View source for Manual:Template expansion process

The '''process of expanding templates''' includes expansion of [[Help:Templates|templates]], [[Help:Parser function|parser function]]s,  [[Help:Variable|variable]]s (on this page collectively referred to as '''''templates''''', in italics), and [[Help:Templates#Parameters|template parameters]] ('''''tplargs''''') is done in [[Help:substitution|substitution]] and also as the first steps in page rendering. It consists of two phases: the creation of an XML parse tree followed by the production of the expanded wikitext. After expansion, two more steps in page rendering are producing HTML and (in the user's browser) rendering the page.

== Overview ==

'''Expansion''' of [[Help:template|template]]s, [[Help:parser function|parser function]]s,  [[Help:variable|variable]]s, and template parameters is done during [[Help:substitution|substitution]] and as first step in page rendering. Expansion is needed because:
* The wikitext may contain template calls such as <tt><nowiki>{{…|…|…=…}}</nowiki></tt> and parser functions such as <tt><nowiki>{{#…:…|…|…}}</nowiki></tt>, which should be recognized and replaced by their content.
* The wikitext may contain variable names such as <tt><nowiki>{{{…}}}</nowiki></tt> or <tt><nowiki>{{{…|…}}}</nowiki></tt>, which need to be identified and replaced by the appropriate values.
Each ellipsis (<tt>…</tt>) may contain other such structures.
The wikitext is therefore parsed with respect to such patterns, and also double square brackets <tt><nowiki>[[…|…]]</nowiki></tt> which delimit links, to determine the limits of the parts of the wikitext that should be substituted.

Expansion of a given level must do two things:
# Delimit the structures that are to be replaced, by determining the corresponding begin-, middle- and end-markers.
# Determine the appropriate replacement wikitext.
This needs to be done while taking into account that the expanded structures and/or the substitution file{{clarify|reason=file??}} may themselves contain templates, variable names or links, and may therefore be nested within one another. Pipes, semicolons,{{clarify|reason=should this actually be colons? -- see Talk}} and equals signs are, as applicable, taken as belonging to the innermost structure in which they occur, and never as part of an outer structure.

The first-level expansion starts with the outermost structures. After fully determining which pairs and triples of braces (or double square brackets) belong together, and which pipes and equals signs belong to the same structure, etc.,{{clarify|reason=what exactly is this etc referring to?}} the parser has identified the outermost structure delimiters and the chunks of wikitext (possibly containing other structures) that are inserted in those structures, between pipes, equals signs or semicolons{{clarify|reason=colons? -- see Talk}} attached to that level. Expansion of the outermost structures now starts.
# The wikitext for the '''name''' is (second level-)expanded for all parser functions and templates, template parameters, and variables.
# It is then determined for which '''parameter values''' the wikitext needs to be (second level-)expanded.
# In the case of templates, in the template body itself (i.e., contents of pages in the Template [[Help:Namespaces|namespace]]):
#* The wikitexts for the names of '''formal parameters''' (''tplargs'') is (second level-)expanded.
#* Each formal parameter is replaced by the wikitext expansion of its value, or (when undefined) the '''default value''' (if specified) is used and (second level-)expanded. Any undefined formal parameter having no default value specified is left unchanged (resulting in something like <tt>{{{1}}}</tt>).
#* The template body is itself (second level-)expanded.

The expansion of a first-level structure therefore relies on the expansion of its second-level substructures, and so on, recursively.
Note that since the structure parsing is done before the expansion, structure markers (pipes, semicolons,{{clarify|reason=colons? -- see Talk}} and equals signs) found in the wikitext that ''result'' from an expansion cannot be taken into account as markers for the embedding structure. A template may therefore return wikicode that would by itself be expanded when inserted directly in a page; this feature is exploiting by templates such as {{template|template}}.

The result of expansion is shown by [[Special:ExpandTemplates]] (provided by [[Extension:ExpandTemplates]]). For studying the expansion step this intermediate result is more useful than the final rendered page, especially in the case of tables or when paying attention to newlines.

In the case of templates, most of the time the wikitext for the name is a simple character string and the parameter values are simple wikitext that need not be recursively expanded. These cases are discussed in [[Help:Template]].
* Special cases for the structure analysis (unbalanced braces, etc.) are discussed below.
* Special cases of name expansion are discussed in [[Manual:Advanced templates]].
* Particularly complicated cases of recursive expansion are discussed in [[Manual:Recursive conversion of wikitext]].

