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## Macro Definitions in WikiText

15th December 2017 at 3:27pm
WikiText

A macro is defined using a \define pragma. Like any pragma, this can only appear at the start of a tiddler.

The first line of the definition specifies the macro name and any parameters. Each parameter has a name, and optionally a default value that is used if no value is supplied on a particular call to the macro. The lines that follow contain the macro's text (i.e. the snippet represented by the macro name), until \end appears on a line by itself:

\define sayhi(name:"Bugs Bunny" address:"Rabbit Hole Hill")
Hi, I'm $name$ and I live in $address$.
\end


Alternatively, the entire definition can be presented on a single line, without an \end marker:

\define sayhi(name:"Bugs Bunny") Hi, I'm $name$.

A more formal presentation of this syntax is also available.

## Placeholders

The snippet can contain placeholders for parameters. These consist of a parameter name between dollar signs, like $this$.

It can also contain placeholders for variables. These consist of a variable name (or macro name) between dollar signs and round brackets, like $(this)$.

The actual value of the parameter or variable is substituted for the placeholder whenever the macro is called:

\define say-hi-using-variables()
Hi, I'm $(name)$ and I live in $(address)$.
\end

\define name() Bugs

<$set name="address" value="Rabbit Hole Hill"> <<say-hi-using-variables>> </$set>

## Parameters as Variables

The parameters to a wikitext macro are also available as special variables named as the parameter name wrapped in double underscores. For example, the example above could also be expressed as:

\define sayhi(name:"Bugs Bunny") Hi, I'm <$text text=<<__name__>>/>. Accessing parameters as variables only works in macros that are wikified and not, for example, when a macro is used as an attribute value. The advantage of the technique is that it avoids the parameter value being substituted into the macro as a literal string, which in turn can help avoid issues with parameters that contain quotes. For example, consider this macro. It is intended to wrap a DIV around another macro invocation, passing through the single parameter to the inner macro: \define related-tags(base-tag) <div class="wrapper"> <$macrocall $name="anothermacro" param="""$base-tag$"""/> </div> \end The code above will fail if the macro is invoked with the argument containing triple double quotes (for example <<related-tags 'Triple """ Quotes'>>). Using parameter variables offers a workaround: \define related-tags(base-tag) <div class="wrapper"> <$macrocall $name="anothermacro" param=<<__base-tag__>>/> </div> \end ## Scope Macros are available to the tiddler that defines them, plus any tiddlers that it transcludes. To make a macro available to all tiddlers, define it in a tiddler that has the tag $:/tags/Macro.

It is also possible to write a macro as a JavaScript module. JavaScript macros are available to all tiddlers, and offer the maximum flexibility.

A tiddler can manually import macro definitions from a selection of other tiddlers by using the \$importvariables widget.