TiddlyWiki5/editions/tw5.com/tiddlers/pragmas/Pragma_ _procedure.tid

created: 20221007132845007
modified: 20230724184009153
tags: Pragmas
title: Pragma: \procedure
type: text/vnd.tiddlywiki

<<.from-version "5.3.0">> The ''\procedure'' [[pragma|Pragmas]] is used to [[define procedures|Procedure Definitions]]. It is a shortcut syntax for the SetVariableWidget with an implicit ParametersWidget.

The usual form allows procedures to span multiple lines:

```
\procedure <procedure-name>(<param-name>[:<param-default-value>],<param-name>[:<param-default-value>]...)
<multiple-line-definition-text>
\end [<procedure-name>]
```

Note that the `\end` marker can optionally specify the name of the procedure to which it relates which allows procedure definitions to be nested.

There is also a single line form for shorter procedures:

```
\procedure <procedure-name>(<param-name>[:<param-default-value>],<param-name>[:<param-default-value>]...) <single-line-definition-text>
```

The first line of the definition specifies the procedure 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 procedure. The lines that follow contain the text of the procedure text (i.e. the snippet represented by the procedure name), until `\end` appears on a line by itself:

For example:

```
\procedure sayhi(name:"Bugs Bunny")
Hi, I'm <<name>>.
\end

<<sayhi "Jeremy">>
```

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

```
\procedure sayhi(name:"Bugs Bunny") Hi, I'm <<name>>.
```

Procedure definitions can be nested by specifying the name of the procedure in the `\end` marker. For example:

<<wikitext-example-without-html src:"""\procedure special-button(caption:"Click me")
\procedure actions()
<$action-sendmessage $message="tm-notify" $param="HelloThere"/>
\end actions
<$button actions=<<actions>>>
<<caption>>
</$button>
\end special-button

<<special-button>>
""">>

! Use of Parameters Inside Procedures
The parameters can be declared inside procedures. The parameters widget is necessary in a procedure if you want to use computed default values. For example:

<<wikitext-example-without-html 
src:"""\procedure myproc()
<$parameters name={{$:/SiteTitle}} desc={{$:/SiteSubtitle}}>
This is <<name>> demonstrates <<desc>>.
</$parameters>
\end

<<myproc>>
""">>

!! Caution in Using Positional Parameters
Procedures are a shortcut syntax for the SetVariableWidget with an implicit ParametersWidget, so generally there is no reason to have multiple parameters widgets within a definition. In the below example when passing `x` to `myproc`, it will also be set to `a`:

<<wikitext-example-without-html 
src:"""\procedure myproc(x:10)
\parameters (a:100, b:200)

x=<<x>>, a=<<a>>, b=<<b>>
\end

<<myproc 50>>
""">>

The reason for that result is clearer if we consider an equivalent with explicit parameters widgets.

<$macrocall $name=wikitext-example-without-html 
src='<$let myprog="""
\parameters (x:10)
\parameters (a:100, b:200)

x=<<x>>, a=<<a>>, b=<<b>>
""">
<<myprog 50>>
</$let>'
/>

This is because those two parameters widgets are entirely independent. They are both processed as if the other parameter widget is not there.

<<.tip "The positional parameters are only required when using the parameterised transclusion shortcut syntax, and that in other cases it is generally clearer to use named parameters.">>

To prevent such situation of above example, pass parameters by name as below.

<<wikitext-example-without-html 
src:"""\procedure myproc(x:10)
\parameters (a:100, b:200)

x=<<x>>, a=<<a>>, b=<<b>>
\end

<<myproc x:50>>
""">>