it has been some years since the [[matching different kinds of links]] issue
was tackled, but hardly a plugin is using it.

in order to enhance on the [[todo/rel attribute for links]] and [[todo/better bug tracking support]]
issues and to provide a more general infrastructure, i'd like to propose a
generic plugin for typed links. it can be also viewed of a way to have
[[todo/structured page data]] that consists of URLs inside the wiki.

following the use case i've developed it for, i'll call it `blocks` for the
moment (but am open to better suggestions).

outline
=======

the plugin has a **configuration option** called `blocks_names`, which consists
of pairs of verbs; the typical example is `blocks/blockedby`, but other values
could be `next/prev up/down` or `owner/owns`.

for each verb in the options, there is a **directive** which is used to state
the relationship; relationships can be declared on both ends, so a page `bugA`
with the contents `\[[!blocks bugB]]` is semantically equivalent to a page
`bugB` with the contents `\[[!blockedby bugA]]`.

for each verb, there is also a **pagespec** which matches all pages that are
the origin of a relationship to a given page. if `developerA` `\[[!owns
bug1]]`, then if `bug1` contains `\[[!map pages="owns(.)"]]`, it will show the
owning developer. these specs match both ways, ie. if `bug1` `\[[!owner
developerA]]`, the said map directive will still produce the same result.

details
=======

* single word relationships vs. symmetric relationships

  with some verbs, it is possible that a relationship is only used in one
  direction (eg `index`, even though one could declare it as
  `index/isindexof`).

  > isindexof is not a very interesting relationship - it just clogs up
  > the link-map, since the index is "the index of" all pages. I can't
  > see any situation in which you'd want to do pagespec matching
  > on it? --[[smcv]]

  >> that's why i used `index` as an example of a one-direction relationship.
  >>
  >> it wouldn't clog up the link map, though: in order to cleanly match both
  >> directions, when the "inverse" term of a relationship is used, the link in
  >> taggedlinks uses the "forward" term, but switches the objects.
  >>
  >> --[[chrysn]]

  other verbs are symmetric, eg. `equivalent`, which need different treatment.

* "taglink" style directives

  the [[plugins/tag]] plugin would be a special case for this plugin (apart
  from the autotag and tagdir features). as there is a `\[[!taglink ...]]`
  directive, there could be an analogous directive for every single directive.

  > This is basically the traillink/trailitem duality, too.
  > I'd be quite tempted to generalize to something like this:
  >
  >     We can't fix [[!link blocks="bug123" text="Bug 123"]] until we do this.
  >
  >     [[!hiddenlink owner="smcv"]]
  >
  > but perhaps that's too wordy?
  >
  > I think both trail and tag need their own special processing beyond the
  > general case, but maybe not? --[[smcv]]

  >> i'd be all in favor of having this unified and deeper; there has been the
  >> idea of a `\[[!link]]` directive [[again|todo/link plugin perhaps too general__63__]]
  >> and [[again|todo/do not make links backwards]].
  >>
  >> i like the `\[[!link text=""]]` and `[[!hiddenlink]]` conventions, but
  >> think that ${REL}="${TARGET}" isn't ideal because it implies that a single
  >> link can have more than one target. instead, i'd go for
  >> `\[[!link to="bug123" rel="blocks" text="Bug 123"]]; as with the html rel
  >> parameter, rel would be a list of whitespace separated values.
  >>
  >> positional parameters (`\[[!link bug123 rel="blocks" text="Bug 123"]]` or
  >> even `\[[!link Bug 123|bug123 rel="blocks"]]`) would be possible, but i
  >> prefer explicit syntax and not joining stings back again with the
  >> whitespace that was split off it before.
  >>
  >> if the '|' character is not widespread in page names (which i assume it is
  >> not), instead of using positional parameters in `\[[!link]]` for
  >> shortcuts, we could extend the regular link syntax; the same relationship
  >> could then be declared as `\[[Bug 123|bug123|blocks]]`; this would be an
  >> easy extension to the original link syntax. it would even work for hidden links
  >> (`\[[|smcv|owner]]`), which previously made no sense because a link with
  >> neither a physicial representation nor metadat is of no use.
  >>
  >> --[[chrysn]]

* implementation notes

  the way pagespec hooks are implemented required some nasty perl tricks, for
  which the people who showed me felt very bad for having spoilt me. indeed,
  `no strict refs;` and `*$forward_name = $forward_match;` are not exactly
  ideal. a change in the pagespec declaration api (why not just `hook` like
  everything else) would make the implementation cleaner.

  > How about replacing `blockedby(bug*)` with `linktype(blockedby bug*)` or
  > something? Then you'd only need one pseudo-hook. --[[smcv]]

  >> there has been the topic of pagespecs like `typedlink(type glob)` back in
  >> the [[matching different kinds of links]] discussion, but it was removed
  >> in favor of per-type matchers. --[[chrysn]]

* configuration location

  i aimed for static configuration of the `block_names` in the setup file. this
  could be made more general like in the [[plugins/shortcut]] plugin, but that
  would make things more complex.

* no html links with `rel=` yet

  as there are no taglink style links between the articles so far, no htmllink
  gets rendered that could carry the relationship name in its rel field.

  having the inverse relationship description in backlinks (as in the link
  created by the map directive in the example above) would be hard to
  implement. (actually, i think it'd be easier to determine the rel values from
  the taggedlinks for *every* htmllink than to influence the backlinks in this
  plugin).

* one direction also creates a normal link

  due to the way add\_link treats relationships, the forward relationship is
  always going to be reflected in the links/backlinks. a section of
  [[todo/matching different kinds of links]] was dismissed with "let's not
  worry about it", this plugin might be reason to worry about it again. (i'd
  consider what is in @links to be a representation of which hyperlinks are
  there, and in this case, none are generated).

  > taglink and traillink already count as wikilinks without generating
  > any visible HTML. --[[smcv]]

implementation
==============

there is a working but slightly incomplete (basically where it comes to the
details mentioned above) implementation in [[blocks.pm]].

--[[chrysn]]