bug#20420: 25.0.50; eieio methods with optional arguments now fail

[Vitalie Spinu <spinuvit@gmail.com>]

 >>> Stefan Monnier on Sun, 26 Apr 2015 00:02:51 -0400 wrote:

 >>> That tells me *how* you used it, not *where*.
 >> I was using it in polymode package for a generic indentation
 >> functionality:
 >> https://github.com/vspinu/polymode/blob/master/polymode-methods.el#L530

 > [ After figuring out that the code now doesn't do that any more and
 >   seeing the old code which does do the "nasty" empty-args defmethod.  ]

Sorry about that. I wasn't expected you will be interested in so much
detail. I would have been more elaborate.

 >> Aha. Cool! I will have a look. Is there a more elaborate documentation
 >> somewhere?

 > No, it's more a wishlist item: add support for formal pseudo-arguments
 > of the form "&context (<exp> <specializer>)".

 >> Particularly I don't see "specializer" and "generalizer"
 >> being properly defined anywhere.

 > "Specializer" is used commonly in CLOS to refer to the "thing" that can
 > be either a class type or (eql <value>).  "Generalizer" is not standard
 > and refers to a thing that takes a value (the actual argument) and finds
 > its corresponding specializers (i.e. its type(s)).  I took the term from
 > an article that extended CLOS method-matching.  cl-generic.el does not
 > directly implement that article, but it's fairly similar.

This sounds like a more involved version of what Clojure does with
multimethods (also implemented in emacs multi.el package [1])

When Clojure's generic is defined you supply a dispatch function that
takes all actual arguments and returns an object (commonly a
symbol). The returned object is then used directly for dispatch. If I
understand correctly the dispatch function is like your generalizer.

  Vitalie

[1] https://github.com/kurisuwhyte/emacs-multi
Better documented at https://github.com/vspinu/emacs-multi