Template:If/doc

This is a documentation subpage for Template:If (see that page for the template itself).
It contains usage information, categories and other content that is not part of the original template page.

With this template the parser functions of the ParserFunctions collection with names starting with "#if" can be used in a way such that they do not 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 ("<nowiki/>") could be used, without Template:If, to allow a lead-space (such as "<nowiki/> xx") anywhere, but the internal storage puts a 43-character marker for "<nowiki/>" in string length. However, there is no extra expansion depth for a nowiki-tag, such as in trailing space, "zz <nowiki/>".

Usage

Where p is implemented in affirmation of the statement and q is implemented in negation of the statement

{{if||x| p | q }} gives " p ". Notice the pair of vertical bars surrounding the empty first parameter.

{{if|expr|2<3| p | q }} gives " p "
{{if|eq| u |u| p | q }} gives " p "
{{if|exist| Help:Link | p | q }} gives " q "
{{if|error|{{#expr:x}}| p | q }} gives " p "

Compare:

{{#if:x| p | q }} gives "p"
{{#ifexpr:2<3| p | q }} gives "p"
{{#ifeq: u |u| p | q }} gives "p"
{{#ifexist: Help:Link | p | q }} gives "q"
{{#iferror:{{#expr:x}}| p | q }} gives "p"

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, "{{" 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:

{{if|{{{1|}}}
|then found parameter 1|else no parameter 1| ← extra "|" ends else-clause
}}

When the else-clause is indented to the next line, a newline (CR/LF) is added:

{{if|{{{1|}}}
|then found parameter 1 ← extra newline here
|else noparameter 1|
}}

To indent the else-clause, split an HTML-form comment, as "|else...". 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 "{#if:" to "{if||" 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 #if, #ifeq, #ifexpr, (etc.) there is a slight overhead when using it. It has been timed to run at speeds of 290 instances per second, so 29 uses would take only 0.1 s, a tenth of a second to run, to make 29 comparisons. Each nested use adds 5 levels to the template expansion depth, so 7 nested if-templates would use 35 levels (5*7) of the 41-level limit.

Using P-if syntax: A similar if-structure can be coded without Template:If, by using the {P1} and {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 #ifexpr: "{{P{{#ifexpr: 4 > 3|1|2}}| then 4 greater| else 4 lower}}"

Result for #ifexpr: " then 4 greater" ← the comparison invoked {{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 #if: "{{P{{#if:{{{2|x}}}|1|2}}| then {2} set| else {2} empty}}"

Result for #if: " else {2} empty" ← the comparison invoked {{P2}}.

Usage

Performance considerations

See also