| * [Hello world](#hello-world) * [Running a project folder](#running-a-project-folder-with-several-files) * [Comments](#comments) * [Functions](#functions) * [Hoistings](#hoistings) * [Returning multiple values](#returning-multiple-values) * [Symbol visibility](#symbol-visibility) * [Variables](#variables) * [Mutable variables](#mutable-variables) * [Initialization vs assignment](#initialization-vs-assignment) * [Declaration errors](#declaration-errors) * [V types](#v-types) * [Primitive types](#primitive-types) * [Strings](#strings) * [Runes](#runes) * [Numbers](#numbers) * [Arrays](#arrays) * [Multidimensional arrays](#multidimensional-arrays) * [Array methods](#array-methods) * [Array slices](#array-slices) * [Fixed size arrays](#fixed-size-arrays) * [Maps](#maps) | * [Module imports](#module-imports) * [Selective imports](#selective-imports) * [Module import aliasing](#module-import-aliasing) * [Statements & expressions](#statements--expressions) * [If](#if) * [Match](#match) * [In operator](#in-operator) * [For loop](#for-loop) * [Defer](#defer) * [Goto](#goto) * [Structs](#structs) * [Heap structs](#heap-structs) * [Default field values](#default-field-values) * [Required fields](#required-fields) * [Short struct literal syntax](#short-struct-literal-syntax) * [Struct update syntax](#struct-update-syntax) * [Trailing struct literal arguments](#trailing-struct-literal-arguments) * [Access modifiers](#access-modifiers) * [Anonymous structs](#anonymous-structs) * [Static type methods](#static-type-methods) * [[noinit] structs](#noinit-structs) * [Methods](#methods) * [Embedded structs](#embedded-structs) * [Unions](#unions) | * [Functions 2](#functions-2) * [Immutable function args by default](#immutable-function-args-by-default) * [Mutable arguments](#mutable-arguments) * [Variable number of arguments](#variable-number-of-arguments) * [Anonymous & higher-order functions](#anonymous--higher-order-functions) * [Closures](#closures) * [Parameter evaluation order](#parameter-evaluation-order) * [References](#references) * [Constants](#constants) * [Required module prefix](#required-module-prefix) * [Builtin functions](#builtin-functions) * [println](#println) * [Printing custom types](#printing-custom-types) * [Dumping expressions at runtime](#dumping-expressions-at-runtime) * [Modules](#modules) * [Create modules](#create-modules) * [init functions](#init-functions) |
| * [Type Declarations](#type-declarations) * [Type aliases](#type-aliases) * [Enums](#enums) * [Function Types](#function-types) * [Interfaces](#interfaces) * [Sum types](#sum-types) * [Option/Result types & error handling](#optionresult-types-and-error-handling) * [Handling options/results](#handling-optionsresults) * [Custom error types](#custom-error-types) * [Generics](#generics) * [Concurrency](#concurrency) * [Spawning Concurrent Tasks](#spawning-concurrent-tasks) * [Channels](#channels) * [Shared Objects](#shared-objects) * [JSON](#json) * [Decoding JSON](#decoding-json) * [Encoding JSON](#encoding-json) * [Testing](#testing) * [Asserts](#asserts) * [Asserts with an extra message](#asserts-with-an-extra-message) * [Asserts that do not abort your program](#asserts-that-do-not-abort-your-program) * [Test files](#test-files) * [Running tests](#running-tests) * [Memory management](#memory-management) * [Control](#control) * [Stack and Heap](#stack-and-heap) * [ORM](#orm) * [Writing documentation](#writing-documentation) * [Newlines in Documentation Comments](#newlines-in-documentation-comments) | * [Tools](#tools) * [v fmt](#v-fmt) * [v shader](#v-shader) * [Profiling](#profiling) * [Package Management](#package-management) * [Package commands](#package-commands) * [Publish package](#publish-package) * [Advanced Topics](#advanced-topics) * [Attributes](#attributes) * [Conditional compilation](#conditional-compilation) * [Compile time pseudo variables](#compile-time-pseudo-variables) * [Compile time reflection](#compile-time-reflection) * [Compile time code](#compile-time-code) * [Compile time types](#compile-time-types) * [Environment specific files](#environment-specific-files) * [Memory-unsafe code](#memory-unsafe-code) * [Structs with reference fields](#structs-with-reference-fields) * [sizeof and __offsetof](#sizeof-and-__offsetof) * [Limited operator overloading](#limited-operator-overloading) * [Performance tuning](#performance-tuning) * [Atomics](#atomics) * [Global Variables](#global-variables) * [Cross compilation](#cross-compilation) * [Debugging](#debugging) * [C Backend binaries Default](#c-backend-binaries-default) * [Native Backend binaries](#native-backend-binaries) * [Javascript Backend](#javascript-backend) | * [V and C](#v-and-c) * [Calling C from V](#calling-c-from-v) * [Calling V from C](#calling-v-from-c) * [Passing C compilation flags](#passing-c-compilation-flags) * [#pkgconfig](#pkgconfig) * [Including C code](#including-c-code) * [C types](#c-types) * [C Declarations](#c-declarations) * [Export to shared library](#export-to-shared-library) * [Translating C to V](#translating-c-to-v) * [Working around C issues](#working-around-c-issues) * [Other V Features](#other-v-features) * [Inline assembly](#inline-assembly) * [Hot code reloading](#hot-code-reloading) * [Cross-platform shell scripts in V](#cross-platform-shell-scripts-in-v) * [Vsh scripts with no extension](#vsh-scripts-with-no-extension) * [Appendices](#appendices) * [Keywords](#appendix-i-keywords) * [Operators](#appendix-ii-operators) |
` (backticks) :
```v
rocket := `🚀`
```
A `rune` can be converted to a UTF-8 string by using the `.str()` method.
