects like semaphores and mutexes require an explicit initialization in place, i.e. not with a value returned from a function call but with a method call by reference. A separate init() function can be used for this purpose – it will be called before main():

import sync

__global (
	sem   sync.Semaphore // needs initialization in `init()`
	mtx   sync.RwMutex // needs initialization in `init()`
	f1    = f64(34.0625) // explicily initialized
	shmap shared map[string]f64 // initialized as empty `shared` map
	f2    f64 // initialized to `0.0`
)

fn init() {
	sem.init(0)
	mtx.init()
}

Be aware that in multi threaded applications the access to global variables is subject to race conditions. There are several approaches to deal with these:

• use shared types for the variable declarations and use lock blocks for access. This is most appropriate for larger objects like structs, arrays or maps.
• handle primitive data types as "atomics" using special C-functions (see above).
• use explicit synchronization primitives like mutexes to control access. The compiler cannot really help in this case, so you have to know what you are doing.
• don't care – this approach is possible but makes only sense if the exact values of global variables do not really matter. An example can be found in the rand module where global variables are used to generate (non cryptographic) pseudo random numbers. In this case data races lead to random numbers in different threads becoming somewhat correlated, which is acceptable considering the performance penalty that using synchonization primitives would represent.

Passing C compilation flags

Add #flag directives to the top of your V files to provide C compilation flags like:

• -I for adding C include files search paths
• -l for adding C library names that you want to get linked
• -L for adding C library files search paths
• -D for setting compile time variables

You can (optionally) use different flags for different targets. Currently the linux, darwin , freebsd, and windows flags are supported.

NB: Each flag must go on its own line (for now)

#flag linux -lsdl2
#flag linux -Ivig
#flag linux -DCIMGUI_DEFINE_ENUMS_AND_STRUCTS=1
#flag linux -DIMGUI_DISABLE_OBSOLETE_FUNCTIONS=1
#flag linux -DIMGUI_IMPL_API=

In the console build command, you can use:

• -cflags to pass custom flags to the backend C compiler.
• -cc to change the default C backend compiler.
• For example: -cc gcc-9 -cflags -fsanitize=thread.

You can define a VFLAGS environment variable in your terminal to store your -cc and -cflags settings, rather than including them in the build command each time.

#pkgconfig

Add #pkgconfig directive is used to tell the compiler which modules should be used for compiling and linking using the pkg-config files provided by the respective dependencies.

As long as backticks can't be used in #flag and spawning processes is not desirable for security and portability reasons, V uses its own pkgconfig library that is compatible with the standard freedesktop one.

If no flags are passed it will add --cflags and --libs, both lines below do the same:

#pkgconfig r_core
#pkgconfig --cflags --libs r_core

The .pc files are looked up into a hardcoded list of default pkg-config paths, the user can add extra paths by using the PKG_CONFIG_PATH environment variable. Multiple modules can be passed.

To check the existence of a pkg-config use $pkgconfig('pkg') as a compile time "if" condition to check if a pkg-config exists. If it exists the branch will be created. Use $else or $else $if to handle other cases.

$if $pkgconfig('mysqlclient') {
	#pkgconfig mysqlclient
} $else $if $pkgconfig('mariadb') {
	#pkgconfig mariadb
}

Including C code

You can also include C code directly in your V module. For example, let's say that your C code is located in a folder named 'c' inside your module folder. Then:

• Put a v.mod file inside the toplevel folder of your module (if you created your module with v new you already have v.mod file). For example:

Module {
	name: 'mymodule',
	description: 'My nice module wraps a simple C library.',
	version: '0.0.1'
	dependencies: []
}

• Add these lines to the top of your module:

#flag -I @VMODROOT/c
#flag @VMODROOT/c/implementation.o
#include "header.h"

NB: @VMODROOT will be replaced by V with the nearest parent folder, where there is a v.mod file. Any .v file beside or below the folder where the v.mod file is, can use #flag @VMODROOT/abc to refer to this folder. The @VMODROOT folder is also prepended to the module lookup path, so you can import other modules under your @VMODROOT, by just naming them.

