%META:TOPICINFO{author="TWikiContributor" date="1301208112" format="1.1" version="1"}%
%META:TOPICPARENT{name="TWikiVariables"}%
---+ Parameterized Variables (Macros)
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It is possible to pass parameters to TWiki variables. This is called a macro in a programming language.

To define a parameterized variable, set a variable that contains other variables, such as:
<verbatim>
   * Set EXAMPLE = Example variable using %DEFAULT%, %PARAM1% and %PARAM2%
   * Set DEMO = Demo using %DEFAULT{ default="(undefined)" }%,
                %PARAM1{ default="(undefined)" }% and %PARAM2{ default="(undefined)" }%
</verbatim>

A special =%<nop>DEFAULT%= variable denotes the default (nameless) parameter of the calling variable. Variables optionally may list a =default="..."= parameter that gets used in case the calling variable does not specify that parameter.

To use a parameterized variable (or call a macro), add parameters within the curly brackets, such as:
<verbatim>
   * %EXAMPLE{ "foo" PARAM1="bar" PARAM2="baz" }%
   * %DEMO{ "demo" PARAM2="parameter 2" }% -- note that PARAM1 is missing
</verbatim>
which resolves to:
   * %EXAMPLE{ "foo" PARAM1="bar" PARAM2="baz" }%
   * %DEMO{ "demo" PARAM2="parameter 2" }% -- note that PARAM1 is missing

Parameters in the variable definition are expanded using the following sequence:

   1. Parameter from variable call. In above example, =%<nop>PARAM1%= gets expanded to =bar=.
   2. Session variable and preferences settings

---++!! Example

*Define variables:*
<verbatim>
   * Set DRINK = red wine
   * Set FAVORITE = My %DEFAULT{default="favorite"}% dish is %DISH{default="steak"}%,
                    my %DEFAULT{default="favorite"}% drink is %DRINK%.
</verbatim>
%T% The default can be defined with a default parameter (=%<nop>DISH{default="steak"}%=), or as a preferences setting (=Set DRINK = ...=).

*Use Variables:*
<verbatim>
%FAVORITE{ DISH="Sushi" DRINK="Sake" }%
</verbatim>
Returns: %BR%
%FAVORITE{ DISH="Sushi" DRINK="Sake" }%

<verbatim>
%FAVORITE{}%
</verbatim>
Returns: %BR%
%FAVORITE{}%

<verbatim>
%FAVORITE{ "preferred" }%
</verbatim>
Returns: %BR%
%FAVORITE{ "preferred" }%
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__Related:__ TWikiVariables, IncludeTopicsAndWebPages