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conditional-compilation.html
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<a class="nav-entry-link" href="appendix-i-keywords.html">Appendix I: Keywords</a>
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<a class="nav-entry-link" href="appendix-ii-operators.html">Appendix II: Operators</a>
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<a class="nav-entry-link" href="other-online-resources.html">Other online resources</a>
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<a class="nav-entry-link" href="other-online-resources.html#v-contributing-guide"> V contributing guide</a>
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<a class="nav-entry-link" href="other-online-resources.html#awesome-v"> Awesome V</a>
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<a class="nav-entry-link" href="other-online-resources.html#the-v-language-discord"> The V language Discord</a>
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<article>
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<main class="v-doc">
<h1 class="main">Conditional compilation</h1>
<p>The goal of this feature, is to tell V to <em>not compile</em> a function, and all its calls, in the final
executable, if a provided custom flag is not passed.</p>
<p>V will still type check the function and all its calls, <em>even</em> if they will not be present in the
final executable, due to the passed -d flags.</p>
<p>In order to see it in action, run the following example with <code>v run example.v</code> once,
and then a second time with <code>v -d trace_logs example.v</code>:</p>
<div class="language-v">@[if trace_logs ?]
fn elog(s string) {
eprintln(s)
}
fn main() {
elog('some expression: ${2 + 2}') // such calls will not be done *at all*, if `-d trace_logs` is not passed
println('hi')
elog('finish')
}
</div>
<p>Conditional compilation, based on custom flags, can also be used to produce slightly different
executables, which share the majority of the same code, but where some of the logic, is needed
only some of the time, for example a network server/client program can be written like so:</p>
<div class="language-v">fn act_as_client() { ... }
fn act_as_server() { ... }
fn main() {
$if as_client ? {
act_as_client()
}
$if as_server ? {
act_as_server()
}
}
</div>
<p>To generate a <code>client.exe</code> executable do: <code>v -d as_client -o client.exe .</code>
To generate a <code>server.exe</code> executable do: <code>v -d as_server -o server.exe .</code></p>
<h2 id="compile-time-pseudo-variables">Compile time pseudo variables <a href="conditional-compilation.html#compile-time-pseudo-variables" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V also gives your code access to a set of pseudo string variables,
that are substituted at compile time:</p>
<ul>
<li><code>@FN</code> => replaced with the name of the current V function.</li>
<li><code>@METHOD</code> => replaced with ReceiverType.MethodName.</li>
<li><code>@MOD</code> => replaced with the name of the current V module.</li>
<li><code>@STRUCT</code> => replaced with the name of the current V struct.</li>
<li><code>@FILE</code> => replaced with the absolute path of the V source file.</li>
<li><code>@LINE</code> => replaced with the V line number where it appears (as a string).</li>
<li><code>@FILE_LINE</code> => like <code>@FILE:@LINE</code>, but the file part is a relative path.</li>
<li><code>@LOCATION</code> => file, line and name of the current type + method; suitable for logging.</li>
<li><code>@COLUMN</code> => replaced with the column where it appears (as a string).</li>
<li><code>@VEXE</code> => replaced with the path to the V compiler.</li>
<li><code>@VEXEROOT</code> => will be substituted with the <em>folder</em>,
where the V executable is (as a string).</li>
<li><code>@VHASH</code> => replaced with the shortened commit hash of the V compiler (as a string).</li>
<li><code>@VCURRENTHASH</code> => Similar to <code>@VHASH</code>, but changes when the compiler is
recompiled on a different commit (after local modifications, or after
using git bisect etc).</li>
<li><code>@VMOD_FILE</code> => replaced with the contents of the nearest v.mod file (as a string).</li>
<li><code>@VMODHASH</code> => is replaced by the shortened commit hash, derived from the .git directory
next to the nearest v.mod file (as a string).</li>
<li><code>@VMODROOT</code> => will be substituted with the <em>folder</em>,
where the nearest v.mod file is (as a string).</li>
<li><code>@BUILD_DATE</code> => replaced with the build date, for example '2024-09-13' .</li>
<li><code>@BUILD_TIME</code> => replaced with the build time, for example '12:32:07' .</li>
<li><code>@BUILD_TIMESTAMP</code> => replaced with the build timestamp, for example '1726219885' .
