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When applyingParserFunctions totemplate 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.
| 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}}}{{#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.}}Parameter 1 is not defined, or is defined and non-null/non-empty.{{{1|}}}{{#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.}}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.
To distinguish a possibly empty parameter from an unspecified one, compare it to itself using{{#ifeq:}} anddifferent 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 definednormal_parameter in all cases. Whennormal_parameter is defined but empty and when it is undefined,wrapped_template receives an emptynormal_parameter.
By contrast, thewrapped_template receives a definedsensitive_parameteronly when it is indeed defined; whensensitive_parameter is undefined, the#if changes the parameter name tosensitive_parameterNULL.The suffixed parameter name must be meaningless to thewrapped_template for this to work properly.