# 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. ## Table of Contents
* [Hello world](#hello-world) * [Comments](#comments) * [Functions](#functions) * [Returning multiple values](#returning-multiple-values) * [Variable number of arguments](#variable-number-of-arguments) * [Symbol visibility](#symbol-visibility) * [Variables](#variables) * [Types](#types) * [Strings](#strings) * [Numbers](#numbers) * [Arrays](#arrays) * [Maps](#maps) * [Module imports](#module-imports) * [Statements & expressions](#statements--expressions) * [If](#if) * [In operator](#in-operator) * [For loop](#for-loop) * [Match](#match) * [Defer](#defer) * [Structs](#structs) * [Embedded structs](#embedded-structs) * [Default field values](#default-field-values) * [Short struct literal syntax](#short-struct-initialization-syntax) * [Access modifiers](#access-modifiers) * [Methods](#methods) * [println and other builtin functions](#println-and-other-builtin-functions) * [Functions 2](#functions-2) * [Pure functions by default](#pure-functions-by-default) * [Mutable arguments](#mutable-arguments) * [Anonymous & high order functions](#anonymous--high-order-functions) * [References](#references) * [Modules](#modules) * [Constants](#constants) * [Types 2](#types-2) * [Interfaces](#interfaces) * [Enums](#enums) * [Sum types](#sum-types) * [Option/Result types & error handling](#optionresult-types-and-error-handling) * [Generics](#generics) * [Concurrency](#concurrency) * [Channels](#channels) * [Decoding JSON](#decoding-json) * [Testing](#testing) * [Memory management](#memory-management) * [ORM](#orm) * [Writing documentation](#writing-documentation) * [Tools](#tools) * [vfmt](#vfmt) * [Profiling](#profiling) * [Advanced](#advanced) * [Memory-unsafe code](#memory-unsafe-code) * [Calling C functions from V](#calling-c-functions-from-v) * [Debugging generated C code](#debugging-generated-c-code) * [Conditional compilation](#conditional-compilation) * [Compile time pseudo variables](#compile-time-pseudo-variables) * [Compile-time reflection](#compile-time-reflection) * [Limited operator overloading](#limited-operator-overloading) * [Inline assembly](#inline-assembly) * [Translating C to V](#translating-c-to-v) * [Hot code reloading](#hot-code-reloading) * [Cross compilation](#cross-compilation) * [Cross-platform shell scripts in V](#cross-platform-shell-scripts-in-v) * [Attributes](#attributes) * [Appendices](#appendices) * [Keywords](#appendix-i-keywords) * [Operators](#appendix-ii-operators)
## Hello World ```v 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](https://github.com/vlang/v/blob/master/README.md#symlinking). 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 ```v println('hello world') ``` ## Comments ```v // This is a single line comment. /* This is a multiline comment. /* It can be nested. */ */ ``` ## Functions ```v 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. 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 ```v nofmt fn foo() (int, int) { return 2, 3 } a, b := foo() println(a) // 2 println(b) // 3 c, _ := foo() // ignore values using `_` ``` ### Variable number of arguments ```v nofmt fn sum(a ...int) int { mut total := 0 for x in a { total += x } return total } println(sum()) // Output: 0 println(sum(1)) // 1 println(sum(2,3)) // 5 ``` ## Symbol visibility ```v 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](#modules). ## Variables ```v 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](#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 ```v 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. ```v failcompile fn main() { age = 21 } ``` This code will not compile, because the variable `age` is not declared. All variables need to be declared in V. ```v fn main() { age := 21 } ``` ### 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). ```v failcompile 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: ```v ignore import ui import gg fn draw(ctx &gg.Context) { gg := ctx.parent.get_ui().gg gg.draw_rect(...) } ``` ## Types ### Primitive types ```v ignore bool string i8 i16 int i64 i128 (soon) byte u16 u32 u64 u128 (soon) rune // represents a Unicode code point f32 f64 any_int, any_float // internal intermediate types of number literals byteptr, voidptr, charptr, size_t // these are 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: ```v ignore 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 `f32` or `u32`. (`f32` would mean precision loss for large values and `u32` would mean loss of the sign for negative values). ### Strings ```v nofmt 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. String values are immutable. You cannot mutate elements: ```v failcompile 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`. Indexes correspond to bytes in the string, not Unicode code points. Character literals have type `rune`. To denote them, use ` ```v 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: ```v nofmt s := r'hello\nworld' println(s) // "hello\nworld" ``` ### 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: ```v nofmt 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`. ```v nofmt x := 123.4567 println('x = ${x:4.2f}') println('[${x:10}]') // pad with spaces on the left println('[${int(x):-10}]') // pad with spaces on the right ``` ### String operators ```v nofmt 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: ```v failcompile 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`: ```v age := 11 println('age = ' + age.str()) ``` or use string interpolation (preferred): ```v age := 12 println('age = $age') ``` ### Numbers ```v 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: ```v 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: ```v nofmt 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: ```v a := i64(123) b := byte(42) c := i16(12345) ``` Assigning floating point numbers works the same way: ```v 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`. ### Arrays ```v nofmt mut nums := [1, 2, 3] println(nums) // "[1, 2, 3]" println(nums[1]) // "2" nums[1] = 5 println(nums) // "[1, 5, 3]" println(nums[0..2]) // slicing gives an array "[1, 5]" println(nums.len) // "3" nums = [] // The array is now empty println(nums.len) // "0" // Declare an empty array: users := []int{} ``` 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 `.len` field returns the length of the array. Note that it's a read-only field, and it can't be modified by the user. Exported fields are read-only by default in V. See [Access modifiers](#access-modifiers). #### Array operations ```v nofmt 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. println(names.len) // "3" println('Alex' in names) // "false" ``` `<<` is an operator that appends a value to the end of the array. It can also append an entire array. `val in array` returns true if the array contains `val`. See [`in` operator](#in-operator). #### Initializing array properties During initialization you can specify the capacity of the array (`cap`), its initial length (`len`), and the default element (`init`): ```v arr := []int{len: 5, init: -1} // `[-1, -1, -1, -1, -1]` ``` Setting the capacity improves performance of insertions, as it reduces the number of reallocations needed: ```v nofmt 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`](#range-for) statement. #### 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()`: ```v nofmt nums := [1, 2, 3] nums_copy := nums.clone() ``` Arrays can be efficiently filtered and mapped with the `.filter()` and `.map()` methods: ```v nofmt 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. #### Multidimensional Arrays Arrays can have more than one dimension. 2d array example: ```v nofmt mut a := [][]int{len:2, init: []int{len:3}} a[0][1] = 2 println(a) // [[0, 2, 0], [0, 0, 0]] ``` 3d array example: ```v nofmt 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]]] ``` #### 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. ```v nofmt mut numbers := [1, 3, 2] numbers.sort() // 1, 2, 3 numbers.sort(a > b) // 3, 2, 1 ``` ```v nofmt 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 ``` ### Maps ```v nofmt mut m := map[string]int // Only maps with string keys are allowed for now 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 m.delete('two') // Short syntax numbers := { 'one': 1 'two': 2 } ``` ## Module imports For information about creating a module, see [Modules](#modules). Modules can be imported using the `import` keyword: ```v 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](https://modules.vlang.io/) 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. ### Selective imports You can also import specific functions and types from modules directly: ```v nofmt import os { input } import crypto.sha256 { sum } import time { Time } ``` Note: This is not allowed for constants - they must always be prefixed. You can import several specific symbols at once: ```v 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` ```v failcompile 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. ```v import time type MyTime = time.Time fn (mut t MyTime) century() int { return 1 + t.year % 100 } fn main() { mut my_time := MyTime{ year: 2020 month: 12 day: 25 } println(time.new_time(my_time).utc_string()) println('Century: $my_time.century()') } ``` ## Statements & expressions ### If ```v a := 10 b := 20 if a < b { println('$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: ```v 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`: ```v nofmt 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`: ```v oksyntax 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: ```v 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 needed to have a type cast despite of mutability. In this case the developer has to mark the expression with a `mut` keyword to tell the compiler that you're aware of what you're doing. It works like this: ```v oksyntax mut x := MySumType(MyStruct{123}) if mut x is MyStruct { // x is casted to MyStruct even 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 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. ```v nofmt nums := [1, 2, 3] println(1 in nums) // true m := {'one': 1, 'two': 2} println('one' in m) // true ``` It's also useful for writing boolean expressions that are clearer and more compact: ```v 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. #### Array `for` ```v nofmt numbers := [1, 2, 3, 4, 5] for num in numbers { println(num) } names := ['Sam', 'Peter'] for i, name in names { println('$i) $name') // Output: 0) Sam } // 1) Peter ``` The `for value in arr` form is used for going through elements of an array. If an index is required, an alternative form `for index, value in arr` can be used. Note, that the value is read-only. If you need to modify the array while looping, you have to use indexing: ```v nofmt mut numbers := [0, 1, 2] for i, _ in numbers { numbers[i]++ } println(numbers) // [1, 2, 3] ``` When an identifier is just a single underscore, it is ignored. #### Map `for` ```v nofmt m := {'one':1, 'two':2} for key, value in m { println("$key -> $value") // Output: one -> 1 } // two -> 2 ``` Either key or value can be ignored by using a single underscore as the identifier. ```v nofmt 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` ```v // 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` ```v nofmt 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` ```v nofmt 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` ```v 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: ```v 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 ```v 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. ```v number := 2 s := match number { 1 { 'one' } 2 { 'two' } else { 'many' } } ``` A match expression returns the final expression from each branch. ```v 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. ```v 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. ```v import os fn read_log() { mut ok := false mut f := os.open('log.txt') or { panic(err) } 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 } ``` ## Structs ```v nofmt 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: ```v 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](#references) to `Point`. References are similar to Go pointers and C++ references. ### Embedded structs V doesn't allow subclassing, but it supports embedded structs: ```v 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: ```v oksyntax button.widget.x = 3 ``` ### Default field values ```v 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 ```v 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: ```v failcompile _ = Foo{} ``` ### Short struct literal syntax ```v struct Point { x int y int } mut p := Point{ x: 10 y: 20 } // you can omit the struct name when it's already known p = { x: 30 y: 4 } assert p.y == 4 ``` 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: ```v 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: ```v ignore new_button(ButtonConfig{text:'Click me', width:100}) ``` This only works for functions that take a struct for the last argument. ### 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: ```v nofmt 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: f int // public and mutable both inside and outside parent module } // (not recommended to use, that's why the 'global' keyword // starts with __) ``` For example, here's the `string` type defined in the `builtin` module: ```v ignore struct string { str byteptr 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: ```v failcompile 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 ```v nofmt 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. 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. ## 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](#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`: ```v nofmt 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: ```v 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. 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. Use `user.register()` or `user = register(user)` instead of `register(mut user)`. V makes it easy to return a modified version of an object: ```v struct User { name string age int is_registered bool } fn register(u User) User { return { u | is_registered: true } } mut user := User{ name: 'abc' age: 23 } user = register(user) println(user) ``` ### Anonymous & high order functions ```v fn sqr(n int) int { return n * n } fn run(value int, op fn (int) int) int { return op(value) } fn main() { 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 }) } ``` ## References ```v 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 `&`: ```v 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: ```v wip struct Node { val T left &Node right &Node } ``` ## Constants ```v 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. V constants are more flexible than in most languages. You can assign more complex values: ```v 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) ``` Global variables are not allowed, so this can be really useful. ```v ignore println('Top cities: $top_cities.filter(.usa)') ``` ## println and other builtin functions `println` is a simple yet powerful builtin function. It can print anything: strings, numbers, arrays, maps, structs. ```v nofmt struct User{ name string age int } println(1) // "1" println('hi') // "hi" println([1,2,3]) // "[1, 2, 3]" println(User{name:'Bob', age:20}) // "User{name:'Bob', age:20}" ``` If you want to define a custom print value for your type, simply define a `.str() string` method: ```v struct Color { r int g int b int } pub fn (c Color) str() string { return '{$c.r, $c.g, $c.b}' } red := Color{ r: 255 g: 0 b: 0 } println(red) ``` If you don't want to print a newline, use `print()` instead. The number of builtin functions is low. Other builtin functions are: ```v ignore fn exit(exit_code int) // terminate the program fn panic(message string) fn print_backtrace() fn eprintln(s string) // same as println, but use stderr ``` ## 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: ```shell cd ~/code/modules mkdir mymodule vim mymodule/myfile.v ``` ```v failcompile // 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: ```v failcompile import mymodule fn main() { mymodule.say_hi() } ``` * Module names should be short, under 10 characters. * 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: ```v 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. ## Types 2 ### Interfaces ```v struct Dog { breed string } struct Cat {} fn (d Dog) speak() string { return 'woof' } fn (c Cat) speak() string { return 'meow' } interface Speaker { speak() string } dog := Dog{'Leonberger'} cat := Cat{} mut arr := []Speaker{} arr << dog arr << cat for item in arr { item.speak() } ``` A type implements an interface by implementing its methods. There is no explicit declaration of intent, no "implements" keyword. We can test