Help:Substitution

Substitution is automatic conversion of wikitext of a page referring to a template, variable, or parser function  when the referring page is saved.

In the case of template substitution the template call is replaced by the template content with substitution of the parameters. Thus a template is used as a macro and the page is macro expanded when the page is saved rather than, as usually happens, when the page is viewed.

In the case of substitution of a variable or parser function the reference to it is replaced by the resulting value.

Substitution is done by putting the modifier  after the double opening braces.

The result (in the form of the difference with the saved wikitext) can be seen before (or without) saving by pressing "Show changes". However, if the text covers more than one paragraph this diff page is not very suitable for copying the result (e.g. for stepwise substitution without saving every step), because of plus signs in the margin.

Applications
Substitution of a template: Substitution of a time-dependent variable: Substitution of a page-dependent variable:
 * Make a rendered page independent of the template:
 * The rendered page does not change when the template is edited.
 * The page can be copied to another project without copying the template.
 * Make page rendering easier and therefore faster for the server.
 * Analyse and demonstrate the working of templates. However, in some cases substitution works differently.
 * Make the correspondence between wikitext and rendered page easier to understand (this may apply, the opposite may also apply).
 * Make a rendered page independent of the time.
 * Make a rendered page independent of renaming of the page and of copying the wikitext to another page (the opposite applies if the variable PAGENAME is used in a no-include part of the page to include the page itself).

Some MediaWiki extensions have the restriction that if they are used in a template with parameters, they only work if the template is substituted.

Overview
For the discussion of substitution, an "ordinary template" is any page included by {&#123;subst:pagename &#160; &#125;} for pages in the template namespace or {&#123;subst:fullpagename &#160; &#125;} for pages in other namespaces. It's also possible to substitute variables and parser functions collectively known as "predefined templates".

Substitution is a separate process before expansion of non-substituted templates, parser functions, variables. and parameters. Substitutions in the expression for the name of the template or parser function, in the parameter definitions of the substituted template or parser function and in the body of the substituted template are done first. Non-substituted expansion is done later, so at the time of the substitution these expressions with pairs of double braces are treated as plain text. Thus during substitution there can be a parameter name with braces in the call of the substituted template (e.g. ad ) that matches a parameter with the same name in the template body.

If one attempts to apply substitution to a non-existing template etc. there is no substitution, the "subst:" prefix is kept in the wikitext.

After the substitution process, expansion of templates etc. and other processing of the resulting wikitext works as usual. However, this cannot undo a mismatch of parameter names during substitution (see also the second header "Partial substitution" below).

Substitution is only possible if the following has been fully evaluated:
 * the name of the template, parser function or variable
 * in the case of a template: the parameter names in the template call and in the template itself
 * in the case of #if, #ifexpr, #ifexist, and #iferror, the parameter after the colon
 * in the case of #ifeq, the parameter after the colon and the next one
 * in the case of #switch, the parameter after the colon and the expressions to the left of the equals signs

Since, as said, substitution is done before other expansion, the abovementioned required evaluation has not taken place if the expressions involve non-substituted expansion.

Also in the case of other parser functions than mentioned, a not fully evaluated parameter after the colon causes the parser function to be applied to the wikitext with braces and not the expanded wikitext, affecting the result.

Examples:
 * using, does not do substitution, because Help:L k is not an existing page, although Help:Lk is rendered as Help:Lk. Thus the resulting wikitext is the same as the original wikitext and rendered as.

In principle the wikitext resulting from full substitution is, immediately after that, rendered the same as the wikitext with ordinary inclusion.


 * There are exceptions, for example in the case of an undefined parameter with a default value:
 * gives, while gives "Expression error: unrecognised punctuation character "{"", because in the case of substitution of #expr,  is not reduced to 3 and therefore #expr concludes that the expression is not valid.  (See also below.)

using is rendered as ;  gives the wikitext a rendered the same as the wikitext, and on the next edit/save changed into start-a -middle--end;  gives the wikitext , rendered as. This is because, both without substitution and in the case of full substitution, the pipe characters in template calls, excluding those inside inner template calls, template parameters, links, and image tags, determine the separation of parameter definitions from each other and from the template name. This separation does not depend on possible extra pipe characters in the expanded form of the template name and parameter definitions. However, if after substitution of an inner template the pipe character is in the outer template call it is one like any other and plays its part in determining the separation. In other words, parsing is done first once for substitution, and then once for rendering, but in both cases not an extra time in between. In the case of substitution of the inner template only, two subsequent parsings are effective.