The instructions above will make V look for an compiled .o file in your module folder/c/implementation.o. If V finds it, the .o file will get linked to the main executable, that used the module. If it does not find it, V assumes that there is a @VMODROOT/c/implementation.c file, and tries to compile it to a .o file, then will use that.

This allows you to have C code, that is contained in a V module, so that its distribution is easier. You can see a complete minimal example for using C code in a V wrapper module here: project_with_c_code. Another example, demonstrating passing structs from C to V and back again: interoperate between C to V to C.

C types

Ordinary zero terminated C strings can be converted to V strings with unsafe { &char(cstring).vstring() } or if you know their length already with unsafe { &char(cstring).vstring_with_len(len) }.

NB: The .vstring() and .vstring_with_len() methods do NOT create a copy of the cstring, so you should NOT free it after calling the method .vstring(). If you need to make a copy of the C string (some libc APIs like getenv pretty much require that, since they return pointers to internal libc memory), you can use cstring_to_vstring(cstring).

On Windows, C APIs often return so called wide strings (utf16 encoding). These can be converted to V strings with string_from_wide(&u16(cwidestring)) .

V has these types for easier interoperability with C:

• voidptr for C's void*,
• &byte for C's byte* and
• &char for C's char*.
• &&char for C's char**

To cast a voidptr to a V reference, use user := &User(user_void_ptr).

voidptr can also be dereferenced into a V struct through casting: user := User(user_void_ptr).

an example of a module that calls C code from V

C Declarations

C identifiers are accessed with the C prefix similarly to how module-specific identifiers are accessed. Functions must be redeclared in V before they can be used. Any C types may be used behind the C prefix, but types must be redeclared in V in order to access type members.

To redeclare complex types, such as in the following C code:

struct SomeCStruct {
	uint8_t implTraits;
	uint16_t memPoolData;
	union {
		struct {
			void* data;
			size_t size;
		};

		DataView view;
	};
};

members of sub-data-structures may be directly declared in the containing struct as below:

struct C.SomeCStruct {
	implTraits  byte
	memPoolData u16
	// These members are part of sub data structures that can't currently be represented in V.
	// Declaring them directly like this is sufficient for access.
	// union {
	// struct {
	data voidptr
	size usize
	// }
	view C.DataView
	// }
}

The existence of the data members is made known to V, and they may be used without re-creating the original structure exactly.

Alternatively, you may embed the sub-data-structures to maintain a parallel code structure.

Debugging

C Backend binaries (Default)

To debug issues in the generated binary (flag: -b c), you can pass these flags:

• -g - produces a less optimized executable with more debug information in it. V will enforce line numbers from the .v files in the stacktraces, that the executable will produce on panic. It is usually better to pass -g, unless you are writing low level code, in which case use the next option -cg.
• -cg - produces a less optimized executable with more debug information in it. The executable will use C source line numbers in this case. It is frequently used in combination with -keepc, so that you can inspect the generated C program in case of panic, or so that your debugger (gdb, lldb etc.) can show you the generated C source code.
• -showcc - prints the C command that is used to build the program.
• -show-c-output - prints the output, that your C compiler produced while compiling your program.
• -keepc - do not delete the generated C source code file after a successful compilation. Also keep using the same file path, so it is more stable, and easier to keep opened in an editor/IDE.

For best debugging experience if you are writing a low level wrapper for an existing C library, you can pass several of these flags at the same time: v -keepc -cg -showcc yourprogram.v, then just run your debugger (gdb/lldb) or IDE on the produced executable yourprogram.

If you just want to inspect the generated C code, without further compilation, you can also use the -o flag (e.g. -o file.c). This will make V produce the file.c then stop.

