Statements & expressions

If

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

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

if can be used as an expression:

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

Type checks and casts

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

You can do it either in an if:

// cgen
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:

// 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:

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)
}
else if x.bar is MyStruct2 as new_var {
    // you can use `as` to create aliases
    println(new_var)
}
match x.bar {
    MyStruct {
        // x.bar is automatically casted
        println(x.bar)
    }
    else {}
}

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

It works like this:

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

In operator

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

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

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

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

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

For loop

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

for/in

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

Array for

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

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

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

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

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

Custom iterators

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

struct SquareIterator {
    arr []int
mut:
    idx int
}
fn (mut iter SquareIterator) next() ?int {
    if iter.idx >= iter.arr.len {
        return error('')
    }
    defer {
        iter.idx++
    }
    return iter.arr[iter.idx] * iter.arr[iter.idx]
}
nums := [1, 2, 3, 4, 5]
iter := SquareIterator{
    arr: nums
}
for squared in iter {
    println(squared)
}

The code above prints:

1
4
9
16
25

Map for

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

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

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

Range for

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

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

Condition for

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

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

Bare for

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

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

C for

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

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

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

Labelled break & continue

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

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

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

4
5
6
7

Match

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

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

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

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

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

or as an unless alternative: unless Ruby

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

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

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

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

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

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

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

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

Defer

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

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

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

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