If a page substitutes itself (e.g. in the noinclude-part of a template page) it substitutes the old version.

Usage considerations
As mentioned, a change of an ordinary template after substitution does not affect the page in which it was substituted, and a substituted variable depending on time no longer depends on time, etc.

The relationship between wikitext of a page and its rendering can become easier to understand after substitution, because one has all wikitext together, and parameter substitutions have been performed.

It can also become more complex. Separately focusing on understanding a template call and understanding the template content can be easier. Wikitext after substitution is often more complex than when the required wikitext would have been written directly.

Unlike a template call (if one knows about templates), wikitext after substitution does not show how one can produce a similar result. The wikitext can be long and complicated, and therefore cumbersome to write directly, or it can be simple, e.g. a number resulting from a computation, but cumbersome to find directly. One may think that the wikitext is what one is supposed to write and find directly to get the result, even in cases where that would be very impractical.

In such cases documentation of the template call is useful. Just like in computer programming we change the source code and/or the data to produce new results, and we do not directly change the object file, here we would change the template calls and/or the templates, instead of changing the wikitext resulting from substitution directly.

Ordinary templates
In the case of substituting an ordinary template, the template tag is replaced by the wikitext of the template, with the parameter values substituted for the parameters. Even parameters in comments are substituted. Substitution removes the noinclude parts and the includeonly tags.

Parameters:
 * A substitution with p=r replaces and  by r; this includes the cases that r is of the form  or.
 * A substitution with undefined p preserves and  ; the latter is not replaced by the default q (see below for a construct that is replaced by p if p is defined, and by q if not).

With "subst:" the replacement of a template tag by wikitext does not work recursively. For full recursive substitution use Special:ExpandTemplates. See also optional substitution below.

Stepwise substitution of templates including other templates including more templates etc. can be useful for analyzing problems or documenting the expected behaviour of complex templates.

Stepwise substitution for template names depending on the parameter default mechanism doesn't work as one may expect, this also affects parameter defaults in the left hand side of parser functions, see below for examples.

In the absence of parameters, template substitution can be compared with copying the wikitext, or the rendering of a previewed or saved inclusion. However, template substitution excludes &lt;noinclude&gt; parts, and removes &lt;includeonly&gt; tags.

Predefined templates
In the case of substituting a predefined template, without parameters depending on other templates, the tag is replaced by the result. Please note that subst: has to be added directly in front of the predefined template name without intervening spaces.

Applying subst to a variable works like applying it to a template. E.g. a timestamp:


 * -- T UTC
 * 2005-03-10 T 08:23 UTC

In the case of substituting a predefined template with a parameter depending on another template, that has to be substituted too, with a separate subst: modifier, otherwise the result is undefined.


 *   gives IN, the same wikitext as  is expanded to; UC: is applied to the output "in" of Tc.
 *   gives Image.
 *       gives wikitext      rendered as.
 * gives the wikitext startHelpend (see t1)
 * gives the wikitext start12end
 * gives the wikitext startABCDEFend
 * gives the wikitext 1331
 * gives the wikitext ABCABCABC
 * gives the wikitext 1e-05 (see LC:)
 * gives (at the time of writing) the wikitext 30
 * gives (at the time of writing) the wikitext THURSDAY

However:
 * 1)   gives the wikitext   </tt> rendered as.
 * 2)  </tt> stays , rendered as  (see ns:).

As mentioned before, on substitution, all calls without substitution of templates, variables, and parser functions are treated as plain text. As a result substitution of the outer x:</tt> in a nested often is only suitable if all inner y:</tt> are also substituted.

In the case of substitution of a predefined template, if the expression for one of its parameters contains with undefined p, this code does not reduce to 3, so a string function like lc can be applied but not e.g. #expr. For example:
 * gives
 * gives
 * gives the wikitext and rendering Expression error: unrecognised punctuation character "{"
 * substituting a template containing or  gives the same error if p is not assigned a value, but twice the number p if it is assigned a value.

Compare:
 * 2* gives 2*
 * 2* gives 2*
 * gives the wikitext 2* rendered as 2*

and also (from above):
 *  </tt> gives IN, just like  does; UC is applied to the output "in" of Tc.
 *  </tt> gives the wikitext   </tt> rendered as.

