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<chapter>
  <header>
    <copyright>
      <year>2001</year><year>2010</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      The contents of this file are subject to the Erlang Public License,
      Version 1.1, (the "License"); you may not use this file except in
      compliance with the License. You should have received a copy of the
      Erlang Public License along with this software. If not, it can be
      retrieved online at http://www.erlang.org/.

      Software distributed under the License is distributed on an "AS IS"
      basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
      the License for the specific language governing rights and limitations
      under the License.

    </legalnotice>

    <title>Functions</title>
    <prepared>Bjorn Gustavsson</prepared>
    <docno></docno>
    <date>2007-11-22</date>
    <rev></rev>
    <file>functions.xml</file>
  </header>

  <section>
    <title>Pattern matching</title>
    <p>Pattern matching in function head and in <c>case</c> and <c>receive</c>
     clauses are optimized by the compiler. With a few exceptions, there is nothing
     to gain by rearranging clauses.</p>

    <p>One exception is pattern matching of binaries. The compiler
    will not rearrange clauses that match binaries. Placing the
    clause that matches against the empty binary <em>last</em> will usually
    be slightly faster than placing it <em>first</em>.</p>

    <p>Here is a rather contrived example to show another exception:</p>

    <p><em>DO NOT</em></p>
    <code type="erl">
atom_map1(one) -> 1;
atom_map1(two) -> 2;
atom_map1(three) -> 3;
atom_map1(Int) when is_integer(Int) -> Int;
atom_map1(four) -> 4;
atom_map1(five) -> 5;
atom_map1(six) -> 6.</code>

     <p>The problem is the clause with the variable <c>Int</c>.
     Since a variable can match anything, including the atoms
     <c>four</c>, <c>five</c>, and <c>six</c> that the following clauses
     also will match, the compiler must generate sub-optimal code that will
     execute as follows:</p>

     <p>First the input value is compared to <c>one</c>, <c>two</c>, and
     <c>three</c> (using a single instruction that does a binary search;
     thus, quite efficient even if there are many values) to select which
     one of the first three clauses to execute (if any).</p>

     <p>If none of the first three clauses matched, the fourth clause
     will match since a variable always matches. If the guard test
     <c>is_integer(Int)</c> succeeds, the fourth clause will be
     executed.</p>

     <p>If the guard test failed, the input value is compared to
     <c>four</c>, <c>five</c>, and <c>six</c>, and the appropriate clause
     is selected. (There will be a <c>function_clause</c> exception if
     none of the values matched.)</p>

     <p>Rewriting to either</p>

     <p><em>DO</em></p>
     <code type="erl"><![CDATA[
atom_map2(one) -> 1;
atom_map2(two) -> 2;
atom_map2(three) -> 3;
atom_map2(four) -> 4;
atom_map2(five) -> 5;
atom_map2(six) -> 6;
atom_map2(Int) when is_integer(Int) -> Int.]]></code>

     <p>or</p> 

     <p><em>DO</em></p>
     <code type="erl"><![CDATA[
atom_map3(Int) when is_integer(Int) -> Int;
atom_map3(one) -> 1;
atom_map3(two) -> 2;
atom_map3(three) -> 3;
atom_map3(four) -> 4;
atom_map3(five) -> 5;
atom_map3(six) -> 6.]]></code>

     <p>will give slightly more efficient matching code.</p>

     <p>Here is a less contrived example:</p>

     <p><em>DO NOT</em></p>
     <code type="erl"><![CDATA[
map_pairs1(_Map, [], Ys) ->
    Ys;
map_pairs1(_Map, Xs, [] ) ->
    Xs;
map_pairs1(Map, [X|Xs], [Y|Ys]) ->
    [Map(X, Y)|map_pairs1(Map, Xs, Ys)].]]></code>

     <p>The first argument is <em>not</em> a problem. It is variable, but it
     is a variable in all clauses. The problem is the variable in the second
     argument, <c>Xs</c>, in the middle clause. Because the variable can
     match anything, the compiler is not allowed to rearrange the clauses,
     but must generate code that matches them in the order written.</p>

