caption: Variables, Parameters, Subsitutions
created: 20141018090608643
modified: 20141018104201050
tags: WikiText
title: Variables, Macros and Subsitutions in WikiText
type: text/vnd.tiddlywiki

! Syntax for Attributes

The syntax for the attributes of [[HTML elements|HTML in WikiText]] and [[widgets|Widgets in WikiText]]:

* `"text"` or `'text'` - passed as a literal value
* `{{text reference}}` - [[transcludes|Transclusion in WikiText]] a text reference
* `<<macro>>` - transcludes a variable or macro invocation.

! Syntax for Filter

Using single brace `{...}` and `<...>` when referenced as operands in [[Filter expression|Introduction to Filters]].

* Text reference as `[search{$:/temp/search}]` or `[prefix{tiddlerA!!fieldname}]`
* Variable reference as  `[search<currentTiddler>]`

! Variables and Macros

[[Variables|Variables in WikiText]] and [[macros|Macros in WikiText]] are indeed the same thing. Macros are just variables that have their parameters substituted before use.

! Textual Substitution

* `$parametername$` - for [[macor parameters|Macros in WikiText]] 
```
\define mysamplemacro(name:"Bugs Bunny",address:"Rabbit Hole Hill")
Hi, I'm $name$ and I live in $address$
\end
```

* `$(variablename)$` - for variable substitutions ''within macros''
```
\define mysamplemacro2()
Hi, I'm $(name)$ and I live in $(address)$
\end

<$set name="name" value="Bugs Bunny">
<$set name="address" value="Rabbit Hole Hill">
<<mysamplemacro2>>
</$set></$set>
```
The differents between textual substitution and other syntax:

* Textual substitution is performed before the macro is parsed, the values are inserted into the target.
* While the `<<...>>` syntax is a transclusion, where the "WikiText" is inserted and rendered separately.
* Textual substitution behaves as text. So that they can be combined with other text, and the usage should include all braces like :
** `filter="[prefix[$:/$(a)$/$(b)$]]"` or 
** `<$reveal ... text="$(c)$"> `