Variables are defined by [[widgets|Widgets]]. Several core widgets define variables, the most common being the <<.wlink SetWidget>>, <<.wlink LetWidget>> and <<.wlink ListWidget>> widgets.
The values of variables are available to descendant widgets, including transcluded content. For example, within each tiddler in the main story river the variable "currentTiddler" is set to the title of the tiddler.
Variables can also be overwritten by descendent widgets defining variables of the same name, thus binding a different snippet to that name for the scope of the children of the widget.
Note that these special kinds of variable can only be created with the associated shortcut definition syntax.
!! Defining Variables
The following core widgets are commonly used to define variables:
* <<.wlink LetWidget>> widget -- the easiest way to define multiple variables
* <<.wlink SetWidget>> widget -- the most flexible way to define a single variable
* <<.wlink ParametersWidget>> widget -- used to declare parameter variables within [[procedures|Procedures]] and [[user defined widgets|User Defined Widgets]]
* <<.wlink ListWidget>> widget -- defines a loop variable and optional counter variable
* <<.wlink SetMultipleVariablesWidget>> widget -- allows creation of multiple variables at once where the names and values are not known in advance
!! Using Variables
Once a variable is defined there are several ways to access it.
!!! Transcluding Variables
Transcluding a variable renders the text contents of the variable as if it replaced the call. It is a shortcut syntax for the <<.wlink TranscludeWidget>> widget with the `$variable` attribute.
```
<<varname>>
```
Parameters can be passed to the transclusion as follows:
```
<<varname "This is a parameter">>
<<varname param:"This is a parameter">>
<<varname param:"This is a parameter" second:"Another parameter">>
```
The handling of these parameters depends on the kind of variable:
* [[Procedures]] assign the parameters to variables that are available within the procedure
* [[Macros]] replace the text of the special markers `$param$` with the values passed to the macro for those parameters (see [[Macro Parameter Handling]] for the details)
The parameters are ignored for other kinds of variable.
!!! Macro Variable Substitutions
Before the text of a macro is used, the special markers `$(variable)$` are replaced with the values of the named variable.
!!! Variable Attributes
Variables can be used as the value of attributes of widgets or HTML elements:
```
<div class=<<varname>>>
```
Parameters can be passed:
```
<div class=<<varname "This is a parameter">>>
...
<div class=<<varname param:"This is a parameter">>>
...
<div class=<<varname param:"This is a parameter" second:"Another parameter">>>
...
```
The handling of these parameters depends on the kind of variable:
* [[Functions]] assign the parameters to variables that are available within the function
* [[Macros]] replace the text of the special markers `$param$` with the values passed to the macro for those parameters (see [[Macro Parameter Handling]] for the details)
The parameters are ignored for other kinds of variable.
!!! Variables in Filters
Variables can be accessed within [[Filters]] using angle brackets to quote the name:
```
[<varname>]
```
Parameters can be passed in the usual way:
```
[<varname "This is a parameter">]
[<varname param:"This is a parameter">]
[<varname param:"This is a parameter" second:"Another parameter">]
...
```
!! See Also
* The <<.mlink dumpvariables>> macro lists all variables that are available at that position in the widget tree
* Complete listing of ~TiddlyWiki's built-in [[Core Variables]]
!! Examples
!!! Example of Defining a Variable
<$macrocall $name=".example" n="1"
eg="""<$set name=animal value=zebra>
<<animal>>
</$set>"""/>
!!! Example of Defining a Macro
The `\define` pragma below [[defines a macro|Macro Definitions]] called <<.var tags-of-current-tiddler>>. The macro returns the value of the tiddler's <<.field tags>> field, and can be accessed from anywhere else in the same tiddler (or in any tiddler that [[imports|ImportVariablesWidget]] it).