the underlying type of an interface using dynamic cast operators: ```v oksyntax fn announce(s Speaker) { if s is Dog { println('a $s.breed') // `s` is automatically cast to `Dog` (smart cast) } else if s is Cat { println('a cat') } else { println('something else') } } ``` For more information, see [Dynamic casts](#dynamic-casts). ### Enums ```v nofmt 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" // enum starts by default at 0 println(color == 1) // true match color { .red { println('the color was red') } .green { println('the color was green') } .blue { println('the color was blue') } } // It's possible to override enum values enum Size { small = -1 medium = 0 large = 1 } mut s := Size.small println(s == -1) // true ``` 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. ### Sum types A sum type instance can hold a value of several different types. Use the `type` keyword to declare a sum type: ```v 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. #### 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`: ```v 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 ```v oksyntax 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*. You can also specify a variable name: ```v ignore if w is Mars as mars { assert typeof(w).name == 'World' if mars.dust_storm() { println('bad weather!') } } ``` `w` keeps its original type. This form is necessary if `w` is a more complex expression than just a variable name. #### Matching sum types You can also use `match` to determine the variant: ```v 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. There are two ways to access the cast variant inside a match branch: - the shadowed match variable - using `as` to specify a variable name ```v ignore 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 {} } // using `as` to specify a name for each value match w as var { Mars { var.shiver() } Venus { var.sweat() } else { // w is of type World assert w is Moon } } } ``` Note: shadowing only works when the match expression is a variable. It will not work on struct fields, array indexes, or map keys. ### Option/Result types and error handling Option types are declared with `?Type`: ```v 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. ```v oksyntax 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: ```v 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: ```v ignore resp := http.get(url) or { return error(err) } return resp.text ``` --- The second method is to break from execution early: ```v oksyntax 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) }` 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. ```v nofmt 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: ```v 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 ```v wip struct Repo { db DB } fn new_repo(db DB) Repo { return Repo{db: db} } // This is a generic function. V will generate it for every type it's used with. fn (r Repo) 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('select * from $table_name where id = ?', id) } db := new_db() users_repo := new_repo(db) // returns Repo posts_repo := new_repo(db) // returns Repo user := users_repo.find_by_id(1)? // find_by_id post := posts_repo.find_by_id(1)? // find_by_id ``` At the moment only one type parameter named `T` is supported. 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 ``, because the receiver argument `r` uses a generic type `T`. Another example: ```v nofmt fn compare(a T, b T) int { if a < b { return -1 } if a > b { return 1 } return 0 } // compare println(compare(1, 0)) // Outputs: 1 println(compare(1, 1)) // 0 println(compare(1, 2)) // -1 // compare println(compare('1', '0')) // Outputs: 1 println(compare('1', '1')) // 0 println(compare('1', '2')) // -1 // compare println(compare(1.1, 1.0)) // Outputs: 1 println(compare(1.1, 1.1)) // 0 println(compare(1.1, 1.2)) // -1 ``` ## Concurrency V's model of concurrency is very similar to Go's. To run `foo()` concurrently, just call it with `go foo()`. Right now, it launches the function on a new system thread. Soon coroutines and a scheduler will be implemented. ```v import sync import time fn task(id int, duration int, mut wg sync.WaitGroup) { println('task $id begin') time.sleep_ms(duration) println('task $id end') wg.done() } fn main() { mut wg := sync.new_waitgroup() wg.add(3) go task(1, 500, mut wg) go task(2, 900, mut wg) go task(3, 100, mut wg) wg.wait() println('done') } // Output: // task 1 begin // task 2 begin // task 3 begin // task 3 end // task 1 end // task 2 end // done ``` ### 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: ```v nofmt 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: ```v import sync 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: ```v nofmt import sync mut ch := chan int{} mut ch2 := chan f64{} n := 5 x := 7.3 ch <- n // push ch2 <- x 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](#handling-optionals)). ```v wip mut ch := chan int{} mut 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](#match) command: ```v wip import time fn main () { mut c := chan f64{} mut ch := chan f64{} mut ch2 := chan f64{} mut ch3 := chan f64{} mut b := 0.0 // ... select { a := <-ch { // do something with `a` } b = <-ch2 { // do something with predeclared variable `b` } ch3 <- c { // do something if `c` was sent } > 500 * time.millisecond { // do something if no channel has become ready within 0.5s } } } ``` 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: ```v wip if select { ch <- a { // ... } } { // channel was open } else { // channel is closed } ``` #### Special Channel Features For special purposes there are some builtin properties and methods: ```v nofmt import sync struct Abc{x int} a := 2.13 mut 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 println(l) println(c) // mut b := Abc{} // mut ch2 := chan f64{} // 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. Use `select` instead. Data can be exchanged between a coroutine and the calling thread via a shared variable. Such variables should be created as references and passed to the coroutine as `mut`. The underlying `struct` should also contain a `mutex` to lock concurrent access: ```v import sync struct St { mut: x int // share data mtx &sync.Mutex } fn (mut b St) g() { b.mtx.m_lock() // read/modify/write b.x b.mtx.unlock() } fn caller() { mut a := &St{ // create as reference so it's on the heap x: 10 mtx: sync.new_mutex() } go a.g() a.mtx.m_lock() // read/modify/write a.x a.mtx.unlock() } ``` ## Decoding JSON ```v import json struct Foo { x int } struct User { name string 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') 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 ```v 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 ```v // hello.v module main fn hello() string { return 'Hello world' } fn main() { println(hello()) } ``` ```v failcompile module main // hello_test.v 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 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. #### 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. ## 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. For developers willing to have more low level control, autofree can be disabled with `-noautofree`. Note: right now autofree is hidden behind the -autofree flag. It will be enabled by default in V 0.3. For example: ```v 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, the first two calls won't result in any allocations at all. These two strings are small, V will use a preallocated buffer for them. ```v 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 } ``` ## ORM (This is still in an alpha state) V has a built-in ORM (object-relational mapping) which supports SQLite, and will soon support MySQL, Postgres, 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.) ```v nofmt import sqlite struct Customer { // struct name has to be the same as the table name (for now) id int // a field named `id` of integer type must be the first field name string nr_orders int country string } db := sqlite.connect('customers.db')? // select count(*) from Customer nr_customers := sql db { select count from Customer } println('number of all customers: $nr_customers') // V syntax can be used to build queries // db.select returns an array uk_customers := sql db { select from Customer where country == 'uk' && nr_orders > 0 } println(uk_customers.len) for customer in uk_customers { println('$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, see vlib/orm/orm_test.v. ## 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: ```v // 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: ```v // 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`. ## Tools ### v fmt You don't need to worry about formatting your code or setting style guidelines. `v fmt` takes care of that: ```shell 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. ### 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: ```v import time fn main() { sw := time.new_stopwatch({}) println('Hello world') println('Greeting the world took: ${sw.elapsed().nanoseconds()}ns') } ``` # Advanced Topics ## 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: ```v wip // 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. ## Calling C functions from V ```v #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, &charptr, &charptr) int fn C.sqlite3_open(charptr, &&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(&sqlite3, charptr, int, &&sqlite3_stmt, &charptr) int fn C.sqlite3_step(&sqlite3_stmt) fn C.sqlite3_finalize(&sqlite3_stmt) fn C.sqlite3_exec(db &sqlite3, sql charptr, FnSqlite3Callback voidptr, cb_arg voidptr, emsg &charptr) int fn C.sqlite3_free(voidptr) fn my_callback(arg voidptr, howmany int, cvalues &charptr, cnames &charptr) 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('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, 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 := charptr(0) query_all_users := 'select * from users' rc := C.sqlite3_exec(db, query_all_users.str, my_callback, 7, &error_msg) if rc != C.SQLITE_OK { eprintln(cstring_to_vstring(error_msg)) C.sqlite3_free(error_msg) } C.sqlite3_close(db) } ``` ### #flag 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 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) ```v oksyntax #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= ``` ### #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: ```v oksyntax #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. ### 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: ```v ignore 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: ```v oksyntax #flag -I @VROOT/c #flag @VROOT/c/implementation.o #include "header.h" ``` NB: @VROOT 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 @VROOT/abc` to refer to this folder. The @VROOT folder is also *prepended* to the module lookup path, so you can *import* other modules under your @VROOT, 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 `@VROOT/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](https://github.com/vlang/v/tree/master/vlib/v/tests/project_with_c_code). Another example, demonstrating passing structs from C to V and back again: [interoperate between C to V to C](https://github.com/vlang/v/tree/master/vlib/v/tests/project_with_c_code_2). You