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Comments (maybe not so helpful - sorry)

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http://www.cse.unsw.edu.au/~willu/ 2009-10-07 02:40:32 -04:00 committed by Joey Hess
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doc/todo/dependency_types.mdwn
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@ -156,6 +156,67 @@ false negatives (though these should be somewhat rare, and no false
positives). Still, it does work, and it makes things like simple maps and
pagecounts much more efficient.
----
#### Will's first pass feedback.
If the API is going to be updated, then it would be good to make it forward compatible.
I'd like for the API to be extendible to what is useful for complex pagespecs, even if we
that is a little redundant at the moment.
My attempt to play with this is in my git repo. [[!template id=gitbranch branch=origin/depends-spec author="[[will]]"]]
That branch is a little out of date, but if you just look at the changes in IkiWiki.pm you'll see the concept I was looking at.
I added an "add_depends_spec()" function that adds a dependency on the pagespec passed to it. If the set of matched pages
changes, then the dependent page is rebuilt. At the moment the implementation uses the same hack used by map and inline -
just add all the pages that currently exist as traditional content dependencies.
Getting back to commenting on your proposal:
Just talking about the definition of a "presence dependency" for the moment, and ignoring implementation. Is a
"presence dependency" supposed to cause an update when a page disappears? I assume so. Is a presence dependency
supposed to cause an update when a pages existence hasn't changed, but it no longer matches the pagespec.
(e.g. you use `created_before(test_page)` in a pagespec, and there was a page, `new_page`, that was created
after `test_page`. `new_page` will not match the spec. Now we'll delete and then re-create `test_page`. Now
`new_page` will match the spec, and yet `new_page` itself hasn't changed. Nor has its 'presence' - it was present
before and it is present now. Should this cause a re-build of any page that has a 'presence' dependency on the spec?
I think that is another version of the problem you encountered with meta-data.
In the longer term I was thinking we'd have to introduce a concept of 'internal pagespec dependencies'. Note that I'm
defining 'internal' pagespec dependencies differently to the pagespec dependencies I defined above. Perhaps an example:
If you had a pagespec that was `created_before(test_page)`, then you could list all pages created before `test_page`
with a `map` directive. The map directive would add a pagespec dependency on `created_before(test_page)`.
Internally, there would be a second page-spec parsing function that discovers which pages a given pagespec
depends on. As well as the function `match_created_before()`, we'd have to add a new function `depend_created_before()`.
This new function would return a list of pages, which when any of them change, the output of `match_created_before()`
would change. In this example, it would just return `test_page`.
These lists of dependent pages could just be concatenated for every `match_...()` function in a pagespec - you can ignore
the boolean formula aspects of the pagespec for this. If a content dependency were added on these pages, then I think
the correct rebuilds would occur.
In all, this is a surprisingly difficult problem to solve perfectly. Consider the following case:
PageA.mdwn:
> [ShavesSelf]
PageB.mdwn
> Doesn't shave self.
ShavedByBob.mdwn:
> [!include pages="!link(ShavesSelf)"]
Does ShavedByBob.mdwn include itself?
(Yeah - in IkiWiki currently links are included by include, but the idea holds. I had a good example a while back, but I can't think of it right now.)
sigh.
-- [[Will]]
----
### Link dependencies