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Moved this relevant discussion to here from
[[tracking_bugs_with_dependencies]]: --[[Joey]]
>> it seems that there are two types of dependency, and ikiwiki
>> currently only handles one of them. The first type is "Rebuild this
>> page when any of these other pages changes" - ikiwiki handles this.
>> The second type is "rebuild this page when set of pages referred to by
>> this pagespec changes" - ikiwiki doesn't seem to handle this. I
>> suspect that named pagespecs would make that second type of dependency
>> more important. I'll try to come up with a good example. -- [[Will]]
>>> Hrm, I was going to build an example of this with backlinks, but it
>>> looks like that is handled as a special case at the moment (line 458 of
>>> render.pm). I'll see if I can breapk
>>> things another way. Fixing this properly would allow removal of that special case. -- [[Will]]
>>>> I can't quite understand the distinction you're trying to draw
>>>> between the two types of dependencies. Backlinks are a very special
>>>> case though and I'll be suprised if they fit well into pagespecs.
>>>> --[[Joey]]
>>>>> The issue is that the existential pagespec matching allows you to build things that have similar
>>>>> problems to backlinks.
>>>>> e.g. the following inline:
\[[!inline pages="define(~done, link(done)) and link(~done)" archive=yes]]
>>>>> includes any page that links to a page that links to done. Now imagine I add a new link to 'done' on
>>>>> some random page somewhere - a page which some other page links to which didn't previously get included - the set of pages accepted by the pagespec, and hence the set of
>>>>> pages inlined, will change. But, there is no dependency anywhere on the page that I altered, so
>>>>> ikiwiki will not rebuild the page with the inline in it. What is happening is that the page that I altered affects
>>>>> the set of pages matched by the pagespec without itself being matched by the pagespec, and hence included in the dependency list.
>>>>> To make this work well, I think you need to recognise two types of dependencies for each page (and no
>>>>> special cases for particular types of links, eg backlinks). The first type of dependency says, "The content of
>>>>> this page depends upon the content of these other pages". The `add_depends()` in the shortcuts
>>>>> plugin is of this form: any time the shortcuts page is edited, any page with a shortcut on it
>>>>> is rebuilt. The inline plugin also needs to add dependencies of this form to detect when the inlined
>>>>> content changes. By contrast, the map plugin does not need a dependency of this form, because it
>>>>> doesn't actually care about the content of any pages, just which pages it needs to include (which we'll handle next).
>>>>> The second type of dependency says, "The content of this page depends upon the exact set of pages matched
>>>>> by this pagespec". The first type of dependency was about the content of some pages, the second type is about
>>>>> which pages get matched by a pagespec. This is the type of dependency tracking that the map plugin needs.
>>>>> If the set of pages matched by map pagespec changes, then the page with the map on it needs to be rebuilt to show a different list of pages.
>>>>> Inline needs this type of dependency as well as the previous type - This type handles a change in which pages
>>>>> are inlined, the previous type handles a change in the content of any of those pages. Shortcut does not need this type of
>>>>> dependency. Most of the places that use `add_depends()` seem to need this type of dependency rather than the first type.
>>>>>> Note that inline and map currently achieve the second type of dependency by
>>>>>> explicitly calling `add_depends` for each page the displayed.
>>>>>> If any of those pages are removed, the regular pagespec would not
>>>>>> match them -- since they're gone. However, the explicit dependency
>>>>>> on them does cause them to match. It's an ugly corner I'd like to
>>>>>> get rid of. --[[Joey]]
>>>>> Implementation Details: The first type of dependency can be handled very similarly to the current
>>>>> dependency system. You just need to keep a list of pages that the content depends upon. You could
>>>>> keep that list as a pagespec, but if you do this you might want to check that the pagespec doesn't change,
>>>>> possibly by adding a dependency of the second type along with the dependency of the first type.
>>>>>> An example of the current system not tracking enough data is
>>>>>> described in [[bugs/transitive_dependencies]].
>>>>>> --[[Joey]]
>>>>> The second type of dependency is a little more tricky. For each page, we'd need a list of pagespecs that
>>>>> the page depended on, and for each pagespec you'd want to store the list of pages that currently match it.
>>>>> On refresh, you'd need to check each pagespec to see if the set of pages that match it has changed, and if
>>>>> that set has changed, then rebuild the dependent page(s). Oh, and for this second type of dependency, I
>>>>> don't think you can merge pagespecs. If I wanted to know if either "\*" or "link(done)" changes, then just checking
>>>>> to see if the set of pages matched by "\* or link(done)" changes doesn't work.
>>>>> The current system works because even though you usually want dependencies of the second type, the set of pages
>>>>> referred to by a pagespec can only change if one of those pages itself changes. i.e. A dependency check of the
>>>>> first type will catch a dependency change of the second type with current pagespecs.
>>>>> This doesn't work with backlinks, and it doesn't work with existential matching. Backlinks are currently special-cased. I don't know
>>>>> how to special-case existential matching - I suspect you're better off just getting the dependency tracking right.
>>>>> I also tried to come up with other possible solutions: e.g. can we find the dependencies for a pagespec? That
>>>>> would be the set of pages where a change on one of those pages could lead to a change in the set of pages matched by the pagespec.
>>>>> For old-style pagespecs without backlinks, the dependency set for a pagespec is the same as the set of pages the pagespec matches.
>>>>> Unfortunately, with existential matching, the set of pages that each
>>>>> pagespec depends upon can quickly become "*", which is not very useful. -- [[Will]]
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