[GHC] #14119: Refactor type patterns

[GHC]

#14119: Refactor type patterns
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        Reporter:  goldfire          |                Owner:  goldfire
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12564, 13910,
                                     |  13938, 14038
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by goldfire):

 * owner:  (none) => goldfire


Comment:

 After a chat at ICFP, Simon and I agreed that this was worthwhile, but in
 a very different way than originally proposed: create a new datatype
 `TypePat`, which would be a ''Core'' type pattern, used in, e.g.,
 `FamInst`. Such a type would have no casts, no type families, and no
 foralls. Casts can be gotten rid of because the pure unifier (the client
 of type patterns) never uses them, unless they wrap a variable. And in
 that case, we can just use a variable with a new, corrected kind, and
 substitute the new one in for the old one (casting appropriately in the,
 e.g., type family equation's RHS). This transformation would happen after
 kind-checking and desugaring.

 Incidentally, it turns out that the underlying problem that this ticket
 was originally opened for was fixed by a small fix in the unifier. (The
 matching substitution must be applied to any cast found in the type
 pattern.) But that fix might not catch every possible case, while the
 proposal in this comment should.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14119#comment:9>
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