Pragma: \procedure

24th July 2023 at 6:40pm

New in v5.3.0 The \procedure pragma is used to define procedures. 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:

\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>>

That renders as:

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:

\procedure myproc()
<$parameters name={{$:/SiteTitle}} desc={{$:/SiteSubtitle}}>
This is <<name>> demonstrates <<desc>>.
</$parameters>
\end

<<myproc>>

That renders as:

This is TiddlyWiki demonstrates a non-linear personal web notebook.

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:

\procedure myproc(x:10)
\parameters (a:100, b:200)

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

<<myproc 50>>

That renders as:

x=50, a=50, b=200

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

<$let myprog="""
\parameters (x:10)
\parameters (a:100, b:200)

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

That renders as:

x=50, a=50, b=200

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.

\procedure myproc(x:10)
\parameters (a:100, b:200)

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

<<myproc x:50>>

That renders as:

x=50, a=100, b=200