path: root/doc/todo/matching_different_kinds_of_links.mdwn

[[!tag wishlist]]
As noted in [[todo/tag_pagespec_function]], there is a "misbehavior" of a tagged() pagespec: it matches even pages which have plain links to the tag page.
And in general, it would be quite useful to be able to distinguish different kinds of links: one more kind, in addition to "tag", is "bug dependency" noted in [[todo/structured_page_data#another_kind_of_links]] and [[todo/tracking_bugs_with_dependencies#another_kind_of_links]].
It could distinguish the links by the rel= attribute. ([[Tags already receive a special rel-class|todo/rel_attribute_for_links]].) This means there is a general need for a syntax to specify user-defined rel-classes on wikilink (then bug deps would simply use their special rel-class, either directly, or through a special directive like \[[!depends ]]), and to refer to them in pagespecs (in forward and backward direction).
Besides pagespecs, the rel= attribute could be used for styles. --Ivan Z.

FWIW, the add_link function introduced in a recent
release adds an abstraction that could be used to get
part of the way there to storing data about different types of
links. That function could easily be extended to take an optional
third parameter specifying the link type.
Then there's the question of how to store and access the data. %links
does not offer a good way to add additional information about links.
Now, we could toss %links entirely and switch to an accessor function,
but let's think about not doing that..
The data that seems to be needed is basically a deep hash, so
one could check $linktype{$page}{tag}{$link} to see if
the page contains a link of the given type. (Note that pages could
contain links that were duplicates except for their types.)
There would be some data duplication, unfortuantly, but if %linktype
is not populated for regular wikilinks, it would at least be limited to
tags and other unusual link types, so not too bad.
%linktype could be stored in %pagestate.. if so
the actual use might look like $pagestate{$page}{linktype}{tag}{$link}.
That could be implemented by the tag plugin right now
with no core changes. (BTW, then I originally wrote tag, pagestate
was not available, which is why I didn't make it differentiate from
normal links.) Might be better to go ahead and add the variable to
core though. --[[Joey]]



I've implemented this with the data structure you suggested, except that
I called it %typedlinks instead of %linktype (it seemed to make more
sense that way). I also ported tag to it, and added a tagged_is_strict
config option. See below! --[[smcv]]


I saw somewhere else here some suggestions for the wiki-syntax for specifying the relation name of a link. One more suggestion---the syntax used in Semantic MediaWiki, like this:

So a part of the effect of [[\[[!taglink TAG\]\]|plugins/tag]] could be represented as something like \[[tag::TAG]] or (more understandable relation name in what concerns the direction) \[[tagged::TAG]].
I don't have any opinion on this syntax (whether it's good or not)...--Ivan Z.



[[!template id=gitbranch author="[[Simon_McVittie|smcv]]" branch=smcv/ready/link-types]]
[[!tag patch]]


Documentation for smcv's branch
added to [[ikiwiki/pagespec]]

"typedlink(type glob)" - matches pages that link to a given page (or glob)
with a given link type. Plugins can create links with a specific type:
for instance, the tag plugin creates links of type tag.

added to [[plugins/tag]]
If the tagged_is_strict config option is set, tagged() will only match
tags explicitly set with [[ikiwiki/directive/tag]] or
[[ikiwiki/directive/taglink]]; if not (the default), it will also match
any other [[WikiLinks|ikiwiki/WikiLink]] to the tag page.
added to [[plugins/write]]
%typedlinks
The %typedlinks hash records links of specific types. Do not modify this
hash directly; call add_link(). The keys are page names, and the values
are hash references. In each page's hash reference, the keys are link types
defined by plugins, and the values are hash references with link targets
as keys, and 1 as a dummy value, something like this:
$typedlinks{"foo"} = {
	tag => { short_word => 1, metasyntactic_variable => 1 },
	next_page => { bar => 1 },
};

