Editing Help:Substitution (section)

== Making templates behave differently when transcluded or substituted ==

Sometimes it is desirable to make a template behave differently when substituted rather than when transcluded. A common trick for doing this is to use an expression like <code><nowiki>{{{{{subst|subst:}}}ns:0}}</nowiki></code>. This evaluates to the empty string if the template is being substituted (since the inner substitution takes effect, giving the prefix of [[WP:namespace|namespace]] 0, which is empty), but to "<nowiki>{{subst:ns:0}}</nowiki>" if the template is being transcluded (since substitution cannot take place after transclusion).

A common application is in templates which are designed only to be substituted, to make them produce a warning if they are mistakenly transcluded instead. This is done in templates like {{tl|prod}}, which are designed to produce a timestamp (e.g. for adding pages to dated categories), and will not be able to do this if transcluded.

A template, {{tl|issubst}}, has been created to simplify this. It returns "yes" if substitution is being performed, or the empty string otherwise. This can then be used as the parameter of a conditional parser function to make the template display a warning method if it is being transcluded, or to otherwise change the behaviour of a template depending on whether it is being transcluded or substituted.