```v
rocket := `🚀`
assert rocket.str() == '🚀'
```
A `rune` can be converted to UTF-8 bytes by using the `.bytes()` method.
```v
rocket := `🚀`
assert rocket.bytes() == [u8(0xf0), 0x9f, 0x9a, 0x80]
```
Hex, Unicode, and Octal escape sequences also work in a `rune` literal:
```v
assert `\x61` == `a`
assert `\141` == `a`
assert `\u0061` == `a`
// multibyte literals work too
assert `\u2605` == `★`
assert `\u2605`.bytes() == [u8(0xe2), 0x98, 0x85]
assert `\xe2\x98\x85`.bytes() == [u8(0xe2), 0x98, 0x85]
assert `\342\230\205`.bytes() == [u8(0xe2), 0x98, 0x85]
```
Note that `rune` literals use the same escape syntax as strings, but they can only hold one unicode
character. Therefore, if your code does not specify a single Unicode character, you will receive an
error at compile time.
Also remember that strings are indexed as bytes, not runes, so beware:
```v
rocket_string := '🚀'
assert rocket_string[0] != `🚀`
assert 'aloha!'[0] == `a`
```
A string can be converted to runes by the `.runes()` method.
```v
hello := 'Hello World 👋'
hello_runes := hello.runes() // [`H`, `e`, `l`, `l`, `o`, ` `, `W`, `o`, `r`, `l`, `d`, ` `, `👋`]
assert hello_runes.string() == hello
```
### 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
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 := u8(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`.
Float literals can also be declared as a power of ten:
```v
f0 := 42e1 // 420
f1 := 123e-2 // 1.23
f2 := 456e+2 // 45600
```
### Arrays
An array is a collection of data elements of the same type. An array literal is a
list of expressions surrounded by square brackets. An individual element can be
accessed using an *index* expression. Indexes start from `0`:
```v
mut nums := [1, 2, 3]
println(nums) // `[1, 2, 3]`
println(nums[0]) // `1`
println(nums[1]) // `2`
nums[1] = 5
println(nums) // `[1, 5, 3]`
```
An element can be appended to the end of an array using the push operator `<<`.
It can also append an entire array.
```v
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]"
```
```v
mut names := ['John']
names << 'Peter'
names << 'Sam'
// names << 10 <-- This will not compile. `names` is an array of strings.
```
`val in array` returns true if the array contains `val`. See [`in` operator](#in-operator).
```v
names := ['John', 'Peter', 'Sam']
println('Alex' in names) // "false"
```
#### Array Fields
There are two fields that control the "size" of an array:
* `len`: *length* - the number of pre-allocated and initialized elements in the array
* `cap`: *capacity* - the amount of memory space which has been reserved for elements,
but not initialized or counted as elements. The array can grow up to this size without
being reallocated. Usually, V takes care of this field automatically but there are
cases where the user may want to do manual optimizations (see [below](#array-initialization)).
```v
mut nums := [1, 2, 3]
println(nums.len) // "3"
println(nums.cap) // "3" or greater
nums = [] // The array is now empty
println(nums.len) // "0"
```
`data` is a field (of type `voidptr`) with the address of the first
element. This is for low-level [`unsafe`](#memory-unsafe-code) code.
> **Note**
> Fields are read-only and can't be modified by the user.
#### Array Initialization
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: `[u8(16), 32, 64, 128]`.
V arrays are homogeneous (all elements must have the same type).
This means that code like `[1, 'a']` will not compile.
The above syntax is fine for a small number of known elements but for very large or empty
arrays there is a second initialization syntax:
```v
mut a := []int{len: 10000, cap: 30000, init: 3}
```
This creates an array of 10000 `int` elements that are all initialized with `3`. Memory
space is reserved for 30000 elements. The parameters `len`, `cap` and `init` are optional;
`len` defaults to `0` and `init` to the default initialization of the element type (`0`
for numerical type, `''` for `string`, etc). The run time system makes sure that the
capacity is not smaller than `len` (even if a smaller value is specified explicitly):
```v
arr := []int{len: 5, init: -1}
// `arr == [-1, -1, -1, -1, -1]`, arr.cap == 5
// Declare an empty array:
users := []int{}
```
Setting the capacity improves performance of pushing elements to the array
as reallocations can be avoided:
```v
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.
You can initialize the array by accessing the `index` variable which gives
the index as shown here:
```v
count := []int{len: 4, init: index}
assert count == [0, 1, 2, 3]
mut square := []int{len: 6, init: index * index}
// square == [0, 1, 4, 9, 16, 25]
```
#### Array Types
An array can be of these types:
| Types | Example Definition |
|--------------|--------------------------------------|
| Number | `[]int,[]i64` |
| String | `[]string` |
| Rune | `[]rune` |
| Boolean | `[]bool` |
| Array | `[][]int` |
| Struct | `[]MyStructName` |
| Channel | `[]chan f64` |
| Function | `[]MyFunctionType` `[]fn (int) bool` |
| Interface | `[]MyInterfaceName` |
| Sum Type | `[]MySumTypeName` |
| Generic Type | `[]T` |
| Map | `[]map[string]f64` |
| Enum | `[]MyEnumType` |
| Alias | `[]MyAliasTypeName` |
| Thread | `[]thread int` |
| Reference | `[]&f64` |
| Shared | `[]shared MyStructType` |
**Example Code:**
This example uses [Structs](#structs) and [Sum Types](#sum-types) to create an array
which can handle different types (e.g. Points, Lines) of data elements.
