update

[git.ikiwiki.info.git] / doc / todo / dependency_types.mdwn

diff --git a/doc/todo/dependency_types.mdwn b/doc/todo/dependency_types.mdwn
index 01205f178a16c77d53285eacfb75965bbb53c31a..cce5997e84d745a6d49a6ac9a7a7bdb340c3cb54 100644 (file)
--- a/doc/todo/dependency_types.mdwn
+++ b/doc/todo/dependency_types.mdwn
@@ -236,6 +236,32 @@ sigh.
 >> dependency graph.  This can lead to situations where the result is just not
 >> well defined.  This is what I was trying to get at above. -- [[Will]]
 
 >> dependency graph.  This can lead to situations where the result is just not
 >> well defined.  This is what I was trying to get at above. -- [[Will]]
 

+>>> Hmm, I'm not seeing cycles be a problem, at least with the current
+>>> pagespec terms. --[[Joey]] 
+
+>>>> Oh, they're not with current pagespec terms.  But this is really close to extending to handle
+>>>> functional pagespecs, etc.  And I think I'd like to think about that now.
+>>>>
+>>>> Having said that, I don't want to hold you up - you seem to be making progress.  The best is
+>>>> the enemy of the good, etc. etc.
+>>>>
+>>>> For my part, I'm imagining we have two more constructs in IkiWiki:
+>>>>
+>>>>  * A map directive that actually wikilinks to the pages it links to, and
+>>>>  * A `match_sharedLink(pageX)` matching function that matches pageY if both pageX and pageY each have links to any same third page, pageZ.
+>>>>
+>>>> With those two constructs, one page changing might change the set of pages included in a map somewhere, which might then change the set of pages matched by some other pagespec, which might then...
+>>>>
+>>>> --[[Will]]
+
+>>>>> I think that should be supported by [[bugs/transitive_dependencies]].
+>>>>> At least in the current implementation, which considers each page
+>>>>> that is rendered to be changed, and rebuilds pages that are dependent
+>>>>> on it, in a loop. An alternate implementation, which could be faster,
+>>>>> is to construct a directed graph and traverse it just once. Sounds
+>>>>> like that would probably not support what you want to do.
+>>>>> --[[Joey]]
+

 ---- 
 
 ### Link dependencies
 ---- 
 
 ### Link dependencies


@@ -276,7 +302,7 @@ One way to fix this is to include with each dependency, a list of pages
 that currently match it. If the list changes, the dependency is triggered.
 
 Should be doable, but may involve more work than
 that currently match it. If the list changes, the dependency is triggered.
 
 Should be doable, but may involve more work than

-currently. Consider that a dependency on "bugs/*" currently
+currently. Consider that a dependency on `bugs/*` currently

 is triggered by just checking until *one* page is found to match it.
 But to store the list, *every* page would have to be tried against it.
 Unless the list can somehow be intelligently updated, looking at only the
 is triggered by just checking until *one* page is found to match it.
 But to store the list, *every* page would have to be tried against it.
 Unless the list can somehow be intelligently updated, looking at only the


@@ -310,22 +336,51 @@ can indirectly influence what pages a pagespec matches.
 
 > Trying to make a formal definition of this: (Note, I'm using the term sets rather than lists, but they're roughly equivalent)
 >
 
 > Trying to make a formal definition of this: (Note, I'm using the term sets rather than lists, but they're roughly equivalent)
 >

->  * Let the *matching set* for a pagespec be the set of pages that the pagespec matches.
->  * Let a *complete influence set* for a pagespec be the set of all pages whose alteration might change the matching set of that pagespec.
->  * Let the *direct influence set* be the intersection of the matching set and the complete influence set.
->  * Let the *indirect influence set* be the compliment of the direct influence set with respect to the complete influence set.
+>  * Let the *matching set* for a pagespec be the set of existing pages that the pagespec matches.
+>  * Let a *influence set* for a pagespec be the set of all pages, *p*, whose alteration might:
+>    * cause the pagespec to include or exclude a page other than *p*, or
+>    * cause the pagespec to exclude *p*.
+>
+>> \[Will snipped some stuff and edited the formal definition]

 >
 >

-> Is that a fair definition?  I don't think it quite matches your examples below unfortunately.
-> I was unsure if I should insert the word 'existing' in there in a few places.  As it stands, these definitions could include sets of pages that don't exist, e.g. "*".
-> The one I'm least sure of is the definition of the direct influence set.  It feels like you want something
-> like "the traditional set of things we thought about that could cause a pagespec to change", but that definition
-> is not very formal and I suspect will lead to problems.  Something like "The set of pages matched by the globs in the pagespec" might be closer?

