Help:Parser functions in templates/pt

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When applying ParserFunctions to template parameters, a pipe symbol ("|") may be used to provide a default value, which is used when a parameter is not defined. Used in an #if parser function, the unexpanded text from the undefined parameter will evaluate as true, which may be an unexpected result.

Testing parameters with and without default values
Parameter {{{1}}}, {{{param}}} {{{1|}}}, {{{param|}}} {{#if:<parameter>|True|False}}
Description Example, unnamed and named {{{1}}}, {{{param}}} {{{1|}}}, {{{param|}}}
Undefined. More appropriate for use in named parameters. {{template}} {{{1}}} True False
Defined, but empty or null. {{template|}}, {{template|1=}}, {{template|param=}} False False
Defined, non-empty, and non-null. {{template|value}}, {{template|1=value}}, {{template|param=value}} value value True True
  • {{{1}}}
Sample A
{{#if: {{{1}}} | Parameter 1 is not defined, or is defined and non-null/non-empty. | Parameter 1 is null. It contains only empty string(s) or breaking space(s) etc.}}
Result
Parameter 1 is not defined, or is defined and non-null/non-empty.
  • {{{1|}}}
Sample B
{{#if: {{{1|}}} | Parameter 1 is defined and non-null/non-empty. | Parameter 1 is not defined, or is defined but null. It contains only empty string(s) or breaking space(s) etc.}}
Result
Parameter 1 is not defined, or is defined but null. It contains only empty string(s) or breaking space(s) etc.

The second usage ({{{1|}}}, sample B) with present empty default is often the desired way to handle situations where a parameter exists, but is comprised only of empty space.

Testing and passing undefined parametersEdit

One way to test whether a parameter was specified in a template call is thus:

{{#ifeq:{{{v|}}}|{{{v|-}}}| v was defined (and may be empty) | v was not defined }}

In rare cases, a template behaves differently when a parameter is unspecified compared to when it is specified but empty. When this template is used by a wrapper template (which uses the same set of named parameters), one way to ensure undefined parameters remain undefined is as follows (the technique also works with numbered parameters):

{{wrapped_template|
normal_parameter={{{normal_parameter|}}}|
sensitive_parameter{{#if:{{{sensitive_parameter|}}}||NULL}}={{{sensitive_parameter}}}
}}

wrapped_template receives a defined normal_parameter in all cases. When normal_parameter is defined but empty and when it is undefined, wrapped_template receives an empty normal_parameter.

By contrast, the wrapped_template receives a defined sensitive_parameter only when it is indeed defined; when sensitive_parameter is undefined, the #if changes the parameter name to sensitive_parameterNULL. The suffixed parameter name must be meaningless to the wrapped_template for this to work properly.

Ver tambémEdit