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<a class="link subtopic" href="v-types.html#map-update-syntax"> Map update syntax</a>
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<a class="link" href="module-imports.html">Module imports</a>
<a class="link subtopic" href="module-imports.html#selective-imports"> Selective imports</a>
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<a class="link" href="statements-&-expressions.html">Statements & expressions</a>
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<a class="link subtopic" href="statements-&-expressions.html#in-operator"> In operator</a>
<a class="link subtopic" href="statements-&-expressions.html#for-loop"> For loop</a>
<a class="link subtopic" href="statements-&-expressions.html#defer"> Defer</a>
<a class="link subtopic" href="statements-&-expressions.html#goto"> Goto</a>
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<a class="link" href="structs.html">Structs</a>
<a class="link subtopic" href="structs.html#heap-structs"> Heap structs</a>
<a class="link subtopic" href="structs.html#default-field-values"> Default field values</a>
<a class="link subtopic" href="structs.html#required-fields"> Required fields</a>
<a class="link subtopic" href="structs.html#short-struct-literal-syntax"> Short struct literal syntax</a>
<a class="link subtopic" href="structs.html#struct-update-syntax"> Struct update syntax</a>
<a class="link subtopic" href="structs.html#trailing-struct-literal-arguments"> Trailing struct literal arguments</a>
<a class="link subtopic" href="structs.html#access-modifiers"> Access modifiers</a>
<a class="link subtopic" href="structs.html#anonymous-structs"> Anonymous structs</a>
<a class="link subtopic" href="structs.html#static-type-methods"> Static type methods</a>
<a class="link subtopic" href="structs.html#noinit-structs"> [noinit] structs</a>
<a class="link subtopic" href="structs.html#methods"> Methods</a>
<a class="link subtopic" href="structs.html#embedded-structs"> Embedded structs</a>
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<a class="link" href="unions.html">Unions</a>
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<a class="link" href="functions-2.html">Functions 2</a>
<a class="link subtopic" href="functions-2.html#immutable-function-args-by-default"> Immutable function args by default</a>
<a class="link subtopic" href="functions-2.html#mutable-arguments"> Mutable arguments</a>
<a class="link subtopic" href="functions-2.html#variable-number-of-arguments"> Variable number of arguments</a>
<a class="link subtopic" href="functions-2.html#anonymous-&-higher-order-functions"> Anonymous & higher order functions</a>
<a class="link subtopic" href="functions-2.html#closures"> Closures</a>
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<a class="link" href="references.html">References</a>
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<a class="link" href="constants.html">Constants</a>
<a class="link subtopic" href="constants.html#required-module-prefix"> Required module prefix</a>
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<a class="link" href="builtin-functions.html">Builtin functions</a>
<a class="link subtopic" href="builtin-functions.html#println"> println</a>
<a class="link subtopic" href="builtin-functions.html#printing-custom-types"> Printing custom types</a>
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<a class="link" href="modules.html">Modules</a>
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<a class="link subtopic" href="type-declarations.html#type-aliases"> Type aliases</a>
<a class="link subtopic" href="type-declarations.html#enums"> Enums</a>
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<a class="link subtopic" href="type-declarations.html#optionresult-types-and-error-handling"> Option/Result types and error handling</a>
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<a class="link" href="concurrency.html">Concurrency</a>
<a class="link subtopic" href="concurrency.html#spawning-concurrent-tasks"> Spawning Concurrent Tasks</a>
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<a class="link subtopic" href="concurrency.html#shared-objects"> Shared Objects</a>
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<a class="link" href="json.html">JSON</a>
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<a class="link" href="testing.html">Testing</a>
<a class="link subtopic" href="testing.html#asserts"> Asserts</a>