Note: <code>@BUILD_DATE</code>, <code>@BUILD_TIME</code>, <code>@BUILD_TIMESTAMP</code> represent times in the UTC timezone.
By default, they are based on the current time of the compilation/build. They can be overriden
by setting the environment variable <code>SOURCE_DATE_EPOCH</code>. That is also useful while making
releases, since you can use the equivalent of this in your build system/script:
<code>export SOURCE_DATE_EPOCH=$(git log -1 --pretty=%ct) ;</code> , and then use <code>@BUILD_DATE</code> etc.,
inside your program, when you for example print your version information to users.
See also <a href="https://reproducible-builds.org/docs/source-date-epoch/">https://reproducible-builds.org/docs/source-date-epoch/</a> .</li>
</ul>
<p>The compile time pseudo variables allow you to do the following
example, which is useful while debugging/logging/tracing your code:</p>
<div class="language-v">eprintln(@LOCATION)
</div>
<p>Another example, is if you want to embed the version/name from v.mod <em>inside</em> your executable:</p>
<div class="language-v">import v.vmod
vm := vmod.decode( @VMOD_FILE ) or { panic(err) }
eprintln('${vm.name} ${vm.version}\n ${vm.description}')
</div>
<p>A program that prints its own source code (a quine):</p>
<div class="language-v">print($embed_file(@FILE).to_string())
</div>
<p>A program that prints the time when it was built:</p>
<div class="language-v">import time
println('This program, was compiled at ${time.unix(@BUILD_TIMESTAMP.i64()).format_ss_milli()} .')
</div>
<blockquote class="tip">
<p>[!NOTE]
you can have arbitrary source code in the file, without problems, since the full file
will be embedded into the executable, produced by compiling it. Also note that printing
is done with <code>print</code> and not <code>println</code>, to not add another new line, missing in the
source code.</p>
</blockquote>
<h2 id="compile-time-reflection">Compile time reflection <a href="conditional-compilation.html#compile-time-reflection" class="header-anchor" aria-hidden="true">#</a></h2>
<p><code>$</code> is used as a prefix for compile time (also referred to as 'comptime') operations.</p>
<p>Having built-in JSON support is nice, but V also allows you to create efficient
serializers for any data format. V has compile time <code>if</code> and <code>for</code> constructs:</p>
<h3 id=".fields"><h4 id="comptime-fields">.fields</h4> <a href="#.fields" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can iterate over struct fields using <code>.fields</code>, it also works with generic types
(e.g. <code>T.fields</code>) and generic arguments (e.g. <code>param.fields</code> where <code>fn gen[T](param T) {</code>).</p>
<div class="language-v">struct User {
name string
age int
}
fn main() {
$for field in User.fields {
$if field.typ is string {
println('${field.name} is of type string')
}
}
}
// Output:
// name is of type string
</div>
<h3 id=".values"><h4 id="comptime-values">.values</h4> <a href="#.values" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can read <a href="type-declarations.html#enums">Enum</a> values and their attributes.</p>
<div class="language-v">enum Color {
red @[RED] // first attribute
blue @[BLUE] // second attribute
}
fn main() {
$for e in Color.values {
println(e.name)
println(e.attrs)
}
}
// Output:
// red
// ['RED']
// blue
// ['BLUE']
</div>
<h3 id=".attributes"><h4 id="comptime-attrs">.attributes</h4> <a href="#.attributes" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can read <a href="structs.html">Struct</a> attributes.</p>
<div class="language-v">@[COLOR]
struct Foo {
a int
}
fn main() {
$for e in Foo.attributes {
println(e)
}
}
// Output:
// StructAttribute{
// name: 'COLOR'
// has_arg: false
// arg: ''
// kind: plain
// }
</div>
<h3 id=".variants"><h4 id="comptime-variants">.variants</h4> <a href="#.variants" class="header-anchor" aria-hidden="true">#</a></h3>
<p>You can read variant types from <a href="type-declarations.html#sum-types">Sum type</a>.</p>
<div class="language-v">type MySum = int | string
fn main() {
$for v in MySum.variants {
$if v.typ is int {
println('has int type')
} $else $if v.typ is string {
println('has string type')
}
}
}
// Output: