Template:If/doc

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 ("&lt;nowiki/>") could be used, without Template:If, to allow a lead-space (such as "&lt;nowiki/> 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 &lt;nowiki/>".

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


 * gives "". Notice the empty first parameter, seen as two adjacent vertical bars.


 * gives " p "
 * gives ""
 * gives ""
 * gives ""

Compare:
 * gives ""
 * gives ""
 * gives ""
 * gives ""
 * gives ""

For example, with a template named Template:Template name containing the code, entering the code   into an article gives "p", but   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: When the else-clause is indented to the next line, a newline (CR/LF) is added: To indent the else-clause, split an HTML-form comment, as " " with next line as " ". 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 " " to " " 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  (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 and  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 :
 * Result for :   "" &larr; 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 :
 * Result for : "" &larr; the comparison invoked { {P2}}.