ikiwiki/doc/todo/conditional_text_based_on_ikiwiki_features.mdwn

I'd like to see some way to conditionally include wiki text based on
whether the wiki enables or disables certain features.  For example,
[[helponformatting]], could use `\[[if (enabled smiley) """Also, because
this wiki has the smiley plugin enabled, you can insert \[[smileys]] and
some other useful symbols."""]]`, and a standard template for [[plugins]]
pages could check for the given plugin name to print "enabled" or
"disabled".

Some potentially useful conditionals:

* `enabled pluginname`
* `disabled pluginname`
* `any pagespec`: true if any of the pages in the [[PageSpec]] exist
* `all pagespec`: true if all of the pages in the [[PageSpec]] exist
* `no pagespec` or `none pagespec`: true if none of the pages in the [[PageSpec]] exist
* `thispage pagespec`: true if pagespec includes the page getting rendered (possibly one including the page with this content on it).
* `sourcepage pagespec`: true if pagespec includes the page corresponding to the file actually containing this content, rather than a page including it.
* `included`: true if included on another page, via [[plugins/inline]], [[plugins/sidebar]], [[plugins/contrib/navbar]], etc.

You may or may not want to include boolean operations (`and`, `or`, and
`not`); if you do, you could replace `disabled` with `not enabled`, and `no
pagespec` or `none pagespec` with `not any pagespec` (but you may want to
keep the aliases for simplicity anyway).  You also may or may not want to
include an `else` clause; if so, you could label the text used if true as
`then`.

Syntax could vary greatly here, both for the [[PreprocessorDirective]] and
for the condition itself.

> I think this is a good thing to consider, although conditionals tend to
> make everything a lot more complicated, so I also want to KISS, and not
> use too many of them.
> 
> I'd probably implement this using the same method as pagespecs, so 'and',
> 'or', '!', and paren groupings work.
> 
> It could be thought of as simply testing to see if a pagespec matches
> anything, using a slightly expanded syntax for the pagespec, which would
> also allow testing for things like link(somepage),
> created_before(somepage), etc.
> 
> That also gives us your "any pagespec" for free: "page or page or page".
> And for "all pagespec", you can do "page and page and page". 
> 
> For plugins testing, maybe just use "enabled(name)"?
> 
> I'm not sure what the use cases are for thispage, sourcepage, and
> included. I don't know if the included test is even doable. I'd be
> inclined to not bother with these three unless there are use cases I'm
> not seeing.
> 
> As to the syntax, to fit it into standard preprocessor syntax, it would
> need to look something like this:
>
>	\[[if test="enabled(smiley)" """foo"""]]
> 
> --[[Joey]]

>> [[PageSpec]] syntax seems perfect, and your proposed syntax for the `if`
>> [[PreprocessorDirective]] looks fine to me.
>>
>> [[PageSpec]]s don't give you `none` for free, since `!foo/*` as a boolean
>> would mean "does any page not matching `foo/*` exist", not "does `foo/*`
>> match nothing"; however, I don't really care much about `none`, since I
>> just threw it in while brainstorming, and I don't know any compelling use
>> cases for it.
>>
>> `enabled(pluginname)` will work perfectly, and `!enabled(pluginname)`
>> makes `disabled` unnecessary.
>>
>> A few use cases for `included`, which I would really like to see:
>>
>> * On the sidebar page, you could say something like \[[if test="!included"
>>   """This page, without this help message, appears as a sidebar on all
>>   pages."""]].  The help text would then only appear on the sidebar page
>>   itself, not the sidebar included on all pages.
>>
>> * On [[blog]] entries, you could use `included` to implement a cut.
>>   (Please don't take that as an argument against. :) ) For instance, you
>>   could use included rather than [[plugins/toggle]] for the detailed
>>   changelogs of ikiwiki, or to embed an image as a link in the feed rather
>>   than an embedded image.
>>
>> Some use cases for `thispage`:
>>
>> * You could use `thispage` to include or exclude parts of the sidebar based
>>   on the page you include it in.  You can already use subpages/sidebar for
>>   subpages/*, but `thispage` seems more flexible, makes it trivial to have
>>   common portions rather than using [[plugins/inline]] with the `raw`
>>   option, and keeps the sidebar in one place.
>>
>> * You could use `thispage` to implement multiple different feeds for the
>>   same content with slightly different presentation.  For instance, using
>>   templates for image inclusion, you could offer a feed with image links
>>   and a feed with embedded images. Similarly, using templates for cuts, you
>>   could offer a feed with cuts and a feed with full content in every post.
>>
>> I don't have any particular attachment to `sourcepage`.  It only makes
>> sense as part of a template, since otherwise you know the source page when
>> typing in the if.
>>
>> --[[JoshTriplett]]