 > --[[Will]]
 
 > --[[Will]]
 

+>> I appreciate the formalism! 
+>>
+>> Only existing pages need to be in these sets, because if a page is added
+>> in the future, the existing dependency code will always test to see
+>> if it matches. So it will be in the maching set (or not) at that point.
+>>
+>>> Hrm, I agree with you in general, but I think I can come up with nasty counter-examples.  What about a pagespec
+>>> of "!backlink(bogus)" where the page bogus doesn't exist?  In this case, the page 'bogus' needs to be in the influence
+>>> set even though it doesn't exist.
+>>>
+>>>> I think you're right, this is a case that the current code is not
+>>>> handling. Actually, I made all the pagespecs return influences
+>>>> even if the influence was not present or did not match. But, it
+>>>> currently only records influences as dependencies when a pagespec
+>>>> successfully matches. Now I'm sure that is wrong, and I've removed
+>>>> that false optimisation. I've updated some of the below. --[[Joey]]
+>>>
+>>> Also, I would really like the formalism to include the whole dependency system, not just any additions to it.  That will make
+>>> the whole thing much easier to reason about.
+>>
+>> The problem with your definition of direct influence set seems to be
+>> that it doesn't allow `link()` and `title()` to have as an indirect
+>> influence, the page that matches. But I'm quite sure we need those.
+>>  --[[Joey]] 
+
+>>> I see what you mean.  Does the revised definition capture this effectively?
+>>> The problem with this revised definition is that it still doesn't match your examples below.
+>>> My revised definition will include pretty much all currently matching pages to be in the influence list
+>>> because deletion of any of them would cause a change in which pages are matched - the removal problem.
+>>> -- [[Will]]
+

 #### Examples
 
 * The pagespec "created_before(foo)" has an influence list that contains foo.
 #### Examples
 
 * The pagespec "created_before(foo)" has an influence list that contains foo.

-  The removal or (re)creation of foo changes what pages match it.
+  The removal or (re)creation of foo changes what pages match it. Note that
+  this is true even if the pagespec currently fails to match.

 
 * The pagespec "foo" has an empty influence list. This is because a
   modification/creation/removal of foo directly changes what the pagespec
 
 * The pagespec "foo" has an empty influence list. This is because a
   modification/creation/removal of foo directly changes what the pagespec


@@ -335,14 +390,30 @@ can indirectly influence what pages a pagespec matches.
   Avoiding including every page in the wiki into its influence list is
   very important!
 
   Avoiding including every page in the wiki into its influence list is
   very important!
 

+>>> So, why don't the above influence lists contain the currently matched pages?
+>>> Don't you need this to handle the removal problem? -- [[Will]]
+
+>>>> The removal problem is slightly confusingly named, since it does not
+>>>> affect pages that were matched by a glob and have been removed. Such
+>>>> pages can be handled without being influences, because ikiwiki knows
+>>>> they have been removed, and so can still match them against the
+>>>> pagespec, and see they used to match; and thus knows that the
+>>>> dependency has triggered.
+>>>>
+>>>> Maybe the thing to do is consider this an optimisation, where such
+>>>> pages are influences, but ikiwiki is able to implicitly find them,
+>>>> so they do not need to be explicitly stored. --[[Joey]]
+

 * The pagespec "title(foo)" has an influence list that contains every page
   that currently matches it. A change to any matching page can change its
 * The pagespec "title(foo)" has an influence list that contains every page
   that currently matches it. A change to any matching page can change its

-  title. Why is that considered an indirect influence? Well, the pagespec
-  might be used in a presence dependency, and so its title changing
-  would not directly affect the dependency.
+  title, making it not match any more, and so the list is needed due to the
+  removal problem. A page that does not have a matching title is not an
+  influence, because modifying the page to change its title directly
+  changes what the pagespec matches.

 
 * The pagespec "backlink(index)" has an influence list
   that contains index (because a change to index changes the backlinks).
 
 * The pagespec "backlink(index)" has an influence list
   that contains index (because a change to index changes the backlinks).

+  Note that this is true even if the backlink currently fails.