     <p>If the function is rewritten like this</p>

     <p><em>DO</em></p>
     <code type="erl"><![CDATA[
map_pairs2(_Map, [], Ys) ->
    Ys;
map_pairs2(_Map, [_|_]=Xs, [] ) ->
    Xs;
map_pairs2(Map, [X|Xs], [Y|Ys]) ->
    [Map(X, Y)|map_pairs2(Map, Xs, Ys)].]]></code>

    <p>the compiler is free rearrange the clauses. It will generate code
    similar to this</p>

    <p><em>DO NOT (already done by the compiler)</em></p>
    <code type="erl"><![CDATA[
explicit_map_pairs(Map, Xs0, Ys0) ->
    case Xs0 of
	[X|Xs] ->
	    case Ys0 of
		[Y|Ys] ->
		    [Map(X, Y)|explicit_map_pairs(Map, Xs, Ys)];
		[] ->
		    Xs0
	    end;
	[] ->
	    Ys0
    end.]]></code>
      
    <p>which should be slightly faster for presumably the most common case
    that the input lists are not empty or very short.
    (Another advantage is that Dialyzer is able to deduce a better type
    for the variable <c>Xs</c>.)</p>
  </section>

  <section>
    <title>Function Calls </title>

    <p>Here is an intentionally rough guide to the relative costs of
    different kinds of calls. It is based on benchmark figures run on
    Solaris/Sparc:</p>

    <list type="bulleted">
    <item>Calls to local or external functions (<c>foo()</c>, <c>m:foo()</c>)
    are the fastest kind of calls.</item>
    <item>Calling or applying a fun (<c>Fun()</c>, <c>apply(Fun, [])</c>)
    is about <em>three times</em> as expensive as calling a local function.</item>
    <item>Applying an exported function (<c>Mod:Name()</c>,
    <c>apply(Mod, Name, [])</c>) is about twice as expensive as calling a fun,
    or about <em>six times</em> as expensive as calling a local function.</item>
    </list>

    <section>
       <title>Notes and implementation details</title>

       <p>Calling and applying a fun does not involve any hash-table lookup.
       A fun contains an (indirect) pointer to the function that implements
       the fun.</p>

       <warning><p><em>Tuples are not fun(s)</em>.
       A "tuple fun", <c>{Module,Function}</c>, is not a fun.
       The cost for calling a "tuple fun" is similar to that
       of <c>apply/3</c> or worse. Using "tuple funs" is <em>strongly discouraged</em>,
       as they may not be supported in a future release,
       and because there exists a superior alternative since the R10B
       release, namely the <c>fun Module:Function/Arity</c> syntax.</p></warning>

       <p><c>apply/3</c> must look up the code for the function to execute
       in a hash table. Therefore, it will always be slower than a
       direct call or a fun call.</p>

       <p>It no longer matters (from a performance point of view)
       whether you write</p>

       <code type="erl">
Module:Function(Arg1, Arg2)</code>

       <p>or</p>

       <code type="erl">
apply(Module, Function, [Arg1,Arg2])</code>

       <p>(The compiler internally rewrites the latter code into the former.)</p>

       <p>The following code</p>

       <code type="erl">
apply(Module, Function, Arguments)</code>

       <p>is slightly slower because the shape of the list of arguments
       is not known at compile time.</p>
    </section>
  </section>

  <section>
    <title>Memory usage in recursion</title>
    <p>When writing recursive functions it is preferable to make them
      tail-recursive so that they can execute in constant memory space.</p>
    <p><em>DO</em></p>
    <code type="none">
list_length(List) ->
    list_length(List, 0).

list_length([], AccLen) -> 
    AccLen; % Base case

list_length([_|Tail], AccLen) ->
    list_length(Tail, AccLen + 1). % Tail-recursive</code>
    <p><em>DO NOT</em></p>
    <code type="none">
list_length([]) ->
    0. % Base case
list_length([_ | Tail]) ->
    list_length(Tail) + 1. % Not tail-recursive</code>
  </section>

</chapter>