In the substitution of UC, the inclusion tag is treated as string just like.

Partial substitution
Inside an ordinary template one can apply substitution to an ordinary template call containing a parameter, to replace it by the direct wikitext containing the parameter. It amounts to automatically merging the two templates (creating a "composite template" like a composite function). It is not possible if the inner and/or outer template is predefined.

This way one can dispense with the optional substitution technique described below, and apply substitution of the resulting outer template by simply using "subst:" (unless there are more nesting levels).

Example:


 * }} gives the wikitext start--end, just that of , without noinclude parts and includeonly tags
 * ab gives the wikitext start-ab-end

Examples with double substitution:
 * gives the wikitext start--endstart--endstart--endstart--end
 * gives the wikitext start--endstart--endstart--end

Multilevel substitution
When substituting a template it may be desirable to carry out a substitution inside the template too. This requires "subst:" in the template. There are two methods to prevent premature substitution (i.e., when the template is saved): the code "subst:" can be the possible value of an expression containing a parameter, or includeonly tags can be used (see below).

With the first-mentioned method (the code "subst:" can be the possible value of an expression) we can:
 * apply multilevel transclusion by making the expression the empty string;
 * apply one-level substitution by substituting the outer template and making the expression the empty string;
 * apply two-level substitution by substituting the outer template and making the expression evaluate to "subst:";
 * apply multi-level substitution by substituting the outer template, making the expression evaluate to "subst:", and calling the inner template with parameters controlling further inner substitutions in the same way; these parameters may depend on the substitution parameter controlling the substitution of the inner template, since if that is not substituted, inner substitutions within that template are not possible.

Similarly, if there are multiple templates, parser functions, and/or variables in the inner template(s) we can control substitution of all, either independently by using different parameters, or with some or all using the same parameter.

The simplest code to control whether we get the empty string of "subst:" is using a parameter which is assigned either value, for example with the name "subst", possibly with a postfix, for independent control of multiple substitutions. If we also specify a default value q by the code, then with substitution and undefined parameter "subst" this evaluates to the literal code "  " (see below), so regardless of q, never to "subst:". Therefore in the case of substitution parameter "subst" has to be defined anyway. In the case of transclusion and undefined parameter "subst" it evaluates to q. Since inner substitution in only carried out if the outer template is also substituted, the only sensible value of q is the empty string. This is indeed convenient because with non-substituted transclusion we can then dispense with the parameter definition "subst=" in the outer template call.

Thus adding the feature of optional substitution to an existing template X can be done as follows:


 * 1) For each template, parser function, and variable Y used within X, say {&#123;Y|a|b|c&#125;}</tt>, replace Y by {&#123;{subst|}&#125;}Y</tt> resulting in {&#123;{&#123;{subst|}&#125;}Y|a|b|c&#125;}</tt>.
 * 2) For ordinary templates add a parameter subst={&#123;{subst|}&#125;}</tt> resulting in {&#123;{&#123;{subst|}&#125;}Y|a|b|c|subst={&#123;{subst|}&#125;}&#125;}</tt> (predefined templates like parser functions and variables don't get the additional parameter).

This allows to use {&#123;X|d|e|f&#125;)</tt> as is, or {&#123;subst:X|d|e|f|subst=subst:&#125;)</tt> with recursive substitution, as far as all ordinary templates Y used within X support this technique. Even if they don't support it they are still substituted, but won't propagate this recursive substitution into templates or variables Z used within Y.

Using {&#123;subst:X|d|e|f&#125;]</tt> without additional parameter subst=subst:</tt> should be okay, otherwise it also won't work with additional parameter. For some templates substitution breaks their function, and for some predefined templates (see below) and variable REVISIONID it won't work as expected.

As mentioned, also multiple parameters like subst can be used, to control which templates, parser functions, and variables are substituted, see e.g. Template:For.

See also Help:Calculation and.

Partial substitution
Using a template prepared for optional <tt>subst=subst:</tt> only with ordinary substitution, without specifying parameter values, allows to insert its code into another template, like copy and paste, but all <tt>&lt;noinclude&gt;</tt> parts and <tt>&lt;includeonly&gt;</tt> keywords automatically stripped. Executing inserted code instead of calling it may be more efficient for the server.