```v
struct Point {
x int
y int
}
struct Line {
p1 Point
p2 Point
}
type ObjectSumType = Line | Point
mut object_list := []ObjectSumType{}
object_list << Point{1, 1}
object_list << Line{
p1: Point{3, 3}
p2: Point{4, 4}
}
dump(object_list)
/*
object_list: [ObjectSumType(Point{
x: 1
y: 1
}), ObjectSumType(Line{
p1: Point{
x: 3
y: 3
}
p2: Point{
x: 4
y: 4
}
})]
*/
```
#### Multidimensional Arrays
Arrays can have more than one dimension.
2d array example:
```v
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
mut a := [][][]int{len: 2, init: [][]int{len: 3, init: []int{len: 2}}}
a[0][1][1] = 2
println(a) // [[[0, 0], [0, 2], [0, 0]], [[0, 0], [0, 0], [0, 0]]]
```
#### Array 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
nums := [1, 2, 3]
nums_copy := nums.clone()
```
Arrays can be efficiently filtered and mapped with the `.filter()` and
`.map()` methods:
```v
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)
```
```v
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 the element currently being
processed in filter/map methods.
Additionally, `.any()` and `.all()` can be used to conveniently test
for elements that satisfy a condition.
```v
nums := [1, 2, 3]
println(nums.any(it == 2)) // true
println(nums.all(it >= 2)) // false
```
There are further built-in methods for arrays:
* `a.repeat(n)` concatenates the array elements `n` times
* `a.insert(i, val)` inserts a new element `val` at index `i` and
shifts all following elements to the right
* `a.insert(i, [3, 4, 5])` inserts several elements
* `a.prepend(val)` inserts a value at the beginning, equivalent to `a.insert(0, val)`
* `a.prepend(arr)` inserts elements of array `arr` at the beginning
* `a.trim(new_len)` truncates the length (if `new_length < a.len`, otherwise does nothing)
* `a.clear()` empties the array without changing `cap` (equivalent to `a.trim(0)`)
* `a.delete_many(start, size)` removes `size` consecutive elements from index `start`
– triggers reallocation
* `a.delete(index)` equivalent to `a.delete_many(index, 1)`
* `a.delete_last()` removes the last element
* `a.first()` equivalent to `a[0]`
* `a.last()` equivalent to `a[a.len - 1]`
* `a.pop()` removes the last element and returns it
* `a.reverse()` makes a new array with the elements of `a` in reverse order
* `a.reverse_in_place()` reverses the order of elements in `a`
* `a.join(joiner)` concatenates an array of strings into one string
using `joiner` string as a separator
See all methods of [array](https://modules.vlang.io/index.html#array)
See also [vlib/arrays](https://modules.vlang.io/arrays.html).
##### 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
mut numbers := [1, 3, 2]
numbers.sort() // 1, 2, 3
numbers.sort(a > b) // 3, 2, 1
```
```v
struct User {
age int
name string
}
mut users := [User{21, 'Bob'}, User{20, 'Zarkon'}, User{25, 'Alice'}]
users.sort(a.age < b.age) // sort by User.age int field
users.sort(a.name > b.name) // reverse sort by User.name string field
```
V also supports custom sorting, through the `sort_with_compare` array method.
Which expects a comparing function which will define the sort order.
Useful for sorting on multiple fields at the same time by custom sorting rules.
The code below sorts the array ascending on `name` and descending `age`.
```v
struct User {
age int
name string
}
mut users := [User{21, 'Bob'}, User{65, 'Bob'}, User{25, 'Alice'}]
custom_sort_fn := fn (a &User, b &User) int {
// return -1 when a comes before b
// return 0, when both are in same order
// return 1 when b comes before a
if a.name == b.name {
if a.age < b.age {
return 1
}
if a.age > b.age {
return -1
}
return 0
}
if a.name < b.name {
return -1
} else if a.name > b.name {
return 1
}
return 0
}
users.sort_with_compare(custom_sort_fn)
```
#### Array Slices
A slice is a part of a parent array. Initially it refers to the elements
between two indices separated by a `..` operator. The right-side index must
be greater than or equal to the left side index.
If a right-side index is absent, it is assumed to be the array length. If a
left-side index is absent, it is assumed to be 0.