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<a class="link subtopic" href="testing.html#asserts-that-do-not-abort-your-program"> Asserts that do not abort your program</a>
<a class="link subtopic" href="testing.html#test-files"> Test files</a>
<a class="link subtopic" href="testing.html#running-tests"> Running tests</a>
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<a class="link" href="memory-management.html">Memory management</a>
<a class="link subtopic" href="memory-management.html#control"> Control</a>
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<a class="link" href="limited-operator-overloading.html">Limited operator overloading</a>
<a class="link subtopic" href="limited-operator-overloading.html#implicitly-generated-overloads"> Implicitly generated overloads</a>
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<a class="link" href="performance-tuning.html">Performance tuning</a>
<a class="link subtopic" href="performance-tuning.html#tuning-operations-details"> Tuning operations details</a>
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<a class="link" href="atomics.html">Atomics</a>
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<a class="link" href="global-variables.html">Global Variables</a>
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<a class="link" href="debugging.html">Debugging</a>
<a class="link subtopic" href="debugging.html#c-backend-binaries-(default)"> C Backend binaries (Default)</a>
<a class="link subtopic" href="debugging.html#native-backend-binaries"> Native Backend binaries</a>
<a class="link subtopic" href="debugging.html#javascript-backend"> Javascript Backend</a>
<div class="items">
<a class="link" href="v-and-c.html">V and C</a>
<a class="link subtopic" href="v-and-c.html#calling-c-from-v"> Calling C from V</a>
<a class="link subtopic" href="v-and-c.html#calling-v-from-c"> Calling V from C</a>
<a class="link subtopic" href="v-and-c.html#passing-c-compilation-flags"> Passing C compilation flags</a>
<a class="link subtopic" href="v-and-c.html#pkgconfig"> pkgconfig</a>
<a class="link subtopic" href="v-and-c.html#including-c-code"> Including C code</a>
<a class="link subtopic" href="v-and-c.html#c-types"> C types</a>
<a class="link subtopic" href="v-and-c.html#c-declarations"> C Declarations</a>
<a class="link subtopic" href="v-and-c.html#export-to-shared-library"> Export to shared library</a>
<a class="link subtopic" href="v-and-c.html#translating-c-to-v"> Translating C to V</a>
<a class="link subtopic" href="v-and-c.html#working-around-c-issues"> Working around C issues</a>
<div class="items">
<a class="link" href="other-v-features.html">Other V Features</a>
<a class="link subtopic" href="other-v-features.html#inline-assembly"> Inline assembly</a>
<a class="link subtopic" href="other-v-features.html#hot-code-reloading"> Hot code reloading</a>
<a class="link subtopic" href="other-v-features.html#cross-platform-shell-scripts-in-v"> Cross-platform shell scripts in V</a>
<a class="link subtopic" href="other-v-features.html#vsh-scripts-with-no-extension"> Vsh scripts with no extension</a>
<div class="items">
<a class="link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
<div class="items">
<a class="link" href="appendix-ii-operators.html">Appendix II: Operators</a>
</div>
</section>
</div>
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</aside>
<article>
<div class="container">
<div class="content">
<div class="content-container">
<main>
<h1 class="main">Type Declarations</h1>
<h2 id="type-aliases">Type aliases <a href="type-declarations.html#type-aliases" class="header-anchor" aria-hidden="true">#</a></h2>
<p>To define a new type <code>NewType</code> as an alias for <code>ExistingType</code>,
do <code>type NewType = ExistingType</code>.<br/>
This is a special case of a <a href="type-declarations.html#sum-types">sum type</a> declaration.</p>
<h2 id="enums">Enums <a href="type-declarations.html#enums" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">enum Color as u8 {
red
green
blue
}
mut color := Color.red
// V knows that `color` is a `Color`. No need to use `color = Color.green` here.
color = .green
println(color) // "green"
match color {
.red { println('the color was red') }
.green { println('the color was green') }
.blue { println('the color was blue') }
}
</div>
<p>The enum type can be any integer type, but can be omitted, if it is <code>int</code>: <code>enum Color {</code>.</p>
<p>Enum match must be exhaustive or have an <code>else</code> branch.