A typical example for this technique is expanding, within another template, a template used as test expression in a #switch: like : parameter tag case 0 etc. parameter tag  case 0 etc. parameter tag  case 0 etc. something&#125;}. This suggests that substitution is prevented by discarding "subst:" on the page itself, but actually substitution is prevented because the subst-syntax is disturbed by the tags.
 * 1) Development code:

It doesn't substitute "something" at the time of the creation of the relevant template, but has the desired effect when the template is substituted. Please note that this works as expected only for substitution, not for normal (unsubstituted) inclusion. For examples see "substitution" in Help:Variable.

Forced substitution
Some templates deliberately refuse to work without substitution. This technique is essential for templates producing some kind of timestamp, e.g. adding pages to dated categories.

URLs
Fullurl and localurl with an undefined parameter cannot be substituted at all, even if a default value is specified. This is a variation of the case of derived template names.


 * <tt> </tt> gives.
 * <tt> </tt> gives.
 * <tt> </tt> gives.
 * <tt> </tt> gives.
 * <tt> </tt> gives http://meta.wikimedia.org/wiki/A.
 * <tt> </tt> gives http://meta.wikimedia.org/wiki/A.

Substitution of part of the parameters
Let template A use parameters 1 and 2. Consider creating a template B with one parameter 1, corresponding to A, with a given value q of parameter 2. Compare and. They look the same on the template page, see e.g., but the first does not work because is treated as text, not as parameter.

However, with substitution (using "subst:" or Special:Expandtemplates) the resulting wikitext is the same, without distinction between a text and a parameter, it is a parameter anyway, so "1=" is not needed.

If A contains e.g. #expr with an expression containing both parameters the same applies, except that we can only substitute the highest level (A), not the parser function, so we cannot use Special:Expandtemplates.

In general, substituting a parameter and applying a template or parser function sometimes gives the same result as substituting the template or parser function with the triple-braced parameter code and then substituting the parameter.

Without defaults (all rendered the same in one-step substitution as without substitution):

Examples with equality:
 * A template containing pqr substituted with 1=u, 2=v gives puqvr; substituted with 2=v it gives pqvr, which itself, substituted with 1=u gives also puqvr.
 * Two-level substitution of a template containing  with 3=u, 4=v gives up; substituted with 4=v it gives p, which itself, substituted with 3=u gives up.

Examples without equality:
 * Two-level substitution of a template containing  with 3=, 4=v gives the empty string; substituted with 4=v it gives vp, which itself, substituted with 3=u remains vp.
 * Two-level substitution of a template containing  with 1=u, 2=v gives up; substituted with 2=v it gives pp (the bug), which itself, substituted with 3=u, gives upp.
 * Two-level substitution of a template containing  with 1=7, 2=8 gives 56; substituted with 2=8 it gives &lt;strong class="error">Expression error: Unrecognised punctuation character "{"&lt;/strong>, which itself, substituted with 1=7, remains the same.

Thus without equality we may or may not get an error message.

One example shows that substitution of one parameter can be affected by the bug mentioned above. However, we can then replace e.g. by  and do full substitution, except that substvoid is undefined, preventing the bug. The result works already correctly with transclusion. Subsequently it can be substituted with substvoid=subst: so that we get the plain.

With defaults:

Rendered the same as without substitution:
 * Two-level substitution of a template containing pqr with 2=v gives pqvr.
 * Two-level substitution of a template containing with 4=v gives p.

Not rendered the same as without substitution:
 * Two-level substitution of a template containing with 4=v gives vp.
 * Two-level substitution of a template containing with 2=v gives pp (the bug).
 * Two-level substitution of a template containing with 2=8 gives &lt;strong class="error">Expression error: Unrecognised punctuation character "{"&lt;/strong>.

After substitution with the parameter definition:
 * vp gives vp.
 * p gives pp (the bug).
 * gives &lt;strong class="error">Expression error: Unrecognised punctuation character "{"&lt;/strong>.

Rewritten:
 * gives the empty string.
 * p gives dp.
 * gives 48.

Documenting substitution
Usage of a template through  does not automatically show up in page histories. Therefore providing the line of wikitext containing "subst:" in the edit summary is especially useful.

Also pages with a substituted template do not show up in backlinks, and the template does not appear in the list of transcluded templates on the edit page. The template could add pages to a category to track substitutions, but listing this category on a page may clutter the list of content-based categories the page is in. Also, comments outside noinclude tags are included in the wikitext. Thus a comment can be used to mention the template. It can even contain the values of the parameters, because substitution of parameters works even in comments.