If you want to see the generated C source code for just a single C function, for example main, you can use: -printfn main -o file.c.

To debug the V executable itself you need to compile from src with ./v -g -o v cmd/v.

You can debug tests with for example v -g -keepc prog_test.v. The -keepc flag is needed, so that the executable is not deleted, after it was created and ran.

To see a detailed list of all flags that V supports, use v help, v help build and v help build-c.

Commandline Debugging

1. compile your binary with debugging info v -g hello.v
2. debug with lldb or GDB e.g. lldb hello

Troubleshooting (debugging) executables created with V in GDB

Visual debugging Setup:

• Visual Studio Code

Native Backend binaries

Currently there is no debugging support for binaries, created by the native backend (flag: -b native).

Javascript Backend

To debug the generated Javascript output you can activate source maps: v -b js -sourcemap hello.v -o hello.js

For all supported options check the latest help: v help build-js

Conditional compilation

Compile time code

$ is used as a prefix for compile-time operations.

$if condition

// Support for multiple conditions in one branch
$if ios || android {
	println('Running on a mobile device!')
}
$if linux && x64 {
	println('64-bit Linux.')
}
// Usage as expression
os := $if windows { 'Windows' } $else { 'UNIX' }
println('Using $os')
// $else-$if branches
$if tinyc {
	println('tinyc')
} $else $if clang {
	println('clang')
} $else $if gcc {
	println('gcc')
} $else {
	println('different compiler')
}
$if test {
	println('testing')
}
// v -cg ...
$if debug {
	println('debugging')
}
// v -prod ...
$if prod {
	println('production build')
}
// v -d option ...
$if option ? {
	println('custom option')
}

If you want an if to be evaluated at compile time it must be prefixed with a $ sign. Right now it can be used to detect an OS, compiler, platform or compilation options. $if debug is a special option like $if windows or $if x32. If you're using a custom ifdef, then you do need $if option ? {} and compile withv -d option. Full list of builtin options:

OS	Compilers	Platforms	Other
windows, linux, macos	gcc, tinyc	amd64, arm64	debug, prod, test
mac, darwin, ios,	clang, mingw	x64, x32	js, glibc, prealloc
android,mach, dragonfly	msvc	little_endian	no_bounds_checking, freestanding
gnu, hpux, haiku, qnx	cplusplus	big_endian	no_segfault_handler, no_backtrace, no_main
solaris

$embed_file

import os
fn main() {
	embedded_file := $embed_file('v.png')
	os.write_file('exported.png', embedded_file.to_string()) ?
}

V can embed arbitrary files into the executable with the $embed_file(<path>) compile time call. Paths can be absolute or relative to the source file.

When you do not use -prod, the file will not be embedded. Instead, it will be loaded the first time your program calls embedded_file.data() at runtime, making it easier to change in external editor programs, without needing to recompile your executable.

When you compile with -prod, the file will be embedded inside your executable, increasing your binary size, but making it more self contained and thus easier to distribute. In this case, embedded_file.data() will cause no IO, and it will always return the same data.

$tmpl for embedding and parsing V template files

V has a simple template language for text and html templates, and they can easily be embedded via $tmpl('path/to/template.txt'):

fn build() string {
	name := 'Peter'
	age := 25
	numbers := [1, 2, 3]
	return $tmpl('1.txt')
}

fn main() {
	println(build())
}

1.txt:

name: @name

age: @age

numbers: @numbers

@for number in numbers
  @number
@end

output:

name: Peter

age: 25

numbers: [1, 2, 3]

1
2
3

$env

module main

fn main() {
	compile_time_env := $env('ENV_VAR')
	println(compile_time_env)
}

V can bring in values at compile time from environment variables. $env('ENV_VAR') can also be used in top-level #flag and #include statements: #flag linux -I $env('JAVA_HOME')/include.

Environment specific files

If a file has an environment-specific suffix, it will only be compiled for that environment.

