Help:Parser functions in templates

<languages /> Template:TNT <translate> When applying [[<tvar|ext>Special:MyLanguage/Help:Extension:ParserFunctions</>|ParserFunctions]] to [[<tvar|templates>Special:MyLanguage/Help:Templates</>|template]] parameters, a pipe symbol ("|") may be used to provide a default value, which is used when a parameter is not defined.</translate> <translate> Used in an #if parser function, the unexpanded text from the undefined parameter will evaluate as true, which may be an unexpected result.</translate>

<translate> Testing parameters with and without default values</translate>

<translate> Parameter</translate>		{{{1}}}, {{{param}}}	{{{1|}}}, {{{param|}}}	{{#if:<<translate> parameter</translate>>|True|False}}
<translate> Description</translate>	<translate> Example, unnamed and named</translate>			{{{1}}}, {{{param}}}	{{{1|}}}, {{{param|}}}
<translate> Undefined. More appropriate for use in named parameters.</translate>	{{template}}	{{{1}}}		True	False
<translate> Defined, but empty or null.</translate>	{{template|}}, {{template|1=}}, {{template|param=}}			False	False
<translate> Defined, non-empty, and non-null.</translate>	{{template|value}}, {{template|1=value}}, {{template|param=value}}	value	value	True	True

• {{{1}}}

<translate>

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

{{#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.}}

</translate>

• {{{1|}}}

<translate>

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

{{#if: | 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.}}

</translate>

<translate> 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 <tvar|ifeq>{{#ifeq:}}</> and different defaults.</translate> <translate> What the defaults are does not matter as long as they are different, so they are typically chosen to be short.</translate> <translate> The following all work equivalently: </translate>

{{#ifeq:{{{v|+}}}|{{{v|-}}}| <translate> v was defined (and may be empty)</translate> | <translate> v was not defined</translate> }}

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

{{#ifeq:{{{v|}}}|{{{v}}}| <translate> v was defined (and may be empty)</translate> | <translate> v was not defined</translate> }}

<translate> 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): </translate>

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

<translate> 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.</translate> <translate> The suffixed parameter name must be meaningless to the wrapped_template for this to work properly.

See also

</translate>

• Template:Ll
• Template:Ll

<translate>

• [[<tvar|ext>Special:MyLanguage/Extension:Loops#.23forargs_.28Experimental.29</>|Extension:Loops#forargs]]

</translate>

[[Category:Extension help{{#translation:}}|Parser functions in templates]]