==XML parse tree==
The wikitext is parsed to an XML parse tree, also called DOM tree. The tree code is shown with the option "Show XML parse tree" of [[Special:ExpandTemplates]] (see [[mw:Extension:ExpandTemplates#XML parse tree]]). The parse tree of a page always starts with <code><root></code> and ends with <code></root></code>. These tags are not shown in the examples.

Parsing is done with respect to pairs of double braces {{cnw|{|{..}|}|}} delimiting a ''template'', and pairs of triple braces {{cnw|{|{|{..}|}|}|}} delimiting a ''tplarg''. If double opening braces are followed by triple closing braces {{cnw|{|{..}|}|}|}} or conversely {{cnw|{|{|{..}|}|}}, this is taken as delimiting a ''template'', with one left-over brace outside it taken as plain text. For any pattern of braces this defines a set of ''templates''  and ''tplargs'' such that any two are either separate or nested (not overlapping).

Unmatched double rectangular closing brackets {{cnw|]|]}} can be in a ''template''  or ''tplarg'', but unmatched double rectangular opening brackets {{cnw|[|[}} cannot. Unmatched double or triple closing braces inside a pair of double rectangular brackets {{cnw|[|[|..}|}..|]|]|}} / {{cnw|[|[|..}|}|}..|]|]|}} are treated as plain text. Other formulation: in ambiguity between ''template'' or ''tplarg'' on one hand, and a link on the other hand, the structure with the rightmost opening takes precedence, even if this is the opening of a link without any closing, so not producing an actual link.

Examples (''title'' is in general a part of a ''template'' or ''tplarg'', see below, but here the whole of it):
----
{{cnw|{|{ {|{|{ }|} }|}|}|}}

{{cnw|{|{|{ {|{ }|}|} }|}|}}

{{cnw|[|[ {|{ ]] }|}|}}

{{cnw|[|[ {|{|{ ]] }|}|}|}}

{{cnw|{|{ [|[ }|}|}}

{{cnw|{|{ [|[ }|} ]]}}

{{cnw|{|{ [|[ }|} ]] }|}|}}

{{cnw|{|{|{ [|[ }|}|}|}}

{{cnw|{|{|{ [|[ }|}|} ]]}}

{{cnw|{|{|{ [|[ }|}|} ]] }|}|}|}}
----
give the parse trees:
----
{{cnw|<template><title> {<template><title> </title></template> </title></template>}|}}

{{cnw|{<template><title> <template><title> </title></template>} </title></template>}}
 
{{cnw|[|[ <template><title> ]] </title></template>}}

{{cnw|[|[ <tplarg><title> ]] </title></tplarg>}}

{{cnw|{|{ [|[ }|}|}}

{{cnw|{|{ [|[ }|} ]]}}

{{cnw|<template><title> [|[ }|} ]] </title></template>}}

{{cnw|{|{|{ [|[ }|}|}|}}

{{cnw|{|{|{ [|[ }|}|} ]]}}

{{cnw|<tplarg><title> [|[ }|}|} ]] </title></tplarg>}}
----


In the case of more than three opening braces the last three are assumed to belong to a ''tplarg'', unless there is no matching triple of closing braces, in which case the last two opening braces are assumed to belong to a ''template''.

Examples:
----
{{cnw|{|{|{|{ }|}|}|}|}}

{{cnw|{|{ {|{ }|}|}|}|}}

{{cnw|{|{|{|{ }|} }|}|}}

{{cnw|{|{|{|{|{ }|}|}|}|}|}}

{{cnw|{|{|{ {|{ }|}|}|}|}|}}

{{cnw|{|{|{|{|{ }|} }|}|}|}}
----
give the parse trees:
----
{{cnw|{<tplarg><title> </title></tplarg>}|}}

{{cnw|<template><title> <template><title> </title></template></title></template>}}

{{cnw|<template><title><template><title> </title></template> </title></template>}}

{{cnw|<template><title><tplarg><title> </title></tplarg></title></template>}}

{{cnw|<tplarg><title> <template><title> </title></template></title></tplarg>}}

{{cnw|<tplarg><title><template><title> </title></template> </title></tplarg>}}
----


Thus, when nesting two double-brace structures, of which the outer structure does not have any pipe, at least one blank space is needed to avoid the pair of inner three braces to be interpreted as a triple-brace structure, and the outer two braces as plain text. This space does not affect the expanded wikitext because in the next processing phase spaces are stripped from the start and end of ''title''.