• .js.v => will be used only by the JS backend. These files can contain JS. code.
• .c.v => will be used only by the C backend. These files can contain C. code.
• .native.v => will be used only by V's native backend.
• _nix.c.v => will be used only on Unix systems (non Windows).
• _${os}.c.v => will be used only on the specific os system. For example, _windows.c.v will be used only when compiling on Windows, or with -os windows.
• _default.c.v => will be used only if there is NOT a more specific platform file. For example, if you have both file_linux.c.v and file_default.c.v, and you are compiling for linux, then only file_linux.c.v will be used, and file_default.c.v will be ignored.

Here is a more complete example: main.v:

module main
fn main() { println(message) }

main_default.c.v:

module main
const ( message = 'Hello world' )

main_linux.c.v:

module main
const ( message = 'Hello linux' )

main_windows.c.v:

module main
const ( message = 'Hello windows' )

With the example above:

• when you compile for windows, you will get 'Hello windows'

• when you compile for linux, you will get 'Hello linux'

• when you compile for any other platform, you will get the non specific 'Hello world' message.

• _d_customflag.v => will be used only if you pass -d customflag to V. That corresponds to $if customflag ? {}, but for a whole file, not just a single block. customflag should be a snake_case identifier, it can not contain arbitrary characters (only lower case latin letters + numbers + _). NB: a combinatorial _d_customflag_linux.c.v postfix will not work. If you do need a custom flag file, that has platform dependent code, use the postfix _d_customflag.v, and then use plaftorm dependent compile time conditional blocks inside it, i.e. $if linux {} etc.

• _notd_customflag.v => similar to _d_customflag.v, but will be used only if you do NOT pass -d customflag to V.

Compile time pseudo variables

V also gives your code access to a set of pseudo string variables, that are substituted at compile time:

• @FN => replaced with the name of the current V function
• @METHOD => replaced with ReceiverType.MethodName
• @MOD => replaced with the name of the current V module
• @STRUCT => replaced with the name of the current V struct
• @FILE => replaced with the path of the V source file
• @LINE => replaced with the V line number where it appears (as a string).
• @COLUMN => replaced with the column where it appears (as a string).
• @VEXE => replaced with the path to the V compiler
• @VEXEROOT => will be substituted with the folder, where the V executable is (as a string).
• @VHASH => replaced with the shortened commit hash of the V compiler (as a string).
• @VMOD_FILE => replaced with the contents of the nearest v.mod file (as a string).
• @VMODROOT => will be substituted with the folder, where the nearest v.mod file is (as a string).

That allows you to do the following example, useful while debugging/logging/tracing your code:

eprintln('file: ' + @FILE + ' | line: ' + @LINE + ' | fn: ' + @MOD + '.' + @FN)

Another example, is if you want to embed the version/name from v.mod inside your executable:

import v.vmod
vm := vmod.decode( @VMOD_FILE ) or { panic(err.msg) }
eprintln('$vm.name $vm.version\n $vm.description')

Performance tuning

The generated C code is usually fast enough, when you compile your code with -prod. There are some situations though, where you may want to give additional hints to the compiler, so that it can further optimize some blocks of code.

NB: These are rarely needed, and should not be used, unless you profile your code, and then see that there are significant benefits for them. To cite gcc's documentation: "programmers are notoriously bad at predicting how their programs actually perform".

[inline] - you can tag functions with [inline], so the C compiler will try to inline them, which in some cases, may be beneficial for performance, but may impact the size of your executable.

[direct_array_access] - in functions tagged with [direct_array_access] the compiler will translate array operations directly into C array operations - omitting bounds checking. This may save a lot of time in a function that iterates over an array but at the cost of making the function unsafe - unless the boundaries will be checked by the user.

if _likely_(bool expression) { this hints the C compiler, that the passed boolean expression is very likely to be true, so it can generate assembly code, with less chance of branch misprediction. In the JS backend, that does nothing.

if _unlikely_(bool expression) { similar to _likely_(x), but it hints that the boolean expression is highly improbable. In the JS backend, that does nothing.

