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<h1>5 Functions</h1>
<h3><a name="id74609">5.1
Function Declaration Syntax</a></h3>
<a name="syntax"></a>
<p>A <strong>function declaration</strong> is a sequence of function
clauses separated by semicolons, and terminated by period (.).</p>
<p>A <strong>function clause</strong> consists of a clause head and a
clause body, separated by <span class="code">-></span>.</p>
<p>A clause <strong>head</strong> consists of the function name, an
argument list, and an optional guard sequence
beginning with the keyword <span class="code">when</span>.</p>
<div class="example"><pre>
Name(Pattern11,...,Pattern1N) [when GuardSeq1] ->
Body1;
...;
Name(PatternK1,...,PatternKN) [when GuardSeqK] ->
BodyK.</pre></div>
<p>The function name is an atom. Each argument is a pattern.</p>
<p>The number of arguments <span class="code">N</span> is the <strong>arity</strong> of
the function. A function is uniquely defined by the module name,
function name and arity. That is, two functions with the same
name and in the same module, but with different arities are two
completely different functions.</p>
<p>A function named <span class="code">f</span> in the module <span class="code">m</span> and with arity
<span class="code">N</span> is often denoted as <span class="code">m:f/N</span>.</p>
<p>A clause <strong>body</strong> consists of a sequence of expressions
separated by comma (,):</p>
<div class="example"><pre>
Expr1,
...,
ExprN</pre></div>
<p>Valid Erlang expressions and guard sequences are described in
<span class="bold_code"><a href="expressions.html">Erlang Expressions</a></span>.</p>
<p>Example:</p>
<div class="example"><pre>
fact(N) when N>0 -> % first clause head
N * fact(N-1); % first clause body
fact(0) -> % second clause head
1. % second clause body</pre></div>
<h3><a name="id74724">5.2
Function Evaluation</a></h3>
<a name="eval"></a>
<p>When a function <span class="code">m:f/N</span> is called, first the code for
the function is located. If the function cannot be found, an
<span class="code">undef</span> run-time error will occur. Note that the function
must be exported to be visible outside the module it is defined
in.</p>
<p>If the function is found, the function clauses are scanned
sequentially until a clause is found that fulfills the following
two conditions:</p>
<ul>
<li>the patterns in the clause head can be successfully
matched against the given arguments, and</li>
<li>the guard sequence, if any, is true.</li>
</ul>
<p>If such a clause cannot be found, a <span class="code">function_clause</span>
run-time error will occur.</p>
<p>If such a clause is found, the corresponding clause body is
evaluated. That is, the expressions in the body are evaluated
sequentially and the value of the last expression is returned.</p>
<p>Example: Consider the function <span class="code">fact</span>:</p>
<div class="example"><pre>
-module(m).
-export([fact/1]).
fact(N) when N>0 ->
N * fact(N-1);
fact(0) ->
1.</pre></div>
<p>Assume we want to calculate factorial for 1:</p>
<div class="example"><pre>
1> <span class="bold_code">m:fact(1).</span></pre></div>
<p>Evaluation starts at the first clause. The pattern <span class="code">N</span> is
matched against the argument 1. The matching succeeds and
the guard (<span class="code">N>0</span>) is true, thus <span class="code">N</span> is bound to 1 and
the corresponding body is evaluated:</p>
<div class="example"><pre>
<span class="bold_code">N * fact(N-1)</span> => (N is bound to 1)
<span class="bold_code">1 * fact(0)</span></pre></div>
<p>Now <span class="code">fact(0)</span> is called and the function clauses are
scanned sequentially again. First, the pattern <span class="code">N</span> is
matched against 0. The matching succeeds, but the guard
(<span class="code">N>0</span>) is false. Second, the pattern 0 is matched against
0. The matching succeeds and the body is evaluated:</p>
<div class="example"><pre>
<span class="bold_code">1 * fact(0)</span> =>
<span class="bold_code">1 * 1</span> =>
<span class="bold_code">1</span></pre></div>
<p>Evaluation has succeed and <span class="code">m:fact(1)</span> returns 1.</p>
<p>If <span class="code">m:fact/1</span> is called with a negative number as
argument, no clause head will match. A <span class="code">function_clause</span>
run-time error will occur.</p>
<h3><a name="id74875">5.3
Tail recursion</a></h3>
<p>If the last expression of a function body is a function call,
a <strong>tail recursive</strong> call is done so that no system
resources for example call stack are consumed. This means
that an infinite loop can be done if it uses tail recursive
calls.</p>
<p>Example:</p>
<div class="example"><pre>
loop(N) ->
io:format("~w~n", [N]),
loop(N+1).</pre></div>
<p>As a counter-example see the factorial example above
that is not tail recursive since a multiplication is done
on the result of the recursive call to <span class="code">fact(N-1)</span>.</p>
<h3><a name="id74908">5.4
Built-In Functions, BIFs</a></h3>
<p><strong>Built-in functions</strong>, BIFs, are implemented in C code in
the runtime system and do things that are difficult or impossible
to implement in Erlang. Most of the built-in functions belong
to the module <span class="code">erlang</span> but there are also built-in functions
belonging to a few other modules, for example <span class="code">lists</span> and
<span class="code">ets</span>.</p>
<p>The most commonly used BIFs belonging to <span class="code">erlang</span> are
<strong>auto-imported</strong>, they do not need to be prefixed with
the module name. Which BIFs are auto-imported is specified in
<span class="code">erlang(3)</span>. For example, standard type conversion BIFs like
<span class="code">atom_to_list</span> and BIFs allowed in guards can be called
without specifying the module name. Examples:</p>
<div class="example"><pre>
1> <span class="bold_code">tuple_size({a,b,c}).</span>
3
2> <span class="bold_code">atom_to_list('Erlang').</span>
"Erlang"</pre></div>
<p>Note that normally it is the set of auto-imported built-in
functions that is referred to when talking about 'BIFs'.</p>
</div>
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