This ensures that if a new enum field is added, it's handled everywhere in the code.</p>
<p>Enum fields cannot re-use reserved keywords. However, reserved keywords may be escaped
with an @.</p>
<div class="language-v">enum Color {
@none
red
green
blue
}
color := Color.@none
println(color)
</div>
<p>Integers may be assigned to enum fields.</p>
<div class="language-v">enum Grocery {
apple
orange = 5
pear
}
g1 := int(Grocery.apple)
g2 := int(Grocery.orange)
g3 := int(Grocery.pear)
println('Grocery IDs: ${g1}, ${g2}, ${g3}')
</div>
<p>Output: <code>Grocery IDs: 0, 5, 6</code>.</p>
<p>Operations are not allowed on enum variables; they must be explicitly cast to <code>int</code>.</p>
<p>Enums can have methods, just like structs.</p>
<div class="language-v">enum Cycle {
one
two
three
}
fn (c Cycle) next() Cycle {
match c {
.one {
return .two
}
.two {
return .three
}
.three {
return .one
}
}
}
mut c := Cycle.one
for _ in 0 .. 10 {
println(c)
c = c.next()
}
</div>
<p>Output:</p>
<pre><code>one
two
three
one
two
three
one
two
three
one
</code></pre>
<p>Enums can be created from string or integer value and converted into string</p>
<div class="language-v">enum Cycle {
one
two = 2
three
}
// Create enum from value
println(Cycle.from(10) or { Cycle.three })
println(Cycle.from('two')!)
// Convert an enum value to a string
println(Cycle.one.str())
</div>
<p>Output:</p>
<pre><code>three
two
one
</code></pre>
<h2 id="function-types">Function Types <a href="type-declarations.html#function-types" class="header-anchor" aria-hidden="true">#</a></h2>
<p>You can use type aliases for naming specific function signatures - for
example:</p>
<div class="language-v">type Filter = fn (string) string
</div>
<p>This works like any other type - for example, a function can accept an
argument of a function type:</p>
<div class="language-v">type Filter = fn (string) string
fn filter(s string, f Filter) string {
return f(s)
}
</div>
<p>V has duck-typing, so functions don't need to declare compatibility with
a function type - they just have to be compatible:</p>
<div class="language-v">fn uppercase(s string) string {
return s.to_upper()
}
// now `uppercase` can be used everywhere where Filter is expected
</div>
<p>Compatible functions can also be explicitly cast to a function type:</p>
<div class="language-v">my_filter := Filter(uppercase)
</div>
<p>The cast here is purely informational - again, duck-typing means that the
resulting type is the same without an explicit cast:</p>
<div class="language-v">my_filter := uppercase
</div>
<p>You can pass the assigned function as an argument:</p>
<div class="language-v">println(filter('Hello world', my_filter)) // prints `HELLO WORLD`
</div>
<p>And you could of course have passed it directly as well, without using a
local variable:</p>
<div class="language-v">println(filter('Hello world', uppercase))
</div>
<p>And this works with anonymous functions as well:</p>
<div class="language-v">println(filter('Hello world', fn (s string) string {
return s.to_upper()
}))
</div>
<p>You can see the complete
<a href="https://github.com/vlang/v/tree/master/examples/function_types.v">example here</a>.</p>
<h2 id="interfaces">Interfaces <a href="type-declarations.html#interfaces" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-v">// interface-example.1
struct Dog {
breed string
}
fn (d Dog) speak() string {
return 'woof'
}
struct Cat {
breed string
}
fn (c Cat) speak() string {
return 'meow'
}
// unlike Go, but like TypeScript, V's interfaces can define both fields and methods.
interface Speaker {
breed string
speak() string
}
fn main() {
dog := Dog{'Leonberger'}
cat := Cat{'Siamese'}
mut arr := []Speaker{}
arr << dog
arr << cat
for item in arr {
println('a ${item.breed} says: ${item.speak()}')
}
}
</div>
<h3 id="implement-an-interface">Implement an interface <a href="type-declarations.html#implement-an-interface" class="header-anchor" aria-hidden="true">#</a></h3>
<p>A type implements an interface by implementing its methods and fields.