 
 * The pagespec "link(done)" has an influence list that
   contains every page that it matches. A change to any matching page can
 
 * The pagespec "link(done)" has an influence list that
   contains every page that it matches. A change to any matching page can


@@ -353,6 +424,10 @@ can indirectly influence what pages a pagespec matches.
 >> 'done' to include a link to 'done', then it will now match...  or is that considered a
 >> 'direct match'? -- [[Will]]
 
 >> 'done' to include a link to 'done', then it will now match...  or is that considered a
 >> 'direct match'? -- [[Will]]
 

+>>> The regular dependency calculation code will check if every changed
+>>> page matches every dependency. So it will notice the link was added.
+>>> --[[Joey]] 
+

 #### Low-level Calculation
 
 One way to calculate a pagespec's influence would be to
 #### Low-level Calculation
 
 One way to calculate a pagespec's influence would be to


@@ -400,17 +475,57 @@ Given that, the `backlink` will always be evalulated, and will put index
 onto the influence list. If we combine the influences from each
 successful match, we get the right result.
 
 onto the influence list. If we combine the influences from each
 successful match, we get the right result.
 

-> This is implemented, seems to work ok. --[[Joey]] 
+> This is implemented, seems to work ok. --[[Joey]]

 
 

-#### High-level Calculation and Storage
+> `or` short-circuits too, but the implementation correctly uses `|`,
+> which I assume is what you meant. --[[smcv]]
+
+>> Er, yeah. --[[Joey]] 

 
 

-Calculating the full influence list for a pagespec requires trying to match
-it against every page in the wiki. 
+----
+
+What about: "!link(done)"
+
+Specifically, I want to make sure it works now that I've changed
+`match_link` to only return a page as an influence if it *does*
+link to done.
+
+So, when matching against page P, that does not link to done,
+there are no influences, and the pagespec matches. If P is later
+changed to add a link to done, then the dependency resolver will directly
+notice that.
+
+When matching against page P, that does link to done, P
+is an influence, and the pagespec does not match. If P is later changed
+to not link to done, the influence will do its job.
+
+Looks good!
+
+#### High-level Calculation and Storage

 
 

-I'd like to avoid doing such expensive matching redundantly. So add a
-`pagespec_match_all`, which returns a list of all pages in the whole
-wiki that match the pagespec, and also adds the pagespec as a dependency,
-and while it's at it, calculates and stores the influence list.
+Naively calculating the full influence list for a pagespec requires trying
+to match it against every page in the wiki. I'd like to avoid doing such
+expensive matching redundantly.
+
+It may be possible, for some types of pagespecs, to just try matching a
+single, arbitrary page against it, and know the full influence list has
+been obtained. It seems to be that case that if a pagespec has any
+influences, matching any page will return at least one. So if none are
+returned, we can skip trying other pages.
+
+If the influence list does not include the page that was tried, we know
+that the pagespec does not things like `link()` and `title()`, that are
+influenced by the page's own content. So it *might* be safe to not try
+matching any more pages in this case too. I think it would work for all
+current pagespec terms. There might be a hypothetical term where this
+optimisation doesn't work. We could add a special case to ensure it can
+work: If a term declares it is unfluenced by "", then it means it is
+always influenced by the matching page.
+
+Anyway, this seems worth doing: Add a `pagespec_match_all`, which returns a
+list of all pages in the whole wiki that match the pagespec, and also adds
+the pagespec as a dependency, and while it's at it, calculates and stores
+the influence list.

 
 It could have an optional sort parameter, and limit parameter, to control
 how many items to return and the sort order. So when inline wants to
 
 It could have an optional sort parameter, and limit parameter, to control
 how many items to return and the sort order. So when inline wants to


@@ -435,7 +550,7 @@ it's calculated more smartly, and is added automatically.
 
 > I've implemented influence calculation in `add_depends`. As expected,
 > it means rather a lot more work, and makes some things much slower.
 
 > I've implemented influence calculation in `add_depends`. As expected,
 > it means rather a lot more work, and makes some things much slower.

-> Optimisation via `pagespec_match_depends` next.. --[[Joey]] 
+> Optimisations next.. --[[Joey]] 

 
 #### Influence types
 
 
 #### Influence types
 


@@ -448,3 +563,5 @@ type. Etc.
 > allow significant computational savings, but I suspect will be tricky
 > to implement. -- [[Will]]
 
 > allow significant computational savings, but I suspect will be tricky
 > to implement. -- [[Will]]
 

+>> It was actually really easy to implement it, assuming I picked the right
+>> dependency types of course. --[[Joey]]