Help:Parser functions in templates

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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\|}}}	{{{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.

Testing and passing undefined parameters

See also