```v
nums := [0, 10, 20, 30, 40]
println(nums[1..4]) // [10, 20, 30]
println(nums[..4]) // [0, 10, 20, 30]
println(nums[1..]) // [10, 20, 30, 40]
```
In V slices are arrays themselves (they are not distinct types). As a result
all array operations may be performed on them. E.g. they can be pushed onto an
array of the same type:
```v
array_1 := [3, 5, 4, 7, 6]
mut array_2 := [0, 1]
array_2 << array_1[..3]
println(array_2) // `[0, 1, 3, 5, 4]`
```
A slice is always created with the smallest possible capacity `cap == len` (see
[`cap` above](#array-initialization)) no matter what the capacity or length
of the parent array is. As a result it is immediately reallocated and copied to another
memory location when the size increases thus becoming independent from the
parent array (*copy on grow*). In particular pushing elements to a slice
does not alter the parent:
```v
mut a := [0, 1, 2, 3, 4, 5]
mut b := a[2..4]
b[0] = 7 // `b[0]` is referring to `a[2]`
println(a) // `[0, 1, 7, 3, 4, 5]`
b << 9
// `b` has been reallocated and is now independent from `a`
println(a) // `[0, 1, 7, 3, 4, 5]` - no change
println(b) // `[7, 3, 9]`
```
Appending to the parent array may or may not make it independent from its child slices.
The behaviour depends on the parent's capacity and is predictable:
```v
mut a := []int{len: 5, cap: 6, init: 2}
mut b := unsafe { a[1..4] }
a << 3
// no reallocation - fits in `cap`
b[2] = 13 // `a[3]` is modified
a << 4
// a has been reallocated and is now independent from `b` (`cap` was exceeded)
b[1] = 3 // no change in `a`
println(a) // `[2, 2, 2, 13, 2, 3, 4]`
println(b) // `[2, 3, 13]`
```
You can call .clone() on the slice, if you do want to have an independent copy right away:
```v
mut a := [0, 1, 2, 3, 4, 5]
mut b := a[2..4].clone()
b[0] = 7 // Note: `b[0]` is NOT referring to `a[2]`, as it would have been, without the .clone()
println(a) // [0, 1, 2, 3, 4, 5]
println(b) // [7, 3]
```
##### Slices with negative indexes
V supports array and string slices with negative indexes.
Negative indexing starts from the end of the array towards the start,
for example `-3` is equal to `array.len - 3`.
Negative slices have a different syntax from normal slices, i.e. you need
to add a `gate` between the array name and the square bracket: `a#[..-3]`.
The `gate` specifies that this is a different type of slice and remember that
the result is "locked" inside the array.
The returned slice is always a valid array, though it may be empty:
```v
a := [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
println(a#[-3..]) // [7, 8, 9]
println(a#[-20..]) // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
println(a#[-20..-8]) // [0, 1]
println(a#[..-3]) // [0, 1, 2, 3, 4, 5, 6]
// empty arrays
println(a#[-20..-10]) // []
println(a#[20..10]) // []
println(a#[20..30]) // []
```
#### Array method chaining
You can chain the calls of array methods like `.filter()` and `.map()` and use
the `it` built-in variable to achieve a classic `map/filter` functional paradigm:
```v
// using filter, map and negatives array slices
files := ['pippo.jpg', '01.bmp', '_v.txt', 'img_02.jpg', 'img_01.JPG']
filtered := files.filter(it#[-4..].to_lower() == '.jpg').map(it.to_upper())
// ['PIPPO.JPG', 'IMG_02.JPG', 'IMG_01.JPG']
```
### Fixed size arrays
V also supports arrays with fixed size. Unlike ordinary arrays, their
length is constant. You cannot append elements to them, nor shrink them.
You can only modify their elements in place.
However, access to the elements of fixed size arrays is more efficient,
they need less memory than ordinary arrays, and unlike ordinary arrays,
their data is on the stack, so you may want to use them as buffers if you
do not want additional heap allocations.
Most methods are defined to work on ordinary arrays, not on fixed size arrays.
You can convert a fixed size array to an ordinary array with slicing:
```v
mut fnums := [3]int{} // fnums is a fixed size array with 3 elements.
fnums[0] = 1
fnums[1] = 10
fnums[2] = 100
println(fnums) // => [1, 10, 100]
println(typeof(fnums).name) // => [3]int
fnums2 := [1, 10, 100]! // short init syntax that does the same (the syntax will probably change)
anums := fnums[..] // same as `anums := fnums[0..fnums.len]`
println(anums) // => [1, 10, 100]
println(typeof(anums).name) // => []int
```
Note that slicing will cause the data of the fixed size array to be copied to
the newly created ordinary array.
### Maps
```v
mut m := map[string]int{} // a map with `string` keys and `int` values
m['one'] = 1
m['two'] = 2
println(m['one']) // "1"
println(m['bad_key']) // "0"
println('bad_key' in m) // Use `in` to detect whether such key exists
println(m.keys()) // ['one', 'two']
m.delete('two')
```
Maps can have keys of type string, rune, integer, float or voidptr.
The whole map can be initialized using this short syntax:
```v
numbers := {
'one': 1
'two': 2
}
println(numbers)
```
If a key is not found, a zero value is returned by default:
```v
sm := {
'abc': 'xyz'
}
val := sm['bad_key']
println(val) // ''
```
```v
intm := {
1: 1234
2: 5678
}
s := intm[3]
println(s) // 0
```
It's also possible to use an `or {}` block to handle missing keys:
```v
mm := map[string]int{}
val := mm['bad_key'] or { panic('key not found') }
```
You can also check, if a key is present, and get its value, if it was present, in one go:
```v
m := {
'abc': 'def'
}
if v := m['abc'] {
println('the map value for that key is: ${v}')
}
```
The same option check applies to arrays:
```v
arr := [1, 2, 3]
large_index := 999
val := arr[large_index] or { panic('out of bounds') }
println(val)
// you can also do this, if you want to *propagate* the access error:
val2 := arr[333]!
println(val2)
```
V also supports nested maps:
```v
mut m := map[string]map[string]int{}
m['greet'] = {
'Hello': 1
}
m['place'] = {
'world': 2
}
m['code']['orange'] = 123
print(m)
```
Maps are ordered by insertion, like dictionaries in Python. The order is a
guaranteed language feature. This may change in the future.
