path: root/system/doc/reference_manual/macros.xml

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<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2009</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>The Preprocessor</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
    <file>macros.xml</file>
  </header>

    <section>
      <title>File Inclusion</title>
      <p>A file can be included in the following way:</p>
      <pre>
-include(File).
-include_lib(File).</pre>
      <p><c>File</c>, a string, should point out a file. The contents of
        this file are included as-is, at the position of the directive.</p>
      <p>Include files are typically used for record and macro
        definitions that are shared by several modules. It is
        recommended that the file name extension <c>.hrl</c> be used
        for include files.</p>
      <p><c>File</c> may start with a path component <c>$VAR</c>, for
        some string <c>VAR</c>. If that is the case, the value of
        the environment variable <c>VAR</c> as returned by
        <c>os:getenv(VAR)</c> is substituted for <c>$VAR</c>. If
        <c>os:getenv(VAR)</c> returns <c>false</c>, <c>$VAR</c> is left
        as is.</p>
      <p>If the filename <c>File</c> is absolute (possibly after
        variable substitution), the include file with that name is
        included. Otherwise, the specified file is searched for in
        the current working directory, in the same directory as
        the module being compiled, and in the directories given by
        the <c>include</c> option, in that order.
        See <c>erlc(1)</c> and <c>compile(3)</c> for details.</p>
      <p>Examples:</p>
      <pre>
-include("my_records.hrl").
-include("incdir/my_records.hrl").
-include("/home/user/proj/my_records.hrl").
-include("$PROJ_ROOT/my_records.hrl").</pre>
      <p><c>include_lib</c> is similar to <c>include</c>, but should not
        point out an absolute file. Instead, the first path component
        (possibly after variable substitution) is assumed to be
        the name of an application. Example:</p>
      <pre>
-include_lib("kernel/include/file.hrl").</pre>
      <p>The code server uses <c>code:lib_dir(kernel)</c> to find
        the directory of the current (latest) version of Kernel, and
        then the subdirectory <c>include</c> is searched for the file
        <c>file.hrl</c>.</p>
    </section>

  <section>
    <title>Defining and Using Macros</title>
    <p>A macro is defined the following way:</p>
    <code type="none">
-define(Const, Replacement).
-define(Func(Var1,...,VarN), Replacement).</code>
    <p>A macro definition can be placed anywhere among the attributes
      and function declarations of a module, but the definition must
      come before any usage of the macro.</p>
    <p>If a macro is used in several modules, it is recommended that
      the macro definition is placed in an include file.</p>
    <p>A macro is used the following way:</p>
    <code type="none">
?Const
?Func(Arg1,...,ArgN)</code>
    <p>Macros are expanded during compilation. A simple macro
      <c>?Const</c> will be replaced with <c>Replacement</c>.
      Example:</p>
    <code type="none">
-define(TIMEOUT, 200).
...
call(Request) ->
    server:call(refserver, Request, ?TIMEOUT).</code>
    <p>This will be expanded to:</p>
    <code type="none">
call(Request) ->
    server:call(refserver, Request, 200).</code>
    <p>A macro <c>?Func(Arg1,...,ArgN)</c> will be replaced with
      <c>Replacement</c>, where all occurrences of a variable <c>Var</c>
      from the macro definition are replaced with the corresponding
      argument <c>Arg</c>. Example:</p>
    <code type="none">
-define(MACRO1(X, Y), {a, X, b, Y}).
...
bar(X) ->
    ?MACRO1(a, b),
    ?MACRO1(X, 123)</code>
    <p>This will be expanded to:</p>
    <code type="none">
bar(X) ->
    {a,a,b,b},
    {a,X,b,123}.</code>
    <p>It is good programming practice, but not mandatory, to ensure
      that a macro definition is a valid Erlang syntactic form.</p>
    <p>To view the result of macro expansion, a module can be compiled
      with the <c>'P'</c> option. <c>compile:file(File, ['P'])</c>.
      This produces a listing of the parsed code after preprocessing
      and parse transforms, in the file <c>File.P</c>.</p>
  </section>

  <section>
    <title>Predefined Macros</title>
    <p>The following macros are predefined:</p>
    <taglist>
      <tag><c>?MODULE</c></tag>
      <item>The name of the current module.</item>
      <tag><c>?MODULE_STRING</c>.</tag>
      <item>The name of the current module, as a string.</item>
      <tag><c>?FILE</c>.</tag>
      <item>The file name of the current module.</item>
      <tag><c>?LINE</c>.</tag>
      <item>The current line number.</item>
      <tag><c>?MACHINE</c>.</tag>
      <item>The machine name, <c>'BEAM'</c>.</item>
    </taglist>
  </section>

  <section>
    <title>Macros Overloading</title>
    <warning>
      <p>This section describes a feature introduced in R13B04. It remains fully
      compatible with old code but previous versions of erlang does not support
      the macros overloading.</p>
    </warning>
    <p>It is possible to have more than one definition of the same macro, except
    for predefined macros. Only user-defined macros are considered here. In
    order to overload a macro, a different signature should be used for each
    overloaded version. A macro signature consists of its name and its number of
    arguments.</p>

