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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2015</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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    <title>Functions</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>functions.xml</file>
  </header>

  <section>
    <marker id="syntax"></marker>
    <title>Function Declaration Syntax</title>
    <p>A <em>function declaration</em> is a sequence of function
      clauses separated by semicolons, and terminated by period (.).</p>
    <p>A <em>function clause</em> consists of a clause head and a
      clause body, separated by <c>-></c>.</p>
    <p>A clause <em>head</em> consists of the function name, an
      argument list, and an optional guard sequence
      beginning with the keyword <c>when</c>:</p>
    <pre>
Name(Pattern11,...,Pattern1N) [when GuardSeq1] ->
    Body1;
...;
Name(PatternK1,...,PatternKN) [when GuardSeqK] ->
    BodyK.</pre>
    <p>The function name is an atom. Each argument is a pattern.</p>
    <p>The number of arguments <c>N</c> is the <em>arity</em> 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
      different functions.</p>
    <p>A function named <c>f</c> in the module <c>m</c> and with arity
      <c>N</c> is often denoted as <c>m:f/N</c>.</p>
    <p>A clause <em>body</em> consists of a sequence of expressions
      separated by comma (,):</p>
    <pre>
Expr1,
...,
ExprN</pre>
    <p>Valid Erlang expressions and guard sequences are described in
      <seealso marker="expressions">Expressions</seealso>.</p>
    <p><em>Example:</em></p>
    <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>
  </section>

  <section>
    <marker id="eval"></marker>
    <title>Function Evaluation</title>
    <p>When a function <c>m:f/N</c> is called, first the code for
      the function is located. If the function cannot be found, an
      <c>undef</c> runtime error occurs. Notice 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 both of
      the following two conditions:</p>
    <list type="ordered">
      <item>The patterns in the clause head can be successfully
       matched against the given arguments.</item>
      <item>The guard sequence, if any, is true.</item>
    </list>
    <p>If such a clause cannot be found, a <c>function_clause</c>
      runtime error occurs.</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>Consider the function <c>fact</c>:</p>
    <pre>
-module(m).
-export([fact/1]).

fact(N) when N>0 ->
    N * fact(N-1);
fact(0) ->
    1.</pre>
    <p>Assume that you want to calculate the factorial for 1:</p>
    <pre>
1> <input>m:fact(1).</input></pre>
    <p>Evaluation starts at the first clause. The pattern <c>N</c> is
      matched against argument 1. The matching succeeds and
      the guard (<c>N>0</c>) is true, thus <c>N</c> is bound to 1, and
      the corresponding body is evaluated:</p>
    <pre>
<input>N * fact(N-1)</input> => (N is bound to 1)
<input>1 * fact(0)</input></pre>
    <p>Now, <c>fact(0)</c> is called, and the function clauses are
      scanned sequentially again. First, the pattern <c>N</c> is
      matched against 0. The matching succeeds, but the guard
      (<c>N>0</c>) is false. Second, the pattern 0 is matched against
      0. The matching succeeds and the body is evaluated:</p>
    <pre>
<input>1 * fact(0)</input> =>
<input>1 * 1</input> =>
<input>1</input></pre>
    <p>Evaluation has succeed and <c>m:fact(1)</c> returns 1.</p>
    <p>If <c>m:fact/1</c> is called with a negative number as
      argument, no clause head matches. A <c>function_clause</c>
      runtime error occurs.</p>
  </section>

  <section>
    <title>Tail recursion</title>
    <p>If the last expression of a function body is a function call,
      a <em>tail recursive</em> call is done.
      This is to ensure 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><em>Example:</em></p>
    <pre>
loop(N) ->
    io:format("~w~n", [N]),
    loop(N+1).</pre>
    <p>The earlier factorial example can act as a counter-example.
      It is not tail-recursive, since a multiplication is done
      on the result of the recursive call to <c>fact(N-1)</c>.</p>
  </section>

  <section>
    <title>Built-In Functions (BIFs)</title>
    <p>BIFs are implemented in C code in
      the runtime system. BIFs do things that are difficult or impossible
      to implement in Erlang. Most of the BIFs belong
      to the module <c>erlang</c> but there are also BIFs
      belonging to a few other modules, for example <c>lists</c> and
      <c>ets</c>.</p>
    <p>The most commonly used BIFs belonging to <c>erlang(3)</c> are
      <em>auto-imported</em>. They do not need to be prefixed with
      the module name. Which BIFs that are auto-imported is specified in the
      <seealso marker="erts:erlang">erlang(3)</seealso> module in ERTS.
      For example, standard-type conversion BIFs like
      <c>atom_to_list</c> and BIFs allowed in guards can be called
      without specifying the module name.</p>
      <p><em>Examples:</em></p>
    <pre>
1> <input>tuple_size({a,b,c}).</input>
3
2> <input>atom_to_list('Erlang').</input>
"Erlang"</pre>
    <p>Notice that it is normally the set of auto-imported BIFs
      that are referred to when talking about 'BIFs'.</p>
  </section>
</chapter>