Compile-time reflection

Having built-in JSON support is nice, but V also allows you to create efficient serializers for any data format. V has compile-time if and for constructs:

struct User {
	name string
	age  int
}

fn main() {
	$for field in User.fields {
		$if field.typ is string {
			println('$field.name is of type string')
		}
	}
}

// Output:
// name is of type string

See examples/compiletime/reflection.v for a more complete example.

Limited operator overloading

struct Vec {
	x int
	y int
}

fn (a Vec) str() string {
	return '{$a.x, $a.y}'
}

fn (a Vec) + (b Vec) Vec {
	return Vec{a.x + b.x, a.y + b.y}
}

fn (a Vec) - (b Vec) Vec {
	return Vec{a.x - b.x, a.y - b.y}
}

fn main() {
	a := Vec{2, 3}
	b := Vec{4, 5}
	mut c := Vec{1, 2}
	println(a + b) // "{6, 8}"
	println(a - b) // "{-2, -2}"
	c += a
	println(c) // "{3, 5}"
}

Operator overloading goes against V's philosophy of simplicity and predictability. But since scientific and graphical applications are among V's domains, operator overloading is an important feature to have in order to improve readability:

a.add(b).add(c.mul(d)) is a lot less readable than a + b + c * d.

To improve safety and maintainability, operator overloading is limited:

• It's only possible to overload +, -, *, /, %, <, >, ==, !=, <=, >= operators.
• == and != are self generated by the compiler but can be overridden.
• Calling other functions inside operator functions is not allowed.
• Operator functions can't modify their arguments.
• When using < and == operators, the return type must be bool.
• !=, >, <= and >= are auto generated when == and < are defined.
• Both arguments must have the same type (just like with all operators in V).
• Assignment operators (*=, +=, /=, etc) are auto generated when the corresponding operators are defined and operands are of the same type.

Inline assembly

a := 100
b := 20
mut c := 0
asm amd64 {
    mov eax, a
    add eax, b
    mov c, eax
    ; =r (c) as c // output
    ; r (a) as a // input
      r (b) as b
}
println('a: $a') // 100
println('b: $b') // 20
println('c: $c') // 120

For more examples, see github.com/vlang/v/tree/master/vlib/v/tests/assembly/asm_test.amd64.v

Translating C to V

TODO: translating C to V will be available in V 0.3.

V can translate your C code to human readable V code and generate V wrappers on top of C libraries.

Let's create a simple program test.c first:

#include "stdio.h"

int main() {
	for (int i = 0; i < 10; i++) {
		printf("hello world\n");
	}
        return 0;
}

Run v translate test.c, and V will generate test.v:

fn main() {
	for i := 0; i < 10; i++ {
		println('hello world')
	}
}

To generate a wrapper on top of a C library use this command:

v wrapper c_code/libsodium/src/libsodium

This will generate a directory libsodium with a V module.

Example of a C2V generated libsodium wrapper:

https://github.com/vlang/libsodium

When should you translate C code and when should you simply call C code from V?

If you have well-written, well-tested C code, then of course you can always simply call this C code from V.

Translating it to V gives you several advantages:

• If you plan to develop that code base, you now have everything in one language, which is much safer and easier to develop in than C.
• Cross-compilation becomes a lot easier. You don't have to worry about it at all.
• No more build flags and include files either.

Hot code reloading

module main

import time

[live]
fn print_message() {
	println('Hello! Modify this message while the program is running.')
}

fn main() {
	for {
		print_message()
		time.sleep(500 * time.millisecond)
	}
}

Build this example with v -live message.v.

Functions that you want to be reloaded must have [live] attribute before their definition.

Right now it's not possible to modify types while the program is running.

