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builtin-functions.html
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<h1 class="main">Builtin functions</h1>
<p>Some functions are builtin like <code>println</code>. Here is the complete list:</p>
<div class="language-v">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
</div>
<blockquote class="tip">
<p>[!NOTE]
Although the <code>print</code> functions take a string, V accepts other printable types too.
See below for details.</p>
</blockquote>
<p>There is also a special built-in function called <a href="builtin-functions.html#dumping-expressions-at-runtime"><code>dump</code></a>.</p>
<h2 id="println">println <a href="builtin-functions.html#println" class="header-anchor" aria-hidden="true">#</a></h2>
<p><code>println</code> is a simple yet powerful builtin function, that can print anything:
strings, numbers, arrays, maps, structs.</p>
<div class="language-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}"
</div>
<p>See also <a href="v-types.html#string-interpolation">String interpolation</a>.</p>
<p><a id='custom-print-of-types'></a></p>
<h2 id="printing-custom-types">Printing custom types <a href="builtin-functions.html#printing-custom-types" class="header-anchor" aria-hidden="true">#</a></h2>
<p>If you want to define a custom print value for your type, simply define a
<code>str() string</code> method:</p>
<div class="language-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)
</div>
<h2 id="dumping-expressions-at-runtime">Dumping expressions at runtime <a href="builtin-functions.html#dumping-expressions-at-runtime" class="header-anchor" aria-hidden="true">#</a></h2>
<p>You can dump/trace the value of any V expression using <code>dump(expr)</code>.
For example, save this code sample as <code>factorial.v</code>, then run it with
<code>v run factorial.v</code>:</p>
<div class="language-v">fn factorial(n u32) u32 {
if dump(n <= 1) {
return dump(1)
}
return dump(n * factorial(n - 1))
}
fn main() {
println(factorial(5))
}
</div>
<p>You will get:</p>
<pre><code>[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
</code></pre>
<p>Note that <code>dump(expr)</code> will trace both the source location,
the expression itself, and the expression value.</p>
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