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}}}{{#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:}} 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.
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