    <taglist>
      <tag>Object-like macro has no arguments</tag>
      <item><code type="none">
-define(M, io:format("object-like macro")).</code>
      </item>
      <tag>Function-like macro has 0 or more arguments.</tag>
      <item><code type="none">
-define(M(),     io:format("function-like macro with 0 argument")).
-define(M(X),    io:format("function-like macro with 1 argument: ~p", [X])).
-define(M(X, Y), io:format("function-like macro with 2 arguments: (~p, ~p)", [X,Y])).</code>
      </item>
    </taglist>
    <p>An object-like macro should not be confused with a function-like macro
    with 0 argument.</p>

    <p>When a macro is used, the preprocessor selects the proper macro following
      these rules: </p>
    <list type="bulleted">
      <item>if there is only the object-like definition, the preprocessor uses
      it. Example:</item>
<code type="none">
-define(FUN, now).
...
call() ->
    {?FUN, ?FUN()}.</code>
This will be expended to:
<code type="none">
call() ->
    {now, now()}.</code>

      <item>if the preprocessor recognizes an object-like macro call, and if its
      definition exists, the preprocessor uses it. Example:</item>
<code type="none">
-define(MOD,    erlang).
-define(MOD(X), X).
...
call() ->
    ?MOD:module_info().</code>
This will be expended to:
<code type="none">
call() ->
    erlang:module_info().</code>

      <item>if the preprocessor recognizes a function-like macro call, the
      preprocessor chooses the definition with the same number of
      arguments. Example: </item>
<code type="none">
-define(CALL(Fun),      Fun()).
-define(CALL(Mod, Fun), Mod:Fun()).
...
call() ->
    ?CALL(kernel, module_info).</code>
This will be expended to:
<code type="none">
call() ->
    kernel:module_info().</code>

      <item>in all other cases, the preprocessor throws an
      error. Example:</item>
<code type="none">
-define(M, erlang).
-define(M(X), X).
...
call() ->
    ?M(kernel, module_info).</code>
The compiler will fail with error <c>argument mismatch for macro 'M'</c>

    </list>
  </section>

  <section>
    <title>Flow Control in Macros</title>
    <p>The following macro directives are supplied:</p>
    <taglist>
      <tag><c>-undef(Macro).</c></tag>
      <item>Causes the macro to behave as if it had never been defined.</item>
      <tag><c>-ifdef(Macro).</c></tag>
      <item>Evaluate the following lines only if <c>Macro</c> is
       defined.</item>
      <tag><c>-ifndef(Macro).</c></tag>
      <item>Evaluate the following lines only if <c>Macro</c> is not
       defined.</item>
      <tag><c>-else.</c></tag>
      <item>Only allowed after an <c>ifdef</c> or <c>ifndef</c>
       directive. If that condition was false, the lines following
      <c>else</c> are evaluated instead.</item>
      <tag><c>-endif.</c></tag>
      <item>Specifies the end of an <c>ifdef</c> or <c>ifndef</c>
       directive.</item>
    </taglist>
    <note>
      <p>The macro directives cannot be used inside functions.</p>
    </note>
    <p>Example:</p>
    <code type="none">
-module(m).
...

-ifdef(debug).
-define(LOG(X), io:format("{~p,~p}: ~p~n", [?MODULE,?LINE,X])).
-else.
-define(LOG(X), true).
-endif.

...</code>
    <p>When trace output is desired, <c>debug</c> should be defined
      when the module <c>m</c> is compiled:</p>
    <pre>
% <input>erlc -Ddebug m.erl</input>

or

1> <input>c(m, {d, debug}).</input>
{ok,m}</pre>
    <p><c>?LOG(Arg)</c> will then expand to a call to <c>io:format/2</c>
      and provide the user with some simple trace output.</p>
  </section>

  <section>
    <title>Stringifying Macro Arguments</title>
    <p>The construction <c>??Arg</c>, where <c>Arg</c> is a macro
      argument, will be expanded to a string containing the tokens of
      the argument. This is similar to the <c>#arg</c> stringifying
      construction in C.</p>
    <p>The feature was added in Erlang 5.0/OTP R7.</p>
    <p>Example:</p>
    <code type="none">
-define(TESTCALL(Call), io:format("Call ~s: ~w~n", [??Call, Call])).

?TESTCALL(myfunction(1,2)),
?TESTCALL(you:function(2,1)).</code>
    <p>results in</p>
    <code type="none">
io:format("Call ~s: ~w~n",["myfunction ( 1 , 2 )",m:myfunction(1,2)]),
io:format("Call ~s: ~w~n",["you : function ( 2 , 1 )",you:function(2,1)]).</code>
    <p>That is, a trace output with both the function called and
      the resulting value.</p>
  </section>
</chapter>