doc: update sum type docs (#5702)
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doc/docs.md
102
doc/docs.md
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@ -1196,70 +1196,84 @@ println(color) // "1" TODO: print "green"?
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### Sum types
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### Sum types
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A sum type instance can hold a value of several different types. Use the `type`
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keyword to declare a sum type:
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```v
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```v
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type Expr = BinaryExpr | UnaryExpr | IfExpr
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struct Moon {}
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struct Mars {}
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struct Venus {}
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struct BinaryExpr{ ... }
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type World = Moon | Mars | Venus
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struct UnaryExpr{ ... }
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struct IfExpr{ ... }
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fn (mut p Parser) expr(precedence int) Expr {
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sum := World(Moon{})
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match p.tok {
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```
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.key_if { return IfExpr{} }
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...
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else { return BinaryExpr{} }
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}
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}
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fn gen(expr Expr) {
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To check whether a sum type instance holds a certain type, use `sum is Type`.
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match expr {
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To cast a sum type to one of its variants you use `sum as Type`:
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IfExpr { gen_if(expr) } // `expr` is cast to the matched type automatically
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...
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}
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}
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fn gen_if(expr IfExpr) {
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```v
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...
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fn (m Mars) dust_storm() bool
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fn main() {
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mut w := World(Moon{})
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assert w is Moon
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w = Mars{}
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// use `as` to access the Mars instance
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mars := w as Mars
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if mars.dust_storm() {
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println('bad weather!')
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}
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}
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}
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```
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```
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To check whether a sum type is a certain type, use `is`:
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You can also use `match` to determine the variant:
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```v
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```v
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println(expr is IfExpr)
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fn open_parachutes(n int)
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fn land(w World) {
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match w {
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Moon {} // no atmosphere
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Mars {
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// light atmosphere
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open_parachutes(3)
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}
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Venus {
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// heavy atmosphere
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open_parachutes(1)
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}
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}
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}
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```
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```
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To cast a sum type to one of its variants you use `as`:
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`match` must have a pattern for each variant or have an `else` branch.
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```v
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There are 2 ways to access the cast variant inside a match branch:
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bin_expr := expr as BinaryExpr
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```
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You can also use match to determine the variant and cast to it at the same time.
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There are 3 ways to access the cast variant inside a match branch:
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- the `it` variable
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- the shadowed match variable
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- the shadowed match variable
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- using `as` to specify a variable name
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- using `as` to specify a variable name
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```v
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```v
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fn binary_expr(bx BinaryExpr) {...}
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fn (m Moon) moon_walk()
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fn unary_expr(ux UnaryExpr) {...}
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fn (m Mars) shiver()
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fn if_expr(ix IfExpr) {...}
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fn (v Venus) sweat()
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// using `it`
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fn pass_time(w World) {
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match expr {
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match w {
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BinaryExpr { binary_expr(it) }
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// using the shadowed match variable, in this case `w`
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...
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Moon { w.moon_walk() }
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Mars { w.shiver() }
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else {}
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}
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// using `as` to specify a variable name
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match w as expr {
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Venus { expr.sweat() }
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else {
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// w is of type World
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assert w !is Venus
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}
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}
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// using the shadowed variable, in this case `expr`
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match expr {
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UnaryExpr { unary_expr(expr) }
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...
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}
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}
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// using `as` to specify a variable
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match expr as actual_expr {
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IfExpr { if_expr(actual_expr) }
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...
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}
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}
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```
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```
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@ -0,0 +1,60 @@
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// Example of sum types
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// Models a landing craft leaving orbit and landing on a world
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import rand
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import time
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struct Moon {
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}
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struct Mars {
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}
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fn (m Mars) dust_storm() bool {
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return rand.int() >= 0
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}
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struct Venus {
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}
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type World = Moon | Mars | Venus
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struct Lander {
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}
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fn (l Lander) deorbit() {
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println('leaving orbit')
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}
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fn (l Lander) open_parachutes(n int) {
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println('opening $n parachutes')
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}
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fn wait() {
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println('waiting...')
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time.sleep(1)
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}
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fn (l Lander) land(w World) {
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if w is Mars {
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m := w as Mars
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for m.dust_storm() {
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wait()
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}
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}
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l.deorbit()
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match w {
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Moon {} // no atmosphere
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Mars {
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// light atmosphere
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l.open_parachutes(3)
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}
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Venus {
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// heavy atmosphere
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l.open_parachutes(1)
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}
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}
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println('landed')
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}
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fn main() {
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l := Lander {}
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l.land(Venus{})
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l.land(Mars{})
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}
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