Template:Exists/doc

This is a documentation subpage for Template:Exists (see that page for the template itself).
It contains usage information, categories and other content that is not part of the original template page.

NOTE: DO NOT USE THIS TEMPLATE unless it is absolutely necessary! It can cause some strange issues. See Talk:Chief Interrogator Washington. It is probably always better to use the #ifexist parser function instead.

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This template tests whether a given page exists. It can be useful in view of the maximum number of #ifexist calls that can be used on a page. (However, in the case of frivolous use of #ifexist which was the reason for the limit, do not replace this use with frivolous use of this template!)

The full syntax for using it is:

{{exists | page=page | then=result if page exists | else=result if page does not exist }}

The parameters then and else are optional; if not given, the default values for then and else are 1 and 0 respectively. The page name may alternatively be given as the first unnamed parameter, as in this short version:

{{exists | page }}

The method is based on transclusion of the possibly existing page. If the page does not exist the code for page transclusion produces a (red) link. Even if this result is used for comparison only, and not for final output, the page counts as being transcluded. Therefore the page appears as red link in the list of transcluded pages.

If the page to be transcluded does not exist, then link brackets are put around the full pagename, with the following conversions (visible with Special:ExpandTemplates):

• a colon is put in front if it is not there yet
• the namespace name (if applicable) and the pagename are capitalized (on most wikis)
• underscores are replaced by spaces

Thus {{help:qq w_x}} and {{:help:qq w_x}} both give [[:Help:Qq w x]].

For the link there is not such an automatic conversion. Therefore the template prefixes a colon to the parameter for the full pagename (it should not already start with a colon) and applies lc and anchorencode (to convert spaces to underscores) on both sides before comparison:

• {{anchorencode:{{lc:[[:help:qq w_x]]}}}} gives help:qq_w_x
• {{anchorencode:{{lc:{{help:qq w_x}}}}}} gives help:qq_w_x

Thus a false-positive for a non-existing page is avoided. A false-negative for an existing page is possible, but typically only for specially constructed examples.