See all methods of
[map](https://modules.vlang.io/index.html#map)
and
[maps](https://modules.vlang.io/maps.html).
## 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.
Cyclic module imports are not allowed, like in Go.
### Selective imports
You can also import specific functions and types from modules directly:
```v
import os { input }
fn main() {
// read text from stdin
name := input('Enter your name: ')
println('Hello, ${name}!')
}
```
> **Note**
> This will import the module as well. Also, this is not allowed for
> constants - they must always be prefixed.
You can import several specific symbols at once:
```v
import os { input, user_os }
name := input('Enter your name: ')
println('Name: ${name}')
current_os := user_os()
println('Your OS is ${current_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
import math
type MyTime = time.Time
fn (mut t MyTime) century() int {
return int(1.0 + math.trunc(f64(t.year) * 0.009999794661191))
}
fn main() {
mut my_time := MyTime{
year: 2020
month: 12
day: 25
}
println(time.new_time(my_time).utc_string())
println('Century: ${my_time.century()}')
}
```
## Statements & expressions
### If
```v
a := 10
b := 20
if a < b {
println('${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"
```
Anywhere you can use `or {}`, you can also use "if unwrapping". This binds the unwrapped value
of an expression to a variable when that expression is not none nor an error.
```v
m := {
'foo': 'bar'
}
// handle missing keys
if v := m['foo'] {
println(v) // bar
} else {
println('not found')
}
```
```v
fn res() !int {
return 42
}
// functions that return a result type
if v := res() {
println(v)
}
```
```v
struct User {
name string
}
arr := [User{'John'}]
// if unwrapping with assignment of a variable
u_name := if v := arr[0] {
v.name
} else {
'Unnamed'
}
println(u_name) // John
```
#### 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 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 cast 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 cast to Abc and can be used here
println(x)
}
Xyz {
// x is automatically cast 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 cast
println(x.bar)
} else if x.bar is MyStruct2 {
new_var := x.bar as MyStruct2
// ... or you can use `as` to create a type cast an alias manually:
println(new_var)
}
match x.bar {
MyStruct {
// x.bar is automatically cast
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:
```v oksyntax
mut x := MySumType(MyStruct{123})
if mut x is MyStruct {
// x is cast 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 cast to MyStruct even if it's mutable
// without the mut keyword that wouldn't work
println(x)
}
}
```
### 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 statement can also to be used as an `if - else if - else` alternative:
```v
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](https://www.tutorialspoint.com/ruby/ruby_if_else.htm)
```v
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.
```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.
```v
const start = 1
const end = 10
c := 2
num := match c {
start...end {
1000
}
else {
0
}
}
println(num)
// 1000
```
Constants can also be used in the range branch expressions.
> **Note**
> `match` as an expression is not usable in `for` loop and `if` statements.
### In operator
`in` allows to check whether an array or a map contains an element.
To do the opposite, use `!in`.
```v
nums := [1, 2, 3]
println(1 in nums) // true
println(4 !in nums) // true
```
> **Note**
> `in` checks if map contains a key, not a value.
```v
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:
```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.
#### `for`/`in`
This is the most common form. You can use it with an array, map or
numeric range.
##### Array `for`
```v
numbers := [1, 2, 3, 4, 5]
for num in numbers {
println(num)
}
names := ['Sam', 'Peter']
for i, name in names {
println('${i}) ${name}')
// Output: 0) Sam
// 1) Peter
}
```
The `for value in arr` form is used for going through elements of an array.
If an index is required, an alternative form `for index, value in arr` can be used.
Note that the value is read-only.
If you need to modify the array while looping, you need to declare the element as mutable:
```v
mut numbers := [0, 1, 2]
for mut num in numbers {
num++
}
println(numbers) // [1, 2, 3]
```
When an identifier is just a single underscore, it is ignored.
##### Custom iterators
Types that implement a `next` method returning an `Option` can be iterated
with a `for` loop.
```v
struct SquareIterator {
arr []int
mut:
idx int
}
fn (mut iter SquareIterator) next() ?int {
if iter.idx >= iter.arr.len {
return none
}
defer {
iter.idx++
}
return iter.arr[iter.idx] * iter.arr[iter.idx]
}
nums := [1, 2, 3, 4, 5]
iter := SquareIterator{
arr: nums
}
for squared in iter {
println(squared)
}
```
The code above prints:
```
1
4
9
16
25
```
##### Map `for`
```v
m := {
'one': 1
'two': 2
}
for key, value in m {
println('${key} -> ${value}')
// Output: one -> 1
// two -> 2
}
```
Either key or value can be ignored by using a single underscore as the identifier.
```v
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
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
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
```
### 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
}
```
If the function returns a value the `defer` block is executed *after* the return
expression is evaluated:
```v
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')
}
```
To access the result of the function inside a `defer` block the `$res()` expression can be used.
`$res()` is only used when a single value is returned, while on multi-return the `$res(idx)`
is parameterized.
