Editing Template:If/doc

{{stack|{{Documentation subpage}}{{risky template}}}}
With this template the [[Help:parser function|parser functions]] of the [[mw:Help:Extension:ParserFunctions|ParserFunctions]] collection with names starting with "#if" can be used in a way such that they do not [[m:Help:Newlines_and_spaces#Stripping_on_expansion|strip spaces and newlines]] from the "then" and "else" part. Spaces still do not affect the outcome of the condition. Parameter 1 selects the if-type as "eq", "expr", "exist" or "error" (for #iferror), or empty "||" for a simple if-there (for #if). The template can be repeatedly nested 6 or 7 levels, one inside the other, because the outer-most is completed before running either the then/else inner levels.

This template can be substituted, when the expression or comparison will not change. Functionally, a lead-space character is stored, internally, as a simple blank character. Note, in many cases a null nowiki tag ("&lt;nowiki/>") could be used, without [[Template:If]], to allow a lead-space (such as "&lt;nowiki/>&nbsp;xx") anywhere, but the internal storage puts a 43-character marker for "&lt;nowiki/>" in string length. However, there is no extra expansion depth for a nowiki-tag, such as in trailing space, "zz&nbsp;&lt;nowiki/>".

==Usage==
'''Where p is output for a true statement and q is output for a false statement.'''

*<syntaxhighlight lang="text" inline>{{if||x| p | q }}</syntaxhighlight> gives "{{if||x| p | q }}". Notice the empty first parameter, seen as two adjacent vertical bars.
*<syntaxhighlight lang="text" inline>{{if|expr|2<3| p | q }}</syntaxhighlight> gives " p "
*<syntaxhighlight lang="text" inline>{{if|eq| u |u| p | q }}</syntaxhighlight> gives "{{if|eq| u |u| p | q }}"
*<syntaxhighlight lang="text" inline>{{if|exist| Help:Link | p | q }}</syntaxhighlight> gives "{{if|exist| Help:Link | p | q }}"
*<syntaxhighlight lang="text" inline>{{if|error|{{#expr:x}}| p | q }}</syntaxhighlight> gives "{{if|error|{{#expr:x}}| p | q }}"

Compare:
*<syntaxhighlight lang="text" inline>{{#if:x| p | q }}</syntaxhighlight> gives "{{#if:x| p | q }}"
*<syntaxhighlight lang="text" inline>{{#ifexpr:2<3| p | q }}</syntaxhighlight> gives "{{#ifexpr:2<3| p | q }}"
*<syntaxhighlight lang="text" inline>{{#ifeq: u |u| p | q }}</syntaxhighlight> gives "{{#ifeq: u |u| p | q }}"
*<syntaxhighlight lang="text" inline>{{#ifexist: Help:Link | p | q }}</syntaxhighlight> gives "{{#ifexist: Help:Link | p | q }}"
*<syntaxhighlight lang="text" inline>{{#iferror:{{#expr:x}}| p | q }}</syntaxhighlight> gives "{{#iferror:{{#expr:x}}| p | q }}"

For example, with a template named ''Template:Template name'' containing the code <syntaxhighlight lang="text" inline>{{#ifeq: {{{parameter}}} |u| p | q }}</syntaxhighlight>, entering the code <syntaxhighlight lang="text" inline>{{Template name|parameter=u}}</syntaxhighlight> into an article gives "p", but <syntaxhighlight lang="text" inline>{{Template name|parameter=v}}</syntaxhighlight> gives "q".

The text of either the then-clause or else-clause is only processed and expanded when triggered. Hence, any templates in use are only expanded once the then-clause or else-clause is matched, otherwise they are skipped as merely paired braces, "&#123;&#123;" with "}}".

===Indentation===
If indenting the markup, care must be taken to avoid extra newlines when indenting "}}" on the next line. An extra bar pipe "|" can be added after the else-clause to complete that text, and allow "}}" to then be placed anywhere without adding a newline into the else-clause. For example:
:::* <syntaxhighlight lang="text">{{if|{{{1|}}}
    |then found parameter 1|else no parameter 1| ← extra "|" ends else-clause
}}</syntaxhighlight >
When the else-clause is indented to the next line, a newline (CR/LF) is added:
:::* <syntaxhighlight lang="text">{{if|{{{1|}}}
    |then found parameter 1 ← extra newline here
    |else noparameter 1|
}}</syntaxhighlight>
To indent the else-clause, split an [[HTML]]-form comment, as "<code>&lt;!--</code>" with next line as "<code>--&gt;|else...</code>". Unless each then-clause and else-clause is carefully tested, to watch for extra newlines, then the results are likely to cause broken lines, with extra line breaks for each newline. For that reason, a global edit with simple search-and-replace of "<code>{#if:</code>" to "<code>{if||</code>" is likely to leave newline problems, wherever the original markup was wrapped to indent either the else-clause or "}}" of each if-structure. Indenting the then-clause is not a problem.

==Performance considerations==
Because [[Template:If]] must prepare the parameters for <code>#if, #ifeq, #ifexpr,</code> (etc.) there is a slight overhead when using it. Each nested use adds 5 levels to the template expansion depth, so 7 nested if-templates would use 35 levels (5*7) of the [[m:Help:Expansion_depth|41-level limit]].

'''Using P-if syntax:''' A similar if-structure can be coded without '''Template:If''', by using the {{tlx|P1}} and {{tlx|P2}} templates in a "P-if" structure. [[Template:P1]] always returns parameter 1, and P2 returns the 2nd. So, a comparison of 4 versus 3 can be coded using P-if syntax form:
:: Example of <code>#ifexpr</code>: <syntaxhighlight lang="text" inline>"{{P{{#ifexpr: 4 > 3|1|2}}| then 4 greater| else 4 lower}}"</syntaxhighlight>
:: Result for <code>#ifexpr</code>: &nbsp; "{{P{{#ifexpr: 4 > 3|1|2}}| then 4 greater| else 4 lower}}" &larr; ''the comparison invoked'' {<nowiki/>{P1}}.
The expansion depth of a P-if is only 4 levels deep, but nesting of the then-clause or else-clause might be more confusing than using nested levels of Template:if.

:: Example of <code>#if</code>: <syntaxhighlight lang="text" inline>"{{P{{#if:{{{2|x}}}|1|2}}| then {2} set| else {2} empty}}"</syntaxhighlight>
:: Result for <code>#if</code>: "{{P{{#if:{{{2|}}}|1|2}}| then {2} set| else {2} empty}}" &larr; ''the comparison invoked'' {<nowiki/>{P2}}.

==See also==
*[[m:Template:If]]
*[[Template:Ifnotempty]] - for simplicity and to reduce the limitations due to the expansion depth limit, this is a separate template corresponding to #if only.
*[[Template:Ifexist not redirect]] - useful because <syntaxhighlight lang="text" inline>{{if|exist| testLink | yesRtn | noRtn }}</syntaxhighlight> returns yesRtn if testLink is a redirect
*[[Help:Conditional expressions]]
<includeonly>{{Sandbox other||
[[Category:If-then-else templates]]
}}</includeonly>