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2011-07-21T12:21:19Z

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When applying [[Help:Extension:ParserFunctions|ParserFunctions]] to [[Help:Template|template]] parameters, a pipe symbol ("|") may be used to provide the empty string as default value, in order to indicate how to handle things if a parameter is or is not passed in.

*<tt><nowiki>{{{1}}}</nowiki></tt>

:;Sample A: <tt><nowiki>{{#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.}}</nowiki></tt>

:;Result: <tt>{{#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.}}</tt>

*<tt><nowiki>{{{1|}}}</nowiki></tt>

:;Sample B: <tt><nowiki>{{#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.}}</nowiki></tt>

:;Result: <tt>{{#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.}}</tt>

;Comparison
{| class="wikitable"
! Example of usage in a template anonymous .. 1 param
! Condition of parameter
! Sample A: <nowiki>{{#if: {{{1}}} |</nowiki> <nowiki>{{#if: {{{param}}} |</nowiki>
! Sample B: <nowiki>{{#if: {{{1|}}} |</nowiki> <nowiki>{{#if: {{{param|}}} |</nowiki>
|-
| <code><nowiki>{{template}}</nowiki></code> <code><nowiki>{{template}}</nowiki></code>||not defined: parameter not used, important for named parameters||'''TRUE'''||''FALSE''
|-
| <code><nowiki>{{template|}} .. {{template|1=}}</nowiki></code> <code><nowiki>{{template|param=}}</nowiki></code>||defined, but null/empty||''FALSE''||''FALSE''
|-
| <code><nowiki>{{template|value}} .. {{template|1=value}}</nowiki></code> <code><nowiki>{{template|param=value}}</nowiki></code>||defined and NOT null neither empty||'''TRUE'''||'''TRUE'''
|}

The second usage (<tt><nowiki>{{{1|}}}</nowiki></tt>, 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.

One way to test whether a parameter was specified in a template call is thus:
<tt><nowiki>{{#ifeq:{{{v|}}}|{{{v|-}}}| v was specified (and may be empty) | v was not specified }}</nowiki></tt>

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):

<pre>

{{wrapped_template|
normal_parameter={{{normal_parameter|}}}|
sensitive_parameter{{#if:{{{sensitive_parameter|}}}||NULL}}={{{sensitive_parameter}}}|
...
}}
</pre>

Note that the <tt>wrapped_template</tt> receives a specified but empty <tt>normal_parameter</tt> in two cases: when <tt>normal_parameter</tt> is specified and empty, and when <tt>normal_parameter</tt> is unspecified (because the default empty value is then specified by the wrapper template).

By contrast, the <tt>wrapped_template</tt> receives a specified <tt>sensitive_parameter</tt> ''only'' when it is indeed specified, because the <tt>#if</tt> changes the <tt>wrapped_template</tt> parameter name to « <tt>sensitive_parameterNULL</tt> » when <tt>sensitive_parameter</tt> is undefined. Note also that <tt>wrapped_template</tt> receives a specified and empty <tt>sensitive_parameter</tt> ''only'' when the wrapper template receives a specified and empty <tt>sensitive_parameter</tt>. Of course, one must make sure the suffixed <tt>wrapped_template</tt> parameter name is meaningless to the <tt>wrapped_template</tt> for this to work properly.

==See also==
*[[Help:Extension:ParserFunctions]]
*[[Help:Template]]
*[[Extension:Loops#.23forargs_.28Experimental.29|Extension:Loops#forargs]]

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