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110 lines
3.5 KiB
Plaintext
110 lines
3.5 KiB
Plaintext
created: 20221007132845007
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modified: 20230724184009153
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tags: Pragmas
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title: Pragma: \procedure
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type: text/vnd.tiddlywiki
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<<.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.
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The usual form allows procedures to span multiple lines:
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```
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\procedure <procedure-name>(<param-name>[:<param-default-value>],<param-name>[:<param-default-value>]...)
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<multiple-line-definition-text>
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\end [<procedure-name>]
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```
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Note that the `\end` marker can optionally specify the name of the procedure to which it relates which allows procedure definitions to be nested.
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There is also a single line form for shorter procedures:
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```
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\procedure <procedure-name>(<param-name>[:<param-default-value>],<param-name>[:<param-default-value>]...) <single-line-definition-text>
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```
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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:
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For example:
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```
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\procedure sayhi(name:"Bugs Bunny")
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Hi, I'm <<name>>.
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\end
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<<sayhi "Jeremy">>
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```
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Alternatively, the entire definition can be presented on a single line without an `\end` marker:
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```
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\procedure sayhi(name:"Bugs Bunny") Hi, I'm <<name>>.
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```
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Procedure definitions can be nested by specifying the name of the procedure in the `\end` marker. For example:
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<<wikitext-example-without-html src:"""\procedure special-button(caption:"Click me")
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\procedure actions()
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<$action-sendmessage $message="tm-notify" $param="HelloThere"/>
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\end actions
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<$button actions=<<actions>>>
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<<caption>>
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</$button>
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\end special-button
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<<special-button>>
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""">>
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! Use of Parameters Inside Procedures
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The parameters can be declared inside procedures. The parameters widget is necessary in a procedure if you want to use computed default values. For example:
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<<wikitext-example-without-html
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src:"""\procedure myproc()
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<$parameters name={{$:/SiteTitle}} desc={{$:/SiteSubtitle}}>
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This is <<name>> demonstrates <<desc>>.
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</$parameters>
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\end
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<<myproc>>
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""">>
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!! Caution in Using Positional Parameters
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Procedures are a shortcut syntax for the SetVariableWidget with an implicit ParametersWidget, so generally there is no reason to have multiple parameters widgets within a definition. In the below example when passing `x` to `myproc`, it will also be set to `a`:
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<<wikitext-example-without-html
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src:"""\procedure myproc(x:10)
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\parameters (a:100, b:200)
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x=<<x>>, a=<<a>>, b=<<b>>
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\end
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<<myproc 50>>
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""">>
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The reason for that result is clearer if we consider an equivalent with explicit parameters widgets.
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<$macrocall $name=wikitext-example-without-html
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src='<$let myprog="""
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\parameters (x:10)
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\parameters (a:100, b:200)
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x=<<x>>, a=<<a>>, b=<<b>>
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""">
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<<myprog 50>>
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</$let>'
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/>
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This is because those two parameters widgets are entirely independent. They are both processed as if the other parameter widget is not there.
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<<.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.">>
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To prevent such situation of above example, pass parameters by name as below.
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<<wikitext-example-without-html
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src:"""\procedure myproc(x:10)
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\parameters (a:100, b:200)
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x=<<x>>, a=<<a>>, b=<<b>>
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\end
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<<myproc x:50>>
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""">> |