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

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE chapter SYSTEM "chapter.dtd">

<chapter>
  <header>
    <copyright>
      <year>2003</year><year>2015</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.

    </legalnotice>

    <title>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 as follows:</p>
      <pre>
-include(File).
-include_lib(File).</pre>
      <p><c>File</c>, a string, is to 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 to use the file name extension <c>.hrl</c> for
        include files.</p>
      <p><c>File</c> can 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>$VAR</c> is left
        as is.</p>
      <p>If the filename <c>File</c> is absolute (possibly after
        variable substitution), the include file with that name is
        included. Otherwise, the specified file is searched for
	in the following directories, and in this order:</p>
	<list type="ordered">
	  <item>The current working directory</item>
	  <item>The directory where the module is being compiled</item>
	  <item>The directories given by the <c>include</c> option</item>
	</list>
	<p>For details, see the
          <seealso marker="erts:erlc">erlc(1)</seealso> manual page
           in ERTS and
           <seealso marker="compiler:compile">compile(3)</seealso>
           manual page in Compiler.</p>
      <p><em>Examples:</em></p>
      <pre>
-include("my_records.hrl").
-include("incdir/my_records.hrl").
-include("/home/user/proj/my_records.hrl").
-include("$PROJ_ROOT/my_records.hrl").</pre>
      <p><c>include_lib</c> is similar to <c>include</c>, but is not to
        point out an absolute file. Instead, the first path component
        (possibly after variable substitution) is assumed to be
        the name of an application.</p>
	<p><em>Example:</em></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 as follows:</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 as follows:</p>
    <code type="none">
?Const
?Func(Arg1,...,ArgN)</code>
    <p>Macros are expanded during compilation. A simple macro
      <c>?Const</c> is replaced with <c>Replacement</c>.</p>
      <p><em>Example:</em></p>
    <code type="none">
-define(TIMEOUT, 200).
...
call(Request) ->
    server:call(refserver, Request, ?TIMEOUT).</code>
    <p>This is expanded to:</p>
    <code type="none">
call(Request) ->
    server:call(refserver, Request, 200).</code>
    <p>A macro <c>?Func(Arg1,...,ArgN)</c> is 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>.</p>
      <p><em>Example:</em></p>
    <code type="none">
-define(MACRO1(X, Y), {a, X, b, Y}).
...
bar(X) ->
    ?MACRO1(a, b),
    ?MACRO1(X, 123)</code>
    <p>This is 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>
      <tag><c>?FUNCTION_NAME</c></tag>
      <item>The name of the current function.</item>
      <tag><c>?FUNCTION_ARITY</c></tag>
      <item>The arity (number of arguments) for the current function.</item>
    </taglist>
  </section>

  <section>
    <title>Macros Overloading</title>
    <p>It is possible to overload macros, except for predefined
      macros. An overloaded macro has more than one definition,
      each with a different number of arguments.</p>
    <p>The feature was added in Erlang 5.7.5/OTP R13B04.</p>
    <p>A macro <c>?Func(Arg1,...,ArgN)</c> with a (possibly empty)
      list of arguments results in an error message if there is at
      least one definition of <c>Func</c> with arguments, but none
      with N arguments.</p>
    <p>Assuming these definitions:</p>
    <code type="none">
-define(F0(), c).
-define(F1(A), A).
-define(C, m:f).</code>
    <p>the following does not work:</p>
    <code type="none">
f0() ->
    ?F0. % No, an empty list of arguments expected.

f1(A) ->
    ?F1(A, A). % No, exactly one argument expected.</code>
    <p>On the other hand,</p>
    <code>
f() ->
    ?C().</code>
    <p>is expanded to</p>
    <code>
f() ->
    m:f().</code>
  </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 is 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><em>Example:</em></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> is to 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> is then expanded 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, is 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><em>Example:</em></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 )",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>