===Pipes===
''Templates'' and ''tplargs'' are decomposed in the same way, with pipes as separator, even though eventually any parts in a ''tplarg'' after the first (the [[Help:Parameter default|parameter default]]) are ignored, and an equals sign in the first part is treated as plain text. Pipes inside inner ''templates'' and ''tplargs'', or inside double rectangular brackets within the ''template'' or ''tplargs'' are not taken into account in this decomposition. The first part is called '''''title'''''<ref>In the case of a parser function, ''title'' (function name, colon, and first parameter) should not be confused with just the function name.</ref>, the other parts are simply called '''''parts'''''. If a ''part'' has one or more equals signs in it, the first equals sign determines the division into ''name'' = ''value''. Equals signs inside inner ''templates'' and ''tplargs'', or inside double rectangular brackets within the ''part'' are not taken into account in this decomposition. ''Parts'' without equals sign are indexed 1, 2, .., given as attribute in the <name> tag. 

Examples:
----<pre>
{{ | | [[ | | ]] }}

{{{ | | [[ | | ]] }}}</pre>
----
give the parse trees:
----
<syntaxhighlight lang="xml"><template><title> </title><part><name index="1"/><value> </value></part><part><name index="2"/><value> [[ | | ]] </value></part></template>

<tplarg><title> </title><part><name index="1"/><value> </value></part><part><name index="2"/><value> [[ | | ]] </value></part></tplarg></syntaxhighlight>
----
and
----<pre>
{{a|b|c=d|e}}

{{{a|b|c=d|e}}}</pre>
----
give the parse trees:
----
<syntaxhighlight lang="xml"><template><title>a</title><part><name index="1"/><value>b</value></part><part><name>c</name>=<value>d</value></part><part><name index="2"/><value>e</value></part></template>

<tplarg><title>a</title><part><name index="1"/><value>b</value></part><part><name>c</name>=<value>d</value></part><part><name index="2"/><value>e</value></part></tplarg></syntaxhighlight>
----
This is applied recursively: the wikitexts a, b, c, d, and e can themselves contain similar structures, and are then represented in the tree by similar code. 

The parse tree does not distinguish between variables, parser functions, and templates. The <code><template></code> tag refers to any of them. The tree is independent of the existence of a variable, parser function, or template with the given name, so when trying out with Special:ExpandTemplates how the first parsing phase works we do not have to bother using names of actual variables, parser functions, and templates.

===Brackets===
Like already demonstrated,
<code><nowiki>{{a|b[[c|d|e=f]]g}}</nowiki></code> gives <syntaxhighlight lang="xml" inline><template><title>a</title><part><name index="1"/><value>b[[c|d|e=f]]g</value></part></template></syntaxhighlight>

However,

<code><nowiki>{{a|b[c|d|e=f]g}}</nowiki></code> gives <syntaxhighlight lang="xml" inline><template><title>a</title><part><name index="1"/><value>b[c</value></part><part><name index="2"/><value>d</value></part><part><name>e</name>=<value>f]g</value></part></template></root></syntaxhighlight>

and

<code><nowiki>{{a|b<c d=e>f</c>g}}</nowiki></code> gives <syntaxhighlight lang="xml" inline><template><title>a</title><part><name>b&amp;lt;c d</name>=<value>e&amp;gt;f&amp;lt;/c&amp;gt;g</value></part></template></syntaxhighlight>

This demonstrates that double rectangular braces shield pipes and equals signs inside from being considered part of an outer structure, but single rectangular braces and angle brackets do not. (Note also that angle brackets which are not part of the XML tags are written <code>&amp;lt;</code> and <code>&amp;gt;</code> to avoid confusion.)

==Expanded wikitext==
The second step is the conversion of the parse tree to expanded wikitext. The result of the first and second step combined is shown with [[Special:ExpandTemplates]], and with the <tt>action=raw&templates=expand</tt> parameters in the URL ([{{fullurl:{{FULLPAGENAME}}|action=raw&templates=expand&ctype=text/css}} like this]); also the wikitext  <code><nowiki>{{#tag:nowiki|...}}</nowiki></code> expands to the expansion of the wikitext at the place of the dots, with the whole inside nowiki tags; hence it is displayed as this expansion of the wikitext at the place of the dots.