There is no explicit declaration of intent, no "implements" keyword.</p>
<p>An interface can have a <code>mut:</code> section. Implementing types will need
to have a <code>mut</code> receiver, for methods declared in the <code>mut:</code> section
of an interface.</p>
<div class="language-v">// interface-example.2
module main
interface Foo {
write(string) string
}
// => the method signature of a type, implementing interface Foo should be:
// `fn (s Type) write(a string) string`
interface Bar {
mut:
write(string) string
}
// => the method signature of a type, implementing interface Bar should be:
// `fn (mut s Type) write(a string) string`
struct MyStruct {}
// MyStruct implements the interface Foo, but *not* interface Bar
fn (s MyStruct) write(a string) string {
return a
}
fn main() {
s1 := MyStruct{}
fn1(s1)
// fn2(s1) -> compile error, since MyStruct does not implement Bar
}
fn fn1(s Foo) {
println(s.write('Foo'))
}
// fn fn2(s Bar) { // does not match
// println(s.write('Foo'))
// }
</div>
<h3 id="casting-an-interface">Casting an interface <a href="type-declarations.html#casting-an-interface" class="header-anchor" aria-hidden="true">#</a></h3>
<p>We can test the underlying type of an interface using dynamic cast operators.</p>
<blockquote class="tip">
<p>[!NOTE]
Dynamic cast converts variable <code>s</code> into a pointer inside the <code>if</code> statements in this example:</p>
</blockquote>
<div class="language-v">// interface-example.3 (continued from interface-example.1)
interface Something {}
fn announce(s Something) {
if s is Dog {
println('a ${s.breed} dog') // `s` is automatically cast to `Dog` (smart cast)
} else if s is Cat {
println('a cat speaks ${s.speak()}')
} else {
println('something else')
}
}
fn main() {
dog := Dog{'Leonberger'}
cat := Cat{'Siamese'}
announce(dog)
announce(cat)
}
</div>
<div class="language-v">// interface-example.4
interface IFoo {
foo()
}
interface IBar {
bar()
}
// implements only IFoo
struct SFoo {}
fn (sf SFoo) foo() {}
// implements both IFoo and IBar
struct SFooBar {}
fn (sfb SFooBar) foo() {}
fn (sfb SFooBar) bar() {
dump('This implements IBar')
}
fn main() {
mut arr := []IFoo{}
arr << SFoo{}
arr << SFooBar{}
for a in arr {
dump(a)
// In order to execute instances that implements IBar.
if a is IBar {
a.bar()
}
}
}
</div>
<p>For more information, see <a href="type-declarations.html#dynamic-casts">Dynamic casts</a>.</p>
<h3 id="interface-method-definitions">Interface method definitions <a href="type-declarations.html#interface-method-definitions" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Also unlike Go, an interface can have its own methods, similar to how
structs can have their methods. These 'interface methods' do not have
to be implemented, by structs which implement that interface.
They are just a convenient way to write <code>i.some_function()</code> instead of
<code>some_function(i)</code>, similar to how struct methods can be looked at, as
a convenience for writing <code>s.xyz()</code> instead of <code>xyz(s)</code>.</p>
<blockquote class="tip">
<p>[!NOTE]
This feature is NOT a "default implementation" like in C#.</p>
</blockquote>
<p>For example, if a struct <code>cat</code> is wrapped in an interface <code>a</code>, that has
implemented a method with the same name <code>speak</code>, as a method implemented by
the struct, and you do <code>a.speak()</code>, <em>only</em> the interface method is called:</p>
<div class="language-v">interface Adoptable {}
fn (a Adoptable) speak() string {
return 'adopt me!'
}
struct Cat {}
fn (c Cat) speak() string {
return 'meow!'
}
struct Dog {}
fn main() {
cat := Cat{}
assert dump(cat.speak()) == 'meow!'
//
a := Adoptable(cat)
assert dump(a.speak()) == 'adopt me!' // call Adoptable's `speak`
if a is Cat {
// Inside this `if` however, V knows that `a` is not just any
// kind of Adoptable, but actually a Cat, so it will use the
// Cat `speak`, NOT the Adoptable `speak`:
dump(a.speak()) // meow!
