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<!DOCTYPE chapter SYSTEM "chapter.dtd">
<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>
