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constants.html
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<a class="link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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<main>
<h1 class="main">Constants</h1>
<div class="language-v">const pi = 3.14
const world = '世界'
println(pi)
println(world)
</div>
<p>Constants are declared with <code>const</code>. 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:</p>
<div class="language-v">const e = 2.71828
</div>
<p>V constants are more flexible than in most languages. You can assign more complex values:</p>
<div class="language-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]
const red = Color{
r: 255
g: 0
b: 0
}
// evaluate function call at compile time*
const blue = rgb(0, 0, 255)
println(numbers)
println(red)
println(blue)
</div>
<p>* WIP - for now function calls are evaluated at program start-up</p>
<p>Global variables are not normally allowed, so this can be really useful.</p>
<p><strong>Modules</strong></p>
<p>Constants can be made public with <code>pub const</code>:</p>
<div class="language-v">module mymodule
pub const golden_ratio = 1.61803
fn calc() {
println(mymodule.golden_ratio)
}
</div>
<p>The <code>pub</code> keyword is only allowed before the <code>const</code> keyword and cannot be used inside
a <code>const ( )</code> block.</p>
<p>Outside from module main all constants need to be prefixed with the module name.</p>
<h2 id="required-module-prefix">Required module prefix <a href="constants.html#required-module-prefix" class="header-anchor" aria-hidden="true">#</a></h2>
<p>When naming constants, <code>snake_case</code> 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 <code>math.pi</code> name must be used both outside the <code>math</code>
module, and inside it. That restriction is relaxed only for the <code>main</code> module
(the one containing your <code>fn main()</code>), where you can use the unqualified name of
constants defined there, i.e. <code>numbers</code>, rather than <code>main.numbers</code>.</p>
<p>vfmt takes care of this rule, so you can type <code>println(pi)</code> inside the <code>math</code> module,
and vfmt will automatically update it to <code>println(math.pi)</code>.</p>
<!--
Many people prefer all caps consts: `TOP_CITIES`. This wouldn't work
well in V, because consts are a lot more powerful than in other languages.
They can represent complex structures, and this is used quite often since there
are no globals:
```v oksyntax
println('Top cities: ${top_cities.filter(.usa)}')
```
-->
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<time datetime="2024-06-04T15:42:48.023Z">Last updated:
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