The wikitext can be considered a [[w:String (computer science)|string]] [[w:Expression (programming)|expression]], consisting of [[w:String literal|string literal]]s without delimiters, and string [[w:Subroutine|functions]] (the brace structures), without operators: the only operation is  [[w:concatenation|concatenation]] by [[wikt:juxtaposition|juxtaposition]]. They have no [[w:side-effect (computer science)|side effect]]s, except if [[W:wp:template limits|template limits]] are exceeded. [[w:Exception handling|Exception handling]] of a function does not stop evaluation of the expression, the function just produces a special return value; e.g. <code>#expr</code> may produce a message instead of a number; when the <code>#ifexist</code> limit is exceeded the else-value is returned; when the expand limit has been reached a brace structure is not expanded but returned as text.

There is no feature for entering a parse tree, to see what expanded wikitext it produces. However, to study what expanded wikitext a subtree of the parse tree of a given wikitext produces, one can identify the corresponding part of the wikitext, and use that as input of Special:ExpandTemplates etc. The only context sensitivity is represented in a possible attribute <code>lineStart="1"</code> in the <code><template></code> tag.

The first expansion step is the expansion of ''title'' (the wikitext before the first pipe).<ref>''Title'' is expanded as a single item, therefore the condition in a call of a branching function with not even a then-part is expanded, even though expansion of the function name would be sufficient to conclude that the result is the empty string, regardless of the condition.</ref> The ''title'' of a ''tplarg'' is always expanded, even in cases where the parameter cannot be defined: if the expansion is for rendering the template itself, and if the template is called without parameters. 
  
===Distinction between variables, parser functions, and templates===
The distinction between variables, parser functions, and templates is made in <tt>function braceSubstitution</tt> in <tt>[[mw:Manual:Parser.php|Parser.php]]</tt>.

*If there are no ''parts'', and ''title'' is a variable name (or <code>safesubst:</code> followed by a variable name), then we have a variable.
*Otherwise, if ''title'' contains a colon and the part before it forms the name of a parser function  (or <code>safesubst:</code> followed by the name of a parser function), then we have a parser function with as first parameter the expanded wikitext of the part of ''title'' after the colon, while the remaining parameter names (if any) and  values are the expanded wikitexts of the names and values of the ''parts''. Some parameters of some parser functions expect plain text as parameters instead of name/value combinations. In that case, if there was a "=" in the text, this text is reconstructed from the supposed name, the "=", and the supposed value. For example, <syntaxhighlight lang="xml" inline><template><title>#tag:math</title><part><name>a</name>=<value>5</value></part></template></syntaxhighlight> gives the expanded wikitext <code><nowiki><math>a=5</math></nowiki></code>. Parameter values are assigned to parameters in two passes. Therefore a parameter name in a template can depend on the value of another parameter of the same template, regardless of the order in which they are specified in the template call, for example, using {{timc|ppp}}, <code><nowiki>{{ppp|p=q|q=r}}</nowiki></code> and even <code><nowiki>{{ppp|q=r|p=q}}</nowiki></code> gives <code>{{ppp|q=r|p=q}}</code>, but using {{timc|tvvv}}, <code><nowiki>{{tvvv|p=q|q=r|r=s}}</nowiki></code> gives <code>{{tvvv|p=q|q=r|r=s}}</code>.
*Otherwise, if ''title'' is a valid template name (in particular this excludes a name starting with "#") or <code>safesubst:</code> followed by the name of an existing template), then we have a template. The representation of the ''parts'' corresponds to the distinction between implicitly numbered parameters (with ''index'' being the number) and named parameters.
*Otherwise we have plain text.

In the cases mentioned, <code>safesubst:</code> is ignored.

There are no variable names with a colon in it.<ref>For some variables there is a strongly related  parser function with the same name. Therefore some overviews of variables and/or parser functions do not strictly distinguish between them.</ref> However, some variable names, e.g. PAGENAME, are also the name of a parser function. Thus:
*If ''title'' is "PAGENAME" and there are no ''parts'', and then we have a variable.
*If ''title'' is "PAGENAME" and there are ''parts'' then it is a reference to [[Template:PAGENAME]].
*If ''title'' is "pAGENAME" then it is anyway a reference to Template:PAGENAME.
*If ''title'' starts with "PAGENAME:" then we have a parser function.
*If ''title'' starts with "pAGENAME:" then it is a reference to a template.
Examples:
*{{xpdoc|PAGENAME}}
*{{xpdop3c|PAGENAME|}}
*{{xpdoc|pAGENAME}}
*{{xpdoc|PAGENAME:}}
*{{xpdop3c|PAGENAME:|}}
*{{xpdoc|PAGENAME:help:def}}
*{{xpdop3c|PAGENAME:help:def|}}
*{{xpdoc|pAGENAME:help:def}}