}
//
b := Adoptable(Dog{})
assert dump(b.speak()) == 'adopt me!' // call Adoptable's `speak`
// if b is Dog {
// dump(b.speak()) // error: unknown method or field: Dog.speak
// }
}
</div>
<h3 id="embedded-interface">Embedded interface <a href="type-declarations.html#embedded-interface" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Interfaces support embedding, just like structs:</p>
<div class="language-v">pub interface Reader {
mut:
read(mut buf []u8) ?int
}
pub interface Writer {
mut:
write(buf []u8) ?int
}
// ReaderWriter embeds both Reader and Writer.
// The effect is the same as copy/pasting all of the
// Reader and all of the Writer methods/fields into
// ReaderWriter.
pub interface ReaderWriter {
Reader
Writer
}
</div>
<h2 id="sum-types">Sum types <a href="type-declarations.html#sum-types" class="header-anchor" aria-hidden="true">#</a></h2>
<p>A sum type instance can hold a value of several different types. Use the <code>type</code>
keyword to declare a sum type:</p>
<div class="language-v">struct Moon {}
struct Mars {}
struct Venus {}
type World = Mars | Moon | Venus
sum := World(Moon{})
assert sum.type_name() == 'Moon'
println(sum)
</div>
<p>The built-in method <code>type_name</code> returns the name of the currently held
type.</p>
<p>With sum types you could build recursive structures and write concise but powerful code on them.</p>
<div class="language-v">// V's binary tree
struct Empty {}
struct Node {
value f64
left Tree
right Tree
}
type Tree = Empty | Node
// sum up all node values
fn sum(tree Tree) f64 {
return match tree {
Empty { 0 }
Node { tree.value + sum(tree.left) + sum(tree.right) }
}
}
fn main() {
left := Node{0.2, Empty{}, Empty{}}
right := Node{0.3, Empty{}, Node{0.4, Empty{}, Empty{}}}
tree := Node{0.5, left, right}
println(sum(tree)) // 0.2 + 0.3 + 0.4 + 0.5 = 1.4
}
</div>
<h3 id="dynamic-casts">Dynamic casts <a href="type-declarations.html#dynamic-casts" class="header-anchor" aria-hidden="true">#</a></h3>
<p>To check whether a sum type instance holds a certain type, use <code>sum is Type</code>.
To cast a sum type to one of its variants you can use <code>sum as Type</code>:</p>
<div class="language-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!')
}
}
</div>
<p><code>as</code> will panic if <code>w</code> doesn't hold a <code>Mars</code> instance.
A safer way is to use a smart cast.</p>
<h3 id="smart-casting">Smart casting <a href="type-declarations.html#smart-casting" class="header-anchor" aria-hidden="true">#</a></h3>
<div class="language-v">if w is Mars {
assert typeof(w).name == 'Mars'
if w.dust_storm() {
println('bad weather!')
}
}
</div>
<p><code>w</code> has type <code>Mars</code> inside the body of the <code>if</code> statement. This is
known as <em>flow-sensitive typing</em>.
If <code>w</code> is a mutable identifier, it would be unsafe if the compiler smart casts it without a warning.
That's why you have to declare a <code>mut</code> before the <code>is</code> expression:</p>
<div class="language-v">if mut w is Mars {
assert typeof(w).name == 'Mars'
if w.dust_storm() {
println('bad weather!')
}
}
</div>
<p>Otherwise <code>w</code> would keep its original type.</p>
<blockquote class="tip">
<p>This works for both simple variables and complex expressions like <code>user.name</code></p>
</blockquote>
<h3 id="matching-sum-types">Matching sum types <a href="type-declarations.html#matching-sum-types" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can also use <code>match</code> to determine the variant:</p>
<div class="language-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)
}
}
}
</div>
<p><code>match</code> must have a pattern for each variant or have an <code>else</code> branch.</p>
<div class="language-v">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 {}
}
}
</div>
<h2 id="optionresult-types-and-error-handling">Option/Result types and error handling <a href="type-declarations.html#optionresult-types-and-error-handling" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Option types are for types which may represent <code>none</code>. Result types may