```v ignore
fn (mut app App) auth_middleware() bool {
defer {
if !$res() {
app.response.status_code = 401
app.response.body = 'Unauthorized'
}
}
header := app.get_header('Authorization')
if header == '' {
return false
}
return true
}
fn (mut app App) auth_with_user_middleware() (bool, string) {
defer {
if !$res(0) {
app.response.status_code = 401
app.response.body = 'Unauthorized'
} else {
app.user = $res(1)
}
}
header := app.get_header('Authorization')
if header == '' {
return false, ''
}
return true, 'TestUser'
}
```
### Goto
V allows unconditionally jumping to a label with `goto`. The label name must be contained
within the same function as the `goto` statement. A program may `goto` a label outside
or deeper than the current scope. `goto` allows jumping past variable initialization or
jumping back to code that accesses memory that has already been freed, so it requires
`unsafe`.
```v ignore
if x {
// ...
if y {
unsafe {
goto my_label
}
}
// ...
}
my_label:
```
`goto` should be avoided, particularly when `for` can be used instead.
[Labelled break/continue](#labelled-break--continue) can be used to break out of
a nested loop, and those do not risk violating memory-safety.
## Structs
```v
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
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](#references) 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.
```v
struct Foo {
mut:
x int
}
fa := Foo{1}
mut a := fa
a.x = 2
assert fa.x == 1
assert a.x == 2
// fb := Foo{ 1 }
// mut b := &fb // error: `fb` is immutable, cannot have a mutable reference to it
// b.x = 2
mut fc := Foo{1}
mut c := &fc
c.x = 2
assert fc.x == 2
assert c.x == 2
println(fc) // Foo{ x: 2 }
println(c) // &Foo{ x: 2 } // Note `&` prefixed.
```
see also [Stack and Heap](#stack-and-heap)
### 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.
In case of reference value, see [here](#structs-with-reference-fields).
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](#attributes), 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
}
p = Point{
x: 30
y: 4
}
assert p.y == 4
//
// array: first element defines type of array
points := [Point{10, 20}, Point{20, 30}, Point{40, 50}]
println(points) // [Point{x: 10, y: 20}, Point{x: 20, y: 30}, Point{x: 40,y: 50}]
```
Omitting the struct name also works for returning a struct literal or passing one
as a function argument.
### Struct update syntax
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 User{
...u
is_registered: true
}
}
mut user := User{
name: 'abc'
age: 23
}
user = register(user)
println(user)
```
### 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
[params]
struct ButtonConfig {
text string
is_disabled bool
width int = 70
height int = 20
}
struct Button {
text string
width int
height int
}
fn new_button(c ButtonConfig) &Button {
return &Button{
width: c.width
height: c.height
text: c.text
}
}
button := new_button(text: 'Click me', width: 100)
// the height is unset, so it's the default value
assert button.height == 20
```
As you can see, both the struct name and braces can be omitted, instead of:
```v oksyntax nofmt
new_button(ButtonConfig{text:'Click me', width:100})
```
This only works for functions that take a struct for the last argument.
> Note the `[params]` tag is used to tell V, that the trailing struct parameter
> can be omitted *entirely*, so that you can write `button := new_button()`.
> Without it, you have to specify *at least* one of the field names, even if it
> has its default value, otherwise the compiler will produce this error message,
> when you call the function with no parameters:
> `error: expected 1 arguments, but got 0`.
### Access modifiers
Struct fields are private and immutable by default (making structs immutable as well).
Their access modifiers can be changed with
`pub` and `mut`. In total, there are 5 possible options:
```v
struct Foo {
a int // private immutable (default)
mut:
b int // private mutable
c int // (you can list multiple fields with the same access modifier)
pub:
d int // public immutable (readonly)
pub mut:
e int // public, but mutable only in parent module
__global:
// (not recommended to use, that's why the 'global' keyword starts with __)
f int // public and mutable both inside and outside parent module
}
```
Private fields are available only inside the same [module](#modules), any attempt
to directly access them from another module will cause an error during compilation.
Public immutable fields are readonly everywhere.
### Anonymous structs
V supports anonymous structs: structs that don't have to be declared separately
with a struct name.
```v
struct Book {
author struct {
name string
age int
}
title string
}
book := Book{
author: struct {
name: 'Samantha Black'
age: 24
}
}
assert book.author.name == 'Samantha Black'
assert book.author.age == 24
```
### Static type methods
V now supports static type methods like `User.new()`. These are defined on a struct via
`fn [Type name].[function name]` and allow to organize all functions related to a struct:
```v oksyntax
struct User {}
fn User.new() User {
return User{}
}
user := User.new()
```
This is an alternative to factory functions like `fn new_user() User {}` and should be used
instead.
Note, that these are not constructors, but simple functions. V doesn't have constructors or
classes.
### `[noinit]` structs
V supports `[noinit]` structs, which are structs that cannot be initialised outside the module
they are defined in. They are either meant to be used internally or they can be used externally
through _factory functions_.
For an example, consider the following source in a directory `sample`:
```v oksyntax
module sample
[noinit]
pub struct Information {
pub:
data string
}
pub fn new_information(data string) !Information {
if data.len == 0 || data.len > 100 {
return error('data must be between 1 and 100 characters')
}
return Information{
data: data
}
}
```
Note that `new_information` is a _factory_ function. Now when we want to use this struct
outside the module:
```v okfmt
import sample
fn main() {
// This doesn't work when the [noinit] attribute is present:
// info := sample.Information{
// data: 'Sample information.'
// }
// Use this instead:
info := sample.new_information('Sample information.')!
println(info)
}
```
### Methods
```v
struct User {
age int
}
fn (u User) can_register() bool {
return u.age > 16
}
user := User{
age: 10
}
println(user.can_register()) // "false"
user2 := User{
age: 20
}
println(user2.can_register()) // "true"
```
V doesn't have classes, but you can define methods on types.
A method is a function with a special receiver argument.
The receiver appears in its own argument list between the `fn` keyword and the method name.
Methods must be in the same module as the receiver type.
In this example, the `can_register` method has a receiver of type `User` named `u`.
The convention is not to use receiver names like `self` or `this`,
but a short, preferably one letter long, name.
### Embedded structs
V supports embedded structs.
```v
struct Size {
mut:
width int
height int
}
fn (s &Size) area() int {
return s.width * s.height
}
struct Button {
Size
title string
}
```
With embedding, the struct `Button` will automatically have get all the fields and methods from
the struct `Size`, which allows you to do:
```v oksyntax
mut button := Button{
title: 'Click me'
height: 2
}
button.width = 3
assert button.area() == 6
assert button.Size.area() == 6
print(button)
```
output :
```
Button{
Size: Size{
width: 3
height: 2
}
title: 'Click me'
}
```
Unlike inheritance, you cannot type cast between structs and embedded structs
(the embedding struct can also has its own fields, and it can also embed multiple structs).
If you need to access embedded structs directly, use an explicit reference like `button.Size`.
Conceptually, embedded structs are similar to [mixin](https://en.wikipedia.org/wiki/Mixin)s
in OOP, *NOT* base classes.
You can also initialize an embedded struct:
```v oksyntax
mut button := Button{
Size: Size{
width: 3
height: 2
}
}
```
or assign values:
```v oksyntax
button.Size = Size{
width: 4
height: 5
}
```
If multiple embedded structs have methods or fields with the same name, or if methods or fields
with the same name are defined in the struct, you can call methods or assign to variables in
the embedded struct like `button.Size.area()`.
When you do not specify the embedded struct name, the method of the outermost struct will be
targeted.
## Unions
Just like structs, unions support embedding.
```v
struct Rgba32_Component {
r u8
g u8
b u8
a u8
}
union Rgba32 {
Rgba32_Component
value u32
}
clr1 := Rgba32{
value: 0x008811FF
}
clr2 := Rgba32{
Rgba32_Component: Rgba32_Component{
a: 128
}
}
sz := sizeof(Rgba32)
unsafe {
println('Size: ${sz}B,clr1.b: ${clr1.b},clr2.b: ${clr2.b}')
}
```
Output: `Size: 4B, clr1.b: 136, clr2.b: 0`
Union member access must be performed in an `unsafe` block.
> **Note**
> Embedded struct arguments are not necessarily stored in the order listed.
## Functions 2
### Immutable function args by default
In V function arguments are immutable by default, and mutable args have to be
marked on call.
Since there are also no globals, that means that the return values of the functions,
are a function of their arguments only, and their evaluation has no side effects
(unless the function uses I/O).
Function arguments are immutable by default, even when [references](#references) are passed.
> **Note**
> However, V is not a purely functional language.
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 declaring them with the keyword `mut`:
```v
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 just the first argument) is explicitly 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,
e.g. `user = register(user)` (or `user.register()`) instead of `register(mut user)`.
Modifying arguments should only be done in performance-critical parts of your application
to reduce allocations and copying.
For this reason V doesn't allow the modification of arguments with primitive types (e.g. integers).
Only more complex types such as arrays and maps may be modified.
### Variable number of arguments
V supports functions that receive an arbitrary, variable amounts of arguments, denoted with the
`...` prefix.
Below, `a ...int` refers to an arbitrary amount of parameters that will be collected
into an array named `a`.
```v
fn sum(a ...int) int {
mut total := 0
for x in a {
total += x
}
return total
}
println(sum()) // 0
println(sum(1)) // 1
println(sum(2, 3)) // 5
// using array decomposition
a := [2, 3, 4]
println(sum(...a)) // <-- using prefix ... here. output: 9
b := [5, 6, 7]
println(sum(...b)) // output: 18
```
### Anonymous & higher order functions
```v
fn sqr(n int) int {
return n * n
}
fn cube(n int) int {
return n * n * n
}
fn run(value int, op fn (int) int) int {
return op(value)
}
fn main() {
// Functions can be passed to other functions
println(run(5, sqr)) // "25"
// Anonymous functions can be declared inside other functions:
double_fn := fn (n int) int {
return n + n
}
println(run(5, double_fn)) // "10"
// Functions can be passed around without assigning them to variables:
res := run(5, fn (n int) int {
return n + n
})
println(res) // "10"
// You can even have an array/map of functions:
fns := [sqr, cube]
println(fns[0](10)) // "100"
fns_map := {
'sqr': sqr
'cube': cube
}
println(fns_map['cube'](2)) // "8"
}
```
### Closures
V supports closures too.
This means that anonymous functions can inherit variables from the scope they were created in.
They must do so explicitly by listing all variables that are inherited.