Similarly for NUMBEROFPAGES, but a difference is that as parser function with anything else than R as parameter (after the colon), it is equivalent with the variable:
*{{xpdoc|NUMBEROFPAGES}}	
*{{xpdop3c|NUMBEROFPAGES|}}
*{{xpdoc|NUMBEROFPAGES:}}	
*{{xpdoc|NUMBEROFPAGES:R}}	
*{{xpdoc|NUMBEROFPAGES:S}}	
*{{xpdop3c|NUMBEROFPAGES:R|}}

Based on the XML tree, expansion starts with evaluating ''title'' for the outermost structures, and in the case of a parser function, the separation between the name and the parameter after the colon.

==Expansion of ''names'' and ''values''==
Which ''names'' and ''values'' are expanded can be demonstrated by putting a template call in each, and checking the list of templates used in the edit page.

For most parser functions all ''names'' and ''values'' are expanded, regardless of what is relevant for the result. The branching functions (#if, #ifeq, #iferror, #ifexist, #ifexpr, #switch) are exceptions: for #if, #iferror, #ifexist, #ifexp, only the ''part'' that is applicable is expanded; for #ifeq the first and the applicable ''part'' are expanded; for #switch, expanded are the ''names''  up to and including the match (or all if there is no match), and the ''value'' in the case of a match or if there is no match, the default, if any. 

All ''names'' in a template call are expanded, and the ''titles'' of the ''tplargs'' in the template body, after which it is determined which ''values'' must be expanded, and for which ''tplargs'' in the template body the first ''part'' (default).

In the case of a ''tplarg'', any ''parts'' beyond the first are never expanded. The possible ''name'' and the ''value'' of the first ''part'' is expanded if the ''title'' does not match a ''name'' in the template call.

==Braces, pipes, and equals signs produced by expansion==
Braces, pipes, and equals signs which are produced by expansion are taken as just characters, not parts of structures, since evaluations happen after the production of the XML tree. A colon does not affect the XML tree anyway, the one after the name of a parser function is just part of ''title''.

For example, using
*[[Template:T1demo]] containing "start-{{{1}}}-end"
*[[Template:1=]] containing "1={{{1}}}"
*[[Template:p1=x]] containing "{{{1=x}}}"

we have:

*<nowiki>{{t1demo|{{{|1=x}}}}}</nowiki> gives {{t1demo|{{{|1=x}}}}}
*<nowiki>{{t1demo|{{#if:x|1=x}}}}</nowiki> gives {{t1demo|{{#if:x|1=x}}}}
*<nowiki>{{t1demo|{{1=|x}}}}</nowiki> gives {{t1demo|{{1=|x}}}}
*<nowiki>{{t1demo|{{1=|x}}=y}}</nowiki> gives {{t1demo|{{1=|x}}=y}}
*<nowiki>{{p1=x|{{{|1=x}}}=y}}</nowiki> gives {{p1=x|{{{|1=x}}}=y}}
*<nowiki>{{p1=x|{{1=|x}}=y}}</nowiki> gives {{p1=x|{{1=|1=x}}=y}}

Compare:
*{{xpd|1=1=|2=x}}
*<nowiki>{{t1demo|1=x}}</nowiki> gives {{t1demo|1=x}}
*<nowiki>{{t1demo|1=x=y}}</nowiki> gives {{t1demo|1=x=y}}

The examples also show that a parameter name "1=x", although impractical, works.

An example of an equals sign in a parameter value not being interpreted in a parameter definition:
*<nowiki>{{t2a1|1=x=y}}</nowiki> (using [[Template:t2a1]] containing "<code><span style="background-color:#c8f0ff; padding:0 3px 2px 3px;">{{msgnw:t2a1}}</span></code>" and [[Template:T2demo]] containing start-{{{1}}}-middle-{{{2}}}-end) gives {{t2a1|1=x=y}}.

