add Subsitutions in WikiText

editions/tw5.com/tiddlers/wikitext/Variables, Macros and Subsitutions in WikiText.tid

@@ -0,0 +1,53 @@
+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="$(c)$"> `