Editing Help/Templates (section)

== Evaluation process == <!--T:198-->
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<translate><!--T:200--> Generally speaking, template parameters are substituted into the template after tokenization, but as is.</translate>
<translate><!--T:201--> They are not evaluated until they are used.</translate>

<translate><!--T:202--> This has a few consequences:</translate>

# <translate><!--T:213--> If you have a <tvar name=1><code>Template:Start</code></tvar> containing <tvar name=2><code><nowiki></nowiki></code></tvar>, and put <tvar name=5><code><nowiki></nowiki></code></tvar> on a page, ''mytemplate'' isn't transcluded, because tokens like "|" cannot be added by a template and keep their special meaning in templates.</translate> <translate><!--T:203--> You can still use templates to control the name of a parameter or template, but you cannot split a template call amongst multiple templates.</translate>
# '''<translate><!--T:204--> Dead-code elimination:</translate>''' <translate><!--T:214--> If you make a template call like <tvar name=1><code><nowiki> }}</nowiki></code></tvar>, and <code>Template:Foo</code> does not contain <tvar name=2><nowiki>}</nowiki></tvar>, then the <tvar name=3><code>DISPLAYTITLE</code></tvar> is not used, since it is only evaluated when needed, and there is no parameter to substitute it into, so it is never evaluated.</translate> <translate><!--T:215--> This usually comes into play when using <tvar name=3></tvar>, and can be especially noticed when used in combination with the <tvar name=2><code><nowiki>int:</nowiki></code></tvar> magic word that varies by user language.</translate> <translate><!--T:318--> This isn't perfect, and in some cases even if the result of expanding a template is not used (because it is part of an if statement condition, for example), the process of evaluating it can still have [[w:Side effect (computer science)|side effects]].</translate> <translate><!--T:319--> For example, any  produced or other templates used will still be added to <tvar name=2>[[Special:WhatLinksHere]]</tvar> even if they are not displayed.</translate>

<translate><!--T:380--> Template parameters are [[w:pass by value|pass by value]], which means a template cannot modify its arguments.</translate>
<translate><!--T:381--> Parameters are treated as [[w:Associative array|associative array]], and parameter names are evaluated before parameter values.</translate>
<translate><!--T:382--> If the same parameter name is given more than once (either as named or unnamed), only the last instance is used, and the page is added to <tvar name=1>[[:Category:Pages using duplicate arguments in template calls]]</tvar>.</translate> 

<translate><!--T:216--> Template calls starting with the magic word <tvar name=1><code>subst:</code></tvar> or <tvar name=2><code>safesubst:</code></tvar> are evaluated in a separate first pass that only happens at save time, along with <tvar name=3><nowiki>~~~~</nowiki></tvar> and links using the [[<tvar name=4>Special:MyLanguage/pipe trick</tvar>|pipe trick]].</translate>
<translate><!--T:217--> If they cannot be evaluated during the first pass, <tvar name=1><code>subst:</code></tvar> calls are ignored, and <tvar name=2><code>safesubst:</code></tvar> are treated as if a normal template.</translate>

<translate><!--T:205--> Many but not all parser functions, parser tags and trancluded special pages are not directly included like templates but instead are replaced by a "strip marker".</translate>
<translate><!--T:206--> This means you cannot manipulate the results with parser functions like <tvar name=1>padleft:</tvar> or similar functions from extensions, as they see the strip marker instead of the result of the parser function.</translate>

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=== Recursion in templates === <!--T:149-->
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<translate><!--T:124--> Including a template in itself won't throw MediaWiki into infinite recursion.</translate>
<translate><!--T:125--> MediaWiki will stop the recursion with the template's name in bold.</translate>
<translate><!--T:126--> For example, if the content of Template:Aaaa is <tvar name=code><code>a <nowiki></nowiki> z</code></tvar>, it'll display "<tvar name=result>a a <span class="error">Template loop detected:  '''Template:Aaaa'''</span> z z</tvar>".</translate>

<translate><!--T:219--> This safeguard precludes a potentially useful template idiom where a template self-normalizes its own calling arguments.</translate>
<translate><!--T:220--> In this forbidden example <tvar name=1><code>template:d</code></tvar> can either be called <tvar name=2><code><nowiki></nowiki></code></tvar> or <tvar name=3><code><nowiki></nowiki></code></tvar>.</translate>
<translate><!--T:221--> If called in the first manner, it recurses into itself with the second argument structure (obtained using string parser functions), which then follows a unified processing path.</translate>

<syntaxhighlight lang="wikitext">
}|}|0|4}}|m=}|4|2}}|d=}|6|2}}}}|<!-- processing path with arguments y,m,d regardless of original call pattern -->}}
</syntaxhighlight>

<translate><!--T:222--> If <tvar name=1><code>template:d</code></tvar> is modified to recurse into <tvar name=2><code>template:d/2</code></tvar> and <tvar name=3><code>template:d/2</code></tvar> is an ''identical manual copy'' of <tvar name=4><code>template:d</code></tvar> this idiom works fine as the self-recursion safeguard operates dynamically and not statically.</translate>

<translate><!--T:223--> A feasible way for the MediaWiki software to loosen the self-recursion rule would be to require that each recursive call have a distinct argument count from all previous active calls, at most once recursing with the argument count non-decreasing.</translate>
<translate><!--T:224--> That would provide a strong guarantee against infinite self-recursion while enabling useful idioms such as the one described here in a flexible manner.</translate>

<translate><!--T:225--> If the processing path is of low complexity, a simple solution using only one template is to handle each calling convention on a separate if/else branch, duplicating the logic of the processing path within each case.</translate>
<translate><!--T:226--> If the processing path is more complex, each call-structure case can delegate to an implementation template with a unified call structure which provides the final template behaviour.</translate>

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=== Tables in parameters === <!--T:227-->

<!--T:240-->
Since the pipe character (<tvar name=1><code>|</code></tvar>) and equality sign (<tvar name=2><code>=</code></tvar>) have different meanings in template calls and wikitables, in order to use table markup in the value of a template parameter one generally needs to "escape" those characters (i.e., protect them from interpretation as template markup) using special sequences:
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* <translate><!--T:255--> the built-in  <tvar name="2"><code><nowiki></nowiki></code></tvar> provides an "escaped" version of <tvar name="3"><code>|</code></tvar> since MediaWiki 1.24</translate>
* <translate><!--T:242--> the built-in magic word <tvar name="1"><code><nowiki></nowiki></code></tvar> provides an "escaped" version of <tvar name="2"><code>=</code></tvar> since MediaWiki 1.39</translate>

<translate><!--T:251--> Before the introduction of these magic words, many wikis used templates to accomplish the same things.</translate>
<translate><!--T:252--> On such a wiki, the magic words take precendence over the same-named templates.</translate>

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====Example table==== <!--T:243-->
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{| class=wikitable
!A!!B!!C
|-
|A1||B1||C1
|-
|A2||B2||C1
|}

'''<translate><!--T:244--> Table code:</translate>'''

<syntaxhighlight lang=wikitext>
{| class=wikitable
!A!!B!!C
|-
|A1||B1||C1
|-
|A2||B2||C1
|}
</syntaxhighlight>

'''<translate><!--T:245--> Escaped table code:</translate>'''

<syntaxhighlight lang=wikitext>
 classwikitable
!A!!B!!C
-
A1B1C1
-
A2B2C2
}
</syntaxhighlight>

<translate><!--T:246--> Note that the first left-brace (<tvar name="1"><code>{</code></tvar>) is interpreted as a literal left-brace character because it is immediately followed by the <tvar name="2"><code><nowiki></nowiki></code></tvar> magic word.</translate>
<translate><!--T:247--> Similarly, the last right-brace (<tvar name="1"><code>}</code></tvar>) is interpreted as a literal right-brace character because it is immediately preceeded by the same magic word.</translate>
<translate><!--T:248--> However, in some cases these brace characters do cause problems, so some wikis provide templates for escaping these characters, as well:</translate>

* <translate><!--T:249--> the template call <tvar name="1"><code><nowiki></nowiki></code></tvar> might provide an "escaped" version of <tvar name="2"><code>{</code></tvar></translate>
* <translate><!--T:250--> the template call <tvar name="1"><code><nowiki></nowiki></code></tvar> might provide an "escaped" version of <tvar name="2"><code>}</code></tvar></translate>

<translate><!--T:253--> Some wikis go even further and provide other convenience templates like <tvar name="1"> (<code><nowiki>{|</nowiki></code>),  (<code>|}</code>),  (<code>||</code>)</tvar>.</translate>
<translate><!--T:254--> On such a wiki, the code can be simplified a bit to this form:</translate>

<syntaxhighlight lang=wikitext>
 classwikitable
!A!!B!!C
-
A1B1C1
-
A2B2C2

</syntaxhighlight>

<translate>