==Balance==
Any wikitext is of the form <b>)(*(</b> or <b>)*)(</b>, where <b>*</b> represents the part from the start of the first structure to the end of the last, and each parenthesis represents zero or more sets of double or triple braces.{{clarify|reason=what in the world is this supposed to mean??}} If there is no such set, neither something similar for double rectangular brackets, then the wikitext is balanced.{{clarify|reason=seems to be saying that only something of the form * would be balanced, which doesn't at all explain the concept -- need examples}}

If a wikitext is unbalanced then inserting that wikitext by hand or by [[Help:Substitution|substitution]] into another wikitext, or concatenating the two, may change some of the structures or add new ones, perhaps unintentionally.{{clarify|reason=again, examples would be nice}} If one wikitext is [[Help:Transclusion|transcluded]] into another, there is no such complication.

==Sensitivity to missing or excess braces==
A wikitext with various pairs of double braces, e.g. <nowiki>{{</nowiki>[[Help:ParserFunctions#.23switch:|#switch:]]<nowiki>r|p=q|r={{tc}}|s=t}}</nowiki> (and what is worse, also a more complicated one) is, of course, very sensitive to missing or excess braces. In the case of missing opening braces of {{tim|tc}}, i.e. "<nowiki>tc}}</nowiki>", its closing braces are taken as closing braces of #switch, so all remaining wikitext intended to be inside the switch is now outside, and therefore shown independent of a switch index match. The latter also happens in the case of excess closing braces of tc: <nowiki>"{{tc}}}}"</nowiki>. The intended closing braces of the switch are taken as plain text.

In the case of missing closing braces in <nowiki>"{{tc}}", i.e. "{{tc"</nowiki>, the intended closing braces of the switch are taken as closing braces of Template:Tc (or more generally, as closing braces of the highest level under switch), so the switch does not have closing braces and is therefore not expanded. The latter also happens in the case of excess opening braces of tc: <nowiki>"{{{{tc}}"</nowiki>.

In the case of missing closing braces or excess opening braces somewhere on a page, and also missing opening braces or excess closing braces further on on the same page, all text in between is considered part of the template call or parser function call etc., so it may not be shown, shown multiple times, etc., e.g.: 

*<nowiki>"{{#switch:p|p=q|r={{tc|s=t}} ... {{#switch:p|p=q|r=tc}}|s=t}}"</nowiki> gives "{{#switch:p|p=q|r={{tc|s=t}} ... {{#switch:p|p=q|r=tc}}|s=t}}" (seemingly unrelated text "..." disappears)

[[Special:ExpandTemplates]] essentially shows how braces are grouped into double and triple braces, but an end tag does not show to which start tag it belongs, and a simple braces error in a wikitext with many braces can be cumbersome to find. Therefore techniques to keep things manageable are useful.

To avoid that a possible braces error in a part of the wikitext of a page affects the expansion of a later part one can temporarily add excess closing braces after the part with the possible errors, at a point in the wikitext that is supposed to be on the top level (outside braces): 

*<nowiki>"{{#switch:p|p=q|r={{tc|s=t}} }}}}}} ... {{#switch:p|p=q|r=tc}}|s=t}}"</nowiki> gives "{{#switch:p|p=q|r={{tc|s=t}} }}}}}} ... {{#switch:p|p=q|r=tc}}|s=t}}" (text "..." is preserved)

This raises the point of insertion to the top level (if enough closing braces are used), so that it is not part of the call of a template etc., so not conditional, not used multiple times, etc.

Further examples:
*<nowiki>{{#switch:p|p=q|r={{tc}}|s=t}}</nowiki> gives {{#switch:p|p=q|r={{tc}}|s=t}}
*<nowiki>{{#switch:q|p=q|r={{tc}}|s=t}}</nowiki> gives {{#switch:q|p=q|r={{tc}}|s=t}}
*<nowiki>{{#switch:r|p=q|r={{tc}}|s=t}}</nowiki> gives {{#switch:r|p=q|r={{tc}}|s=t}}

*<nowiki>{{#switch:p|p=q|r=tc}}|s=t}}</nowiki> gives {{#switch:p|p=q|r=tc}}|s=t}}
*<nowiki>{{#switch:q|p=q|r=tc}}|s=t}}</nowiki> gives {{#switch:q|p=q|r=tc}}|s=t}}
*<nowiki>{{#switch:r|p=q|r=tc}}|s=t}}</nowiki> gives {{#switch:r|p=q|r=tc}}|s=t}}

*<nowiki>{{#switch:p|p=q|r={{tc|s=t}}</nowiki> gives {{#switch:p|p=q|r={{tc|s=t}}
*<nowiki>{{#switch:q|p=q|r={{tc|s=t}}</nowiki> gives {{#switch:q|p=q|r={{tc|s=t}}
*<nowiki>{{#switch:r|p=q|r={{tc|s=t}}</nowiki> gives {{#switch:r|p=q|r={{tc|s=t}}

