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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.
An array returned from an immutable argument remains immutable, including when returned through
a local function value, a narrowed <code>if</code> or <code>match</code> branch, or after an exiting <code>if</code> guard.
Use <code>.clone()</code> for a mutable copy.</p>
<p>A cloned map has separate entry storage. After replacing a copied struct's map field with
a clone, replacing entries leaves the original map unchanged. The compiler may reject such
writes if a reference to the containing struct could rebind the field, including references
taken before the clone. A callback that may rebind a map also prevents subsequent entry writes
from being treated as independent of the original; a callback that shadows the enclosing
function's name still counts as a callback.</p>
<p>A pointer returned through a callback can still refer to an immutable argument, even if the
callback returns <code>voidptr</code>. Converting that result to a typed reference does not make the
underlying object mutable. Different pointee types do not prove separate storage: a <code>voidptr</code>
can erase the type of an existing reference. For an opaque container lookup that guarantees
separate mutable component storage, place the conversion in <code>unsafe { ... }</code>. The caller must
ensure the returned pointer does not provide mutable access to an immutable argument.</p>
<p>When a readable function returns a stored pointer to separate storage, or newly allocated storage
that contains no references or other shared storage from its arguments, the returned reference does
not borrow the containing object or the function's other arguments. A new outer object can still
borrow an argument through a reference-bearing field, such as <code>&Box{item: item}</code>.</p>
<p>A scalar passed by value to a callback is independent of the caller's storage. If a callback
parameter is a reference, an implicitly referenced scalar remains borrowed from its immutable
argument; the scalar's expression type alone does not establish a by-value copy.
Scalar fields supplied with collapsed struct argument syntax and scalar elements decomposed
into by-value parameters are copied too. Pointer fields and elements can still share storage.</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>
<p>A method with a <code>mut</code> receiver requires a mutable value receiver even when it only
changes private fields in another module. An immutable value parameter cannot call
such a method: mutations would affect its copy and be lost when the function returns.
Declare the parameter or receiver with <code>mut</code> when its changes must reach the caller.</p>
<p>An explicit <code>mut param &T</code> takes mutable pointer storage. Passing <code>mut &value</code>
creates temporary pointer storage for that call, including when <code>value</code> is a
<code>mut value T</code> parameter. Changes to fields reach <code>value</code>; rebinding the temporary
pointer does not rebind the caller's value.</p>
<div class="language-v">struct User {
name string
mut:
is_registered bool
}
fn (mut u User) register() {