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<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 run 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>@DIR</code> => replaced with the absolute path of the <em>folder</em>, where the V source file is.</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 module + function or 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>