Another method of containing the propagation of any braces error in some wikitext is putting it in a template. For example, if the wikitext with the braces error is "{{", which is the content of {{tim|lb}}, then <nowiki>{{1x|{{#if:|{{lb}}}} ...}}</nowiki> gives "{{1x|{{#if:|{{lb}}}} ...}}", while <nowiki>{{1x|{{#if:|{{}} ...}}</nowiki> gives "{{1x|{{#if:|{{}} ...}}" (the text "..." is preserved in the first case, but not in the second).

We can apply this method e.g. for a switch, i.e. make each case a template call. If the overall template only contains this switch, based on a parameter, we can dispense with the overall template alltogether and call the subtemplates directly, and they can have one parameter less, except possibly for the default case. If the name of the subtemplate is obtained by concatenation of the name of the overall template and the value of the parameter the template call gets even shorter: <nowiki>{{p|q=r|s=t}} or {{p|r|s=t}} becomes {{pr|s=t}}</nowiki> for supported values of r. In this case there is no mechanism to handle an unsupported r: while with the switch we get nothing or the default, here we would get a call of a non-existing template, giving a red link. Alternatively we could use something like <nowiki>{{p{{#switch:r|.|.|.=r|default}}|s=t}}</nowiki> or <nowiki>{{ {{#switch:r|.|.|.=pr|void}}|s=t}}</nowiki>.

Compare the choice between using switch for an [[Manual:Representing arrays|array]] and using a template for each element.

Advantages of having only a single template:
*easier to copy (to another wiki, or to make a different version)
*easier to get an overview of everything; however, this can be obtained on a separate page showing the contents of the templates with msgnw, see e.g. {{tim|xpd/msgnw}}
*easier to make overall find/replace changes, etc.
*a common part before or after the switch is possible without duplication
*in the case of a common part, although it can be made a template, with subtemplates we at least need a template call in each

Advantages of having a template for each case:
*no propagation of wikitext errors
*less server load

==Efficiency==
Suppose a string S is the result of some expansion, and needed multiple times. For example, S is the municipality a given town belongs to, which is looked up in a table, and it is used for display, and for looking up the province it belongs to. Then we want to avoid that the server needs to determine the municipality twice. In a computer program we could use a variable S, but in MediaWiki (without [[mw:Extension:VariablesExtension|VariablesExtension]]) the way to do this is with a template that uses a parameter for S. This way S is expanded only once. While extra layers of templates can increase [[w:wp:Template limits|page counts]], they can in a case like this reduce them. In particular, if the lookup is done in a [[Manual:Representing arrays|data template]] containing many data items, the preprocessor node count is often large, unless the data template stores the data in the form of values of unnamed parameters (which would not be possible in the example because we start from a given town name, not an integer), so avoiding duplication is worthwhile.

Some other cases where a template is needed to avoid duplicate expansion:
*finding a number and determining its order of magnitude to determine the desired rounding
*finding a population figure for display and for computation of the population density
*if S represents a number, computing S + 1 / S
There are also cases where a formula can be rewritten, such that S occurs only once. For example, in an expression we can write <code>(S+.5)^2-.25</code> instead of <code>S*(1+S)</code>.

See also [[Help:Template#Page counts]].

==Intermediate results==
In the process from wikitext to rendering, the XML tree and the expanded wikitext are intermediate results which help to understand the process. Another intermediate result that can similarly be helpful is the HTML sent to the browser.

Using [[Help:XML-style tags|XML-style tag]] nowiki, <nowiki><nowiki>{{t1demo| ''p'' > 1 }}</nowiki></nowiki> displays the wikitext, and with [[mw:Help:Magic_words#Miscellaneous|#tag]], <nowiki>{{#tag:nowiki|{{t1demo| ''p'' > 1 }}}}</nowiki> displays the expanded wikitext.

