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<h1 class="main">Functions 2</h1>
<h2 id="immutable-function-args-by-default">Immutable function args by default <a href="functions-2.html#immutable-function-args-by-default" class="header-anchor" aria-hidden="true">#</a></h2>
<p>In V function arguments are immutable by default, and mutable args have to be
marked on call.</p>
<p>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).</p>
<p>Function arguments are immutable by default, even when <a href="references.html">references</a> are passed.</p>
<blockquote class="tip">
<p>[!NOTE]
However, V is not a purely functional language.</p>
</blockquote>
<p>There is a compiler flag to enable global variables (<code>-enable-globals</code>), but this is
intended for low-level applications like kernels and drivers.</p>
<h2 id="mutable-arguments">Mutable arguments <a href="functions-2.html#mutable-arguments" class="header-anchor" aria-hidden="true">#</a></h2>
<p>It is possible to modify function arguments by declaring them with the keyword <code>mut</code>:</p>
<div class="language-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"
</div>
<p>In this example, the receiver (which is just the first argument) is explicitly marked as mutable,
so <code>register()</code> can change the user object. The same works with non-receiver arguments:</p>
<div class="language-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]"
</div>
<p>Note that you have to add <code>mut</code> before <code>nums</code> when calling this function. This makes
it clear that the function being called will modify the value.</p>
<p>It is preferable to return values instead of modifying arguments,
e.g. <code>user = register(user)</code> (or <code>user.register()</code>) instead of <code>register(mut user)</code>.
Modifying arguments should only be done in performance-critical parts of your application
to reduce allocations and copying.</p>
<p>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.</p>
<h2 id="variable-number-of-arguments">Variable number of arguments <a href="functions-2.html#variable-number-of-arguments" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V supports functions that receive an arbitrary, variable amounts of arguments, denoted with the
<code>...</code> prefix.
Below, <code>a ...int</code> refers to an arbitrary amount of parameters that will be collected
into an array named <code>a</code>.</p>
<div class="language-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
</div>
<h2 id="anonymous-&-higher-order-functions">Anonymous & higher order functions <a href="#anonymous-&-higher-order-functions" class="header-anchor" aria-hidden="true">#</a></h2>
<div class="language-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"
}
</div>
<h2 id="lambda-expressions">Lambda expressions <a href="functions-2.html#lambda-expressions" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Lambda expressions in V are small anonymous functions, defined using
the <code>|variables| expression</code> syntax. Note: this syntax is valid only inside calls to higher
order functions.</p>
<p>Here are some examples:</p>
<div class="language-v">mut a := [1, 2, 3]
a.sort(|x, y| x > y) // sorts the array, defining the comparator with a lambda expression
println(a.map(|x| x * 10)) // prints [30, 20, 10]
</div>
<div class="language-v">// Lambda function can be used as callback
fn f(cb fn (a int) int) int {
return cb(10)