```v oksyntax
my_int := 1
my_closure := fn [my_int] () {
println(my_int)
}
my_closure() // prints 1
```
Inherited variables are copied when the anonymous function is created.
This means that if the original variable is modified after the creation of the function,
the modification won't be reflected in the function.
```v oksyntax
mut i := 1
func := fn [i] () int {
return i
}
println(func() == 1) // true
i = 123
println(func() == 1) // still true
```
However, the variable can be modified inside the anonymous function.
The change won't be reflected outside, but will be in the later function calls.
```v oksyntax
fn new_counter() fn () int {
mut i := 0
return fn [mut i] () int {
i++
return i
}
}
c := new_counter()
println(c()) // 1
println(c()) // 2
println(c()) // 3
```
If you need the value to be modified outside the function, use a reference.
```v oksyntax
mut i := 0
mut ref := &i
print_counter := fn [ref] () {
println(*ref)
}
print_counter() // 0
i = 10
print_counter() // 10
```
### Parameter evaluation order
The evaluation order of the parameters of function calls is *NOT* guaranteed.
Take for example the following program:
```v
fn f(a1 int, a2 int, a3 int) {
dump(a1 + a2 + a3)
}
fn main() {
f(dump(100), dump(200), dump(300))
}
```
V currently does not guarantee that it will print 100, 200, 300 in that order.
The only guarantee is that 600 (from the body of `f`) will be printed after all of them.
This *may* change in V 1.0 .
## 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
struct Node[T] {
val T
left &Node[T]
right &Node[T]
}
```
To dereference a reference, use the `*` operator, just like in C.
## 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. You can also declare a single
constant separately:
```v
const e = 2.71828
```
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)
```
\* WIP - for now function calls are evaluated at program start-up
Global variables are not normally allowed, so this can be really useful.
**Modules**
Constants can be made public with `pub const`:
```v oksyntax
module mymodule
pub const golden_ratio = 1.61803
fn calc() {
println(mymodule.golden_ratio)
}
```
The `pub` keyword is only allowed before the `const` keyword and cannot be used inside
a `const ( )` block.
Outside from module main all constants need to be prefixed with the module name.
### Required module prefix
When naming constants, `snake_case` must be used. In order to distinguish consts
from local variables, the full path to consts must be specified. For example,
to access the PI const, full `math.pi` name must be used both outside the `math`
module, and inside it. That restriction is relaxed only for the `main` module
(the one containing your `fn main()`), where you can use the unqualified name of
constants defined there, i.e. `numbers`, rather than `main.numbers`.
vfmt takes care of this rule, so you can type `println(pi)` inside the `math` module,
and vfmt will automatically update it to `println(math.pi)`.
## Builtin functions
Some functions are builtin like `println`. Here is the complete list:
```v ignore
fn print(s string) // prints anything on stdout
fn println(s string) // prints anything and a newline on stdout
fn eprint(s string) // same as print(), but uses stderr
fn eprintln(s string) // same as println(), but uses stderr
fn exit(code int) // terminates the program with a custom error code
fn panic(s string) // prints a message and backtraces on stderr, and terminates the program with error code 1
fn print_backtrace() // prints backtraces on stderr
```
> **Note**
> Although the `print` functions take a string, V accepts other printable types too.
> See below for details.
There is also a special built-in function called [`dump`](#dumping-expressions-at-runtime).
### println
`println` is a simple yet powerful builtin function, that can print anything:
strings, numbers, arrays, maps, structs.
```v
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}"
```
See also [Array methods](#array-methods).
### Printing custom types
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)
```
### Dumping expressions at runtime
You can dump/trace the value of any V expression using `dump(expr)`.
For example, save this code sample as `factorial.v`, then run it with
`v run factorial.v`:
```v
fn factorial(n u32) u32 {
if dump(n <= 1) {
return dump(1)
}
return dump(n * factorial(n - 1))
}
fn main() {
println(factorial(5))
}
```
You will get:
```
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: false
[factorial.v:2] n <= 1: true
[factorial.v:3] 1: 1
[factorial.v:5] n * factorial(n - 1): 2
[factorial.v:5] n * factorial(n - 1): 6
[factorial.v:5] n * factorial(n - 1): 24
[factorial.v:5] n * factorial(n - 1): 120
120
```
Note that `dump(expr)` will trace both the source location,
the expression itself, and the expression value.
## 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'.
See [symbol visibility](#symbol-visibility), [Access modifiers](#access-modifiers).
### Create modules
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!')
}
```
All items inside a module can be used between the files of a module regardless of whether or
not they are prefaced with the `pub` keyword.
```v failcompile
// myfile2.v
module mymodule
pub fn say_hi_and_bye() {
say_hi() // from myfile.v
println('goodbye from mymodule')
}
```
You can now use `mymodule` in your code:
```v failcompile
import mymodule
fn main() {
mymodule.say_hi()
mymodule.say_hi_and_bye()
}
```
* Module names should be short, under 10 characters.
* Module names must use `snake_case`.
* Circular imports are not allowed.
* You can have as many .v files in a module as you want.
* You can create modules anywhere.
* All modules are compiled statically into a single executable.
### `init` functions
If you want a module to automatically call some setup/initialization code when it is imported,
you can use a module `init` function:
```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.
## Type Declarations
### Type aliases
To define a new type `NewType` as an alias for `ExistingType`,
do `type NewType = ExistingType`.