Thus we have e.g.:

{| class=wikitable
|-
!wikitext 
|
 <nowiki>{{t1demo| ''p'' > 1 }}</nowiki>
|
 <nowiki>{{#tag:nowiki|{{t1demo| ''p'' > 1 }}}}</nowiki>
|
 <nowiki><nowiki>{{t1demo| ''p'' > 1 }}</nowiki></nowiki>
|-
!expanded wikitext 
|
 {{#tag:nowiki|{{t1demo| ''p'' > 1 }}}}
|
 <nowiki><nowiki>start- ''p'' > 1 -end</nowiki></nowiki>
|
 <nowiki><nowiki>{{t1demo| ''p'' > 1 }}</nowiki></nowiki>
|-
!HTML 
|
 start- &lt;i>p&lt;/i> &amp;gt; 1 -end
|
 <nowiki>start- ''p'' &amp;gt; 1 -end</nowiki>
|
 <nowiki>{{t1demo| ''p'' &amp;gt; 1 }}</nowiki>
|-
!rendering 
|{{t1demo| ''p'' > 1 }}||<nowiki>start- ''p'' > 1 -end</nowiki>||<nowiki>{{t1demo| ''p'' > 1 }}</nowiki>
|}

The step from HTML to rendering depends on CSS: common and personal CSS files referred to in the HTML code, as well as possibly a CSS file specified in the browser.

==Substitution==
If a save command is applied to a page containing one or more ''titles'' starting with <code>subst:</code> or <code>safesubst:</code>,  the wikitext is modified before saving. This [[help:substitution|substitution]] process treats all ''templates'' without <code>subst:</code> or <code>safesubst:</code> on the page and in directly or indirectly substituted templates as plain text. It also treats all ''tplargs'' on the page as plain text. The new wikitext is equal to what the expanded wikitext resulting from the wikitext without these occurrences of <code>subst:</code> or <code>safesubst:</code> would be if the other structures were treated as plain texts. For any substitution to occur there has to be at least one explicit occurrence of <code>subst:</code> or <code>safesubst:</code> on the page itself. If the substituted ''title'' is inside another ''title'' it may construct a text <code>subst:</code> or <code>safesubst:</code> causing that other title to be substituted too, etc. A text <code>subst:</code> or <code>safesubst:</code> in a substituted template can also be constructed with a ''tplarg'' (parameter value or default value). 

Examples: 

<code><nowiki>{{subst:lc:{{TC}}}}</nowiki></code> changes to <code><nowiki>{{tc}}</nowiki></code>.

<code><nowiki>{{subst:urlencode:{{tc}}}}</nowiki></code> changes to <code>%7B%7Btc%7D%7D</code>.

<code><nowiki>{{subst:lc:{{{A|B}}}}}</nowiki></code> changes to <code><nowiki>{{{a|b}}}</nowiki></code>.

<code><nowiki>{{su{{su{{subst:1x|bs}}t:1x|bs}}t:tc}}</nowiki></code> changes to <code>{{tc}}</code>. 

[[Special:ExpandTemplates]] can be used as usual to find the parse tree of wikitext containing "subst:" or "safesubst:", but it cannot be used to find the new wikitext. For this, one should use "show changes", or save a test page and press edit to see the result. In Special:ExpandTemplates the ''title'' starting with "subst:" is treated as an invalid template name, so "{{subst:" and the corresponding "}}" are treated as plain text. Prefixes "safesubst:" are ignored, like when they are in a transcluded page.

If a template contains a ''template'' call with <code>subst:</code>, then if the outer template is transcluded the inner ''template'' call is not expanded, while if the outer template is substituted the inner ''template'' call ''is'' expanded. This can be used for branching depending on whether the outer template is transcluded or substituted.

Example:
*<code><nowiki>{{#if:{{{{{|subst:}}}ns:0}}|transcluded|substituted}}</nowiki></code>

==Expansion depth limit==
There is an "expansion depth limit" of 40, see [[Help:Expansion depth]].

==Internal code for parts in XML-style tags==
XML-style tags, e.g. <code>&lt;nowiki&gt;</code> tags and <code>&lt;math&gt;</code> tags, together with their content, are temporarily replaced by a so-called strip marker, a unique code with a length of ca. 37 characters plus the length of the tag name (independent of the length of the content). If a string operation is applied to it, only fully intact codes in the result are converted back to the original text, the rest is exposed. For example:

<code><nowiki>{{formatnum:&lt;nowiki>&lt;/nowiki>}}</nowiki></code> gives {{formatnum:<nowiki></nowiki>}}.

==See also==
*[[Manual:Newlines and spaces#Automatic newline]]
*[[Preprocessor ABNF]]
*[[Markup spec/EBNF#Includes]]
* [[Manual:Parser]]
* [[Help:Template]]

== References ==
{{reflist}}

[[Category:Parser manuals]]