More examples, including a graphical application: github.com/vlang/v/tree/master/examples/hot_code_reload.

Cross compilation

To cross compile your project simply run

v -os windows .

or

v -os linux .

(Cross compiling for macOS is temporarily not possible.)

If you don't have any C dependencies, that's all you need to do. This works even when compiling GUI apps using the ui module or graphical apps using gg.

You will need to install Clang, LLD linker, and download a zip file with libraries and include files for Windows and Linux. V will provide you with a link.

Cross-platform shell scripts in V

V can be used as an alternative to Bash to write deployment scripts, build scripts, etc.

The advantage of using V for this is the simplicity and predictability of the language, and cross-platform support. "V scripts" run on Unix-like systems as well as on Windows.

Use the .vsh file extension. It will make all functions in the os module global (so that you can use mkdir() instead of os.mkdir(), for example).

An example deploy.vsh:

#!/usr/bin/env -S v run
// The shebang above associates the file to V on Unix-like systems,
// so it can be run just by specifying the path to the file
// once it's made executable using `chmod +x`.

// Remove if build/ exits, ignore any errors if it doesn't
rmdir_all('build') or { }

// Create build/, never fails as build/ does not exist
mkdir('build') ?

// Move *.v files to build/
result := exec('mv *.v build/') ?
if result.exit_code != 0 {
	println(result.output)
}
// Similar to:
// files := ls('.') ?
// mut count := 0
// if files.len > 0 {
//     for file in files {
//         if file.ends_with('.v') {
//              mv(file, 'build/') or {
//                  println('err: $err')
//                  return
//              }
//         }
//         count++
//     }
// }
// if count == 0 {
//     println('No files')
// }

Now you can either compile this like a normal V program and get an executable you can deploy and run anywhere: v deploy.vsh && ./deploy

Or just run it more like a traditional Bash script: v run deploy.vsh

On Unix-like platforms, the file can be run directly after making it executable using chmod +x: ./deploy.vsh

Attributes

V has several attributes that modify the behavior of functions and structs.

An attribute is a compiler instruction specified inside [] right before a function/struct/enum declaration and applies only to the following declaration.

// [flag] enables Enum types to be used as bitfields

[flag]
enum BitField {
	read
	write
	other
}

fn main() {
	assert 1 == int(BitField.read)
	assert 2 == int(BitField.write)
	mut bf := BitField.read
	assert bf.has(.read | .other) // test if *at least one* of the flags is set
	assert !bf.all(.read | .other) // test if *all* of the flags is set
	bf.set(.write | .other)
	assert bf.has(.read | .write | .other)
	assert bf.all(.read | .write | .other)
	bf.toggle(.other)
	assert bf == BitField.read | .write
	assert bf.all(.read | .write)
	assert !bf.has(.other)
}

// Calling this function will result in a deprecation warning
[deprecated]
fn old_function() {
}

// It can also display a custom deprecation message
[deprecated: 'use new_function() instead']
fn legacy_function() {}

// You can also specify a date, after which the function will be
// considered deprecated. Before that date, calls to the function
// will be compiler notices - you will see them, but the compilation
// is not affected. After that date, calls will become warnings,
// so ordinary compiling will still work, but compiling with -prod
// will not (all warnings are treated like errors with -prod).
// 6 months after the deprecation date, calls will be hard
// compiler errors.
[deprecated: 'use new_function2() instead']
[deprecated_after: '2021-05-27']
fn legacy_function2() {}

// This function's calls will be inlined.
[inline]
fn inlined_function() {
}

// This function's calls will NOT be inlined.
[noinline]
fn function() {
}

// This function will NOT return to its callers.
// Such functions can be used at the end of or blocks,
// just like exit/1 or panic/1. Such functions can not
// have return types, and should end either in for{}, or
// by calling other `[noreturn]` functions.
[noreturn]
fn forever() {
	for {}
}

