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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<TITLE>Macros</TITLE>
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<A NAME="7"><!-- Empty --></A>
<H2>7 Macros</H2>
<A NAME="7.1"><!-- Empty --></A>
<H3>7.1 Defining and Using Macros</H3>
<P>A macro is defined the following way:
<PRE>
-define(Const, Replacement).
-define(Func(Var1,...,VarN), Replacement).
</PRE>
<P>A macro definition can be placed anyhere among the attributes
and function declarations of a module, but the definition must
come before any usage of the macro.
<P>If a macro is used in several modules, it is recommended that
the macro definition is placed in an include file.
<P>A macro is used the following way:
<PRE>
?Const
?Func(Arg1,...,ArgN)
</PRE>
<P>Macros are expanded during compilation. A simple macro
<CODE>?Const</CODE> will be replaced with <CODE>Replacement</CODE>.
Example:
<PRE>
-define(TIMEOUT, 200).
...
call(Request) ->
server:call(refserver, Request, ?TIMEOUT).
</PRE>
<P>This will be expanded to:
<PRE>
call(Request) ->
server:call(refserver, Request, 200).
</PRE>
<P>A macro <CODE>?Func(Arg1,...,ArgN)</CODE> will be replaced with
<CODE>Replacement</CODE>, where all occurences of a variable <CODE>Var</CODE>
from the macro definition are replaced with the corresponding
argument <CODE>Arg</CODE>. Example:
<PRE>
-define(MACRO1(X, Y), {a, X, b, Y}).
...
bar(X) ->
?MACRO1(a, b),
?MACRO1(X, 123)
</PRE>
<P>This will be expanded to:
<PRE>
bar(X) ->
{a,a,b,b},
{a,X,b,123}.
</PRE>
<P>It is good programming practice, but not mandatory, to ensure
that a macro definition is a valid Erlang syntactic form.
<P>To view the result of macro expansion, a module can be compiled
with the <CODE>'P'</CODE> option. <CODE>compile:file(File, ['P'])</CODE>.
This produces a listing of the parsed code after preprocessing
and parse transforms, in the file <CODE>File.P</CODE>.
<A NAME="7.2"><!-- Empty --></A>
<H3>7.2 Predefined Macros</H3>
<P>The following macros are predefined:
<P>
<DL>
<DT>
<CODE>?MODULE</CODE>
</DT>
<DD>
The name of the current module.
</DD>
<DT>
<CODE>?MODULE_STRING</CODE>.
</DT>
<DD>
The name of the current module, as a string.
</DD>
<DT>
<CODE>?FILE</CODE>.
</DT>
<DD>
The file name of the current module.
</DD>
<DT>
<CODE>?LINE</CODE>.
</DT>
<DD>
The current line number.
</DD>
<DT>
<CODE>?MACHINE</CODE>.
</DT>
<DD>
The machine name, <CODE>'BEAM'</CODE>.
</DD>
</DL>
<A NAME="7.3"><!-- Empty --></A>
<H3>7.3 Flow Control in Macros</H3>
<P>The following macro directives are supplied:
<P>
<DL>
<DT>
<CODE>-undef(Macro).</CODE>
</DT>
<DD>
Causes the macro to behave as if it had never been defined.
</DD>
<DT>
<CODE>-ifdef(Macro).</CODE>
</DT>
<DD>
Evaluate the following lines only if <CODE>Macro</CODE> is
defined.
</DD>
<DT>
<CODE>-ifndef(Macro).</CODE>
</DT>
<DD>
Evaluate the following lines only if <CODE>Macro</CODE> is not
defined.
</DD>
<DT>
<CODE>-else.</CODE>
</DT>
<DD>
Only allowed after an <CODE>ifdef</CODE> or <CODE>ifndef</CODE>
directive. If that condition was false, the lines following
<CODE>else</CODE> are evaluated instead.
</DD>
<DT>
<CODE>-endif.</CODE>
</DT>
<DD>
Specifies the end of an <CODE>ifdef</CODE> or <CODE>ifndef</CODE>
directive.
</DD>
</DL>
<P>Example:
<PRE>
-module(m).
...
-ifdef(debug).
-define(LOG(X), io:format("{~p,~p}: ~p~n", [?MODULE,?LINE,X])).
-else.
-define(LOG(X), true).
-endif.
...
</PRE>
<P>When trace output is desired, <CODE>debug</CODE> should be defined
when the module <CODE>m</CODE> is compiled:
<PRE>
% <STRONG>erlc -Ddebug m.erl</STRONG>
or
1> <STRONG>c(m, {d, debug}).</STRONG>
{ok,m}
</PRE>
<P><CODE>?LOG(Arg)</CODE> will then expand to a call to <CODE>io:format/2</CODE>
and provide the user with some simple trace output.<A NAME="7.4"><!-- Empty --></A>
<H3>7.4 Stringifying Macro Arguments</H3>
<P>The construction <CODE>??Arg</CODE>, where <CODE>Arg</CODE> is a macro
argument, will be expanded to a string containing the tokens of
the argument. This is similar to the <CODE>#arg</CODE> stringifying
construction in C.
<P>The feature was added in Erlang 5.0/OTP R7.
<P>Example:
<PRE>
-define(TESTCALL(Call), io:format("Call ~s: ~w~n", [??Call, Call])).
?TESTCALL(myfunction(1,2)),
?TESTCALL(you:function(2,1)).
</PRE>
<P>results in
<PRE>
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)]).
</PRE>
<P>That is, a trace output with both the function called and
the resulting value.<CENTER>
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