From: Joey Hess <joey@gnu.kitenet.net>
Date: Wed, 7 Oct 2009 17:00:54 +0000 (-0400)
Subject: Merge branch 'master' into dependency-types
X-Git-Url: https://sipb.mit.edu/gitweb.cgi/ikiwiki.git/commitdiff_plain/b6b7dc3a43266515822c31b177bfd26e8d59f359?hp=-c

Merge branch 'master' into dependency-types
---

b6b7dc3a43266515822c31b177bfd26e8d59f359
diff --combined doc/todo/dependency_types.mdwn
index 32c9187be,0503b47af..4bc38e9c0
--- a/doc/todo/dependency_types.mdwn
+++ b/doc/todo/dependency_types.mdwn
@@@ -156,13 -156,77 +156,74 @@@ false negatives (though these should b
  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+ That branch is a little out of date, but if you just look at the changes in IkiWiki.pm you'll see the concept I was looking at.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
  
  * `add_depends($page, $spec, links => 1, presence => 1)`
    adds a links + presence dependency.
 -* `refresh` only rebuilds a page with a links dependency if
 -  pages matched by the pagespec gain or lose links. (What the link
 -  actually points to may change independent of this, due to changes
 -  elsewhere, without it firing.)
 +* Use backlinks change code to detect changes to link dependencies too.
  * So, brokenlinks can fire whenever any links in any of the
    pages it's tracking change, or when pages are added or
    removed.
@@@ -172,7 -236,6 +233,7 @@@
    that the page links to, which is just what link dependencies are
    triggered on.
  
 +[[done]]
  ----
  
  ### the removal problem