can use `-cflags` to pass custom flags to the backend C compiler. You can also use `-cc` to change the default C backend compiler. For example: `-cc gcc-9 -cflags -fsanitize=thread`. ### C types Ordinary zero terminated C strings can be converted to V strings with `unsafe { charptr(cstring).vstring() }` or if you know their length already with `unsafe { charptr(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*`, - `byteptr` for C's `byte*` and - `charptr` for C's `char*`. - `&charptr` 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)`. [socket.v has an example which calls C code from V](https://github.com/vlang/v/blob/master/vlib/net/socket.v) . ## Debugging generated C code To debug issues in the generated C code, you can pass these flags: - `-cg` - produces a less optimized executable with more debug information in it. - `-showcc` - prints the C command that is used to build the program. For the best debugging experience, you can pass all of them at the same time: `v -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 see a detailed list of all flags that V supports, use `v help`, `v help build` and `v help build-c`. ## Conditional compilation ```v // 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 -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 with`v -d option`. Full list of builtin options: | OS | Compilers | Platforms | Other | | --- | --- | --- | --- | | `windows`, `linux`, `macos` | `gcc`, `tinyc` | `amd64`, `aarch64` | `debug`, `test`, `js` | | `mac`, `darwin`, `ios`, | `clang`, `mingw` | `x64`, `x32` | `glibc`, `prealloc` | | `android`,`mach`, `dragonfly` | `msvc` | `little_endian` | `no_bounds_checking` | | `gnu`, `hpux`, `haiku`, `qnx` | `cplusplus` | `big_endian` | | | `solaris`, `linux_or_macos` | | | | ## 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 - `@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 - `@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). That allows you to do the following example, useful while debugging/logging/tracing your code: ```v 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: ```v ignore import v.vmod vm := vmod.decode( @VMOD_FILE ) or { panic(err) } 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 - omiting 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: ```v wip // TODO: not fully implemented struct User { name string age int } // Note: T should be passed a struct name only fn decode(data string) T { mut result := T{} // compile-time `for` loop // T.fields gives an array of a field metadata type $for field in T.fields { $if field.typ is string { // $(string_expr) produces an identifier result.$(field.name) = get_string(data, field.name) } $else $if field.typ is int { result.$(field.name) = get_int(data, field.name) } } return result } // `decode` generates: fn decode_User(data string) User { mut result := User{} result.name = get_string(data, 'name') result.age = get_int(data, 'age') return result } ``` ## Limited operator overloading ```v 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} println(a + b) // "{6, 8}" println(a - b) // "{-2, -2}" } ``` 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. - Calling other functions inside operator functions is not allowed. - Operator functions can't modify their arguments. - Both arguments must have the same type (just like with all operators in V). ## Inline assembly TODO: not implemented yet ```v failcompile fn main() { a := 10 asm x64 { mov eax, [a] add eax, 10 mov [a], eax } } ``` ## 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: ```c #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`: ```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: ```bash 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/medvednikov/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 ```v live module main import time import os [live] fn print_message() { println('Hello! Modify this message while the program is running.') } fn main() { for { print_message() time.sleep_ms(500) } } ``` 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](https://github.com/vlang/v/tree/master/examples/hot_reload). ## Cross compilation To cross compile your project simply run ```shell v -os windows . ``` or ```shell 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 `ls()` instead of `os.ls()`, for example). ```v wip #!/usr/local/bin/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`. rm('build/*') // Same as: for file in ls('build/') { rm(file) } mv('*.v', 'build/') // Same as: for file in ls('.') { if file.ends_with('.v') { mv(file, 'build/') } } ``` 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 specified inside `[]` right before a function/struct declaration and applies only to the following declaration. ```v // Calling this function will result in a deprecation warning [deprecated] fn old_function() { } // This function's calls will be inlined. [inline] fn inlined_function() { } // The following struct can only be used as a reference (`&Window`) and allocated on the heap. [ref_only] 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 } // 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) ``` # Appendices ## Appendix I: Keywords V has 41 reserved keywords (3 are literals): ```v ignore 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 ``` See also [Types](#types). ## Appendix II: Operators This lists operators for [primitive types](#primitive-types) only. ```v ignore + 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 Precedence Operator 5 * / % << >> & 4 + - | ^ 3 == != < <= > >= 2 && 1 || Assignment Operators += -= *= /= %= &= |= ^= >>= <<= ```