Ordinary [[WikiLinks|ikiwiki/WikiLink]] appear in %links, but not in
%typedlinks.
add_link($$;$)
This adds a link to %links, ensuring that duplicate links are not
added. Pass it the page that contains the link, and the link text.
An optional third parameter sets the link type (undef produces an ordinary
[[ikiwiki/WikiLink]]).
Review
Some code refers to oldtypedlinks, and other to oldlinktypes. --[[Joey]]

Oops, I'll fix that. That must mean missing test coverage, too :-(
--s



A test suite for the dependency resolver would be nice. --[[Joey]]





Bug fixed, I think. A test suite for the dependency resolver seems
more ambitious than I want to get into right now, but I added a
unit test for this part of it... --s



I'm curious what your reasoning was for adding a new variable
rather than using pagestate. Was it only because you needed
the old version to detect change, or was there other complexity?
--J

You seemed to be more in favour of adding it to the core in
your proposal above, so I assumed that'd be more likely to be
accepted :-) I don't mind one way or the other - %typedlinks
costs one core variable, but saves one level of hash nesting. If
you're not sure either, then I think the decision should come down
to which one is easier to document clearly - I'm still unhappy with
my docs for %typedlinks, so I'll try to write docs for it as
pagestate and see if they work any better. --s



On reflection, I don't think it's any better as a pagestate, and
the contents of pagestates (so far) aren't documented for other
plugins' consumption, so I'm inclined to leave it as-is, unless
you want to veto that. Loose rationale: it needs special handling
in the core to be a dependency type (I re-used the existing link
type), it's API beyond a single plugin, and it's really part of
the core parallel to pagestate rather than being tied to a
specific plugin. Also, I'd need to special-case it to have
ikiwiki not delete it from the index, unless I introduced a
dummy typedlinks plugin (or just hook) that did nothing... --s


I have not convinced myself this is a real problem, but..
If a page has a typed link, there seems to be no way to tell
if it also has a separate, regular link. add_link will add
to @links when adding a typed, or untyped link. If only untyped
links were recorded there, one could tell the difference. But then
typed links would not show up at all in eg, a linkmap,
unless it was changed to check for typed links too.
(Or, regular links could be recorded in typedlinks too,
with a empty type. (Bloaty.)) --J

I think I like the semantics as-is - I can't think of any
reason why you'd want to ask the question "does A link to B,
not counting tags and other typed links?". A typed link is
still a link, in my mind at least. --s



Me neither, let's not worry about it. --[[Joey]]


I suspect we could get away without having tagged_is_strict
without too much transitional trouble. --[[Joey]]

If you think so, I can delete about 5 LoC. I don't particularly
care either way; [[Jon]] expressed concern about people relying
on the current semantics, on one of the pages requesting this
change. --s



Removed in a newer version of the branch. --s


I might have been wrong to introduce typedlink(tag foo). It's not
very user-friendly, and is more useful as a backend for other plugins
that as a feature in its own right - any plugin introducing a link
type will probably also want to have its own preprocessor directive
to set that link type, and its own pagespec function to match it.
I wonder whether to make a typedlink plugin that has the typedlink
pagespec match function and a new \[[!typedlink to="foo" type="bar"]]
though... --[[smcv]]

I agree, per-type matchers are more friendly and I'm not enamored of the
multi-parameter pagespec syntax. --[[Joey]]



Removed in a newer version of the branch. I re-introduced it as a
plugin in smcv/typedlink, but I don't think we really need it. --s



I am ready to merge this, but I noticed one problem -- since match_tagged
now only matches pages with the tag linktype, a wiki will need to be
rebuilt on upgrade in order to get the linktype of existing tags in it
recorded. So there needs to be a NEWS item about this and
the postinst modified to force the rebuild.

Done, although you'll need to plug in an appropriate version number when
you release it. Is there a distinctive reminder string you grep for
during releases? I've used UNRELEASED for now. --[[smcv]]

Also, the ready branch adds typedlink() to [[ikiwiki/pagespec]],
but you removed that feature as documented above.
--[[Joey]]

[[Done]]. --s