Help:Parser functions in templates/pl
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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.
| Parametr | {{{1}}}, {{{param}}} | {{{1|}}}, {{{param|}}} | {{#if:<parametr>|True|False}} | ||
|---|---|---|---|---|---|
| Opis | 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 parameters
To distinguish a possibly empty parameter from an unspecified one, compare it to itself using {{#ifeq:}} and different defaults.
What the defaults are does not matter as long as they are different, so they are typically chosen to be short.
The following all work equivalently:
{{#ifeq:{{{v|+}}}|{{{v|-}}}| v was defined (and may be empty) | v was not defined }}{{#ifeq:{{{v|}}}|{{{v|-}}}| v was defined (and may be empty) | v was not defined }}{{#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.