// The following struct must be allocated on the heap. Therefore, it can only be used as a
// reference (`&Window`) or inside another reference (`&OuterStruct{ Window{...} }`).
// See section "Stack and Heap"
[heap]
struct Window {
}

// V will not generate this function and all its calls if the provided flag is false.
// To use a flag, use `v -d flag`
[if debug]
fn foo() {
}

fn bar() {
	foo() // will not be called if `-d debug` is not passed
}

// The memory pointed to by the pointer arguments of this function will not be
// freed by the garbage collector (if in use) before the function returns
[keep_args_alive]
fn C.my_external_function(voidptr, int, voidptr) int

// Calls to following function must be in unsafe{} blocks.
// Note that the code in the body of `risky_business()` will still be
// checked, unless you also wrap it in `unsafe {}` blocks.
// This is useful, when you want to have an `[unsafe]` function that
// has checks before/after a certain unsafe operation, that will still
// benefit from V's safety features.
[unsafe]
fn risky_business() {
	// code that will be checked, perhaps checking pre conditions
	unsafe {
		// code that *will not be* checked, like pointer arithmetic,
		// accessing union fields, calling other `[unsafe]` fns, etc...
		// Usually, it is a good idea to try minimizing code wrapped
		// in unsafe{} as much as possible.
		// See also [Memory-unsafe code](#memory-unsafe-code)
	}
	// code that will be checked, perhaps checking post conditions and/or
	// keeping invariants
}

// V's autofree engine will not take care of memory management in this function.
// You will have the responsibility to free memory manually yourself in it.
[manualfree]
fn custom_allocations() {
}

// For C interop only, tells V that the following struct is defined with `typedef struct` in C
[typedef]
struct C.Foo {
}

// Used in Win32 API code when you need to pass callback function
[windows_stdcall]
fn C.DefWindowProc(hwnd int, msg int, lparam int, wparam int)

// Windows only:
// If a default graphics library is imported (ex. gg, ui), then the graphical window takes
// priority and no console window is created, effectively disabling println() statements.
// Use to explicitly create console window. Valid before main() only.
[console]
fn main() {
}

Goto

V allows unconditionally jumping to a label with goto. The label name must be contained within the same function as the goto statement. A program may goto a label outside or deeper than the current scope. goto allows jumping past variable initialization or jumping back to code that accesses memory that has already been freed, so it requires unsafe.

if x {
	// ...
	if y {
		unsafe {
			goto my_label
		}
	}
	// ...
}
my_label:

goto should be avoided, particularly when for can be used instead. Labelled break/continue can be used to break out of a nested loop, and those do not risk violating memory-safety.

Appendices

Appendix I: Keywords

V has 41 reserved keywords (3 are literals):

as
asm
assert
atomic
break
const
continue
defer
else
embed
enum
false
fn
for
go
goto
if
import
in
interface
is
lock
match
module
mut
none
or
pub
return
rlock
select
shared
sizeof
static
struct
true
type
typeof
union
unsafe
volatile
__offsetof

See also V Types.

Appendix II: Operators

This lists operators for primitive types only.

+    sum                    integers, floats, strings
-    difference             integers, floats
*    product                integers, floats
/    quotient               integers, floats
%    remainder              integers

~    bitwise NOT            integers
&    bitwise AND            integers
|    bitwise OR             integers
^    bitwise XOR            integers

!    logical NOT            bools
&&   logical AND            bools
||   logical OR             bools
!=   logical XOR            bools

<<   left shift             integer << unsigned integer
>>   right shift            integer >> unsigned integer
>>>  unsigned right shift	integer >> unsigned integer


Precedence    Operator
    5             *  /  %  <<  >> >>> &
    4             +  -  |  ^
    3             ==  !=  <  <=  >  >=
    2             &&
    1             ||


Assignment Operators
+=   -=   *=   /=   %=
&=   |=   ^=
>>=  <<=  >>>=