[GHC] #11450: Associated types at wrong type in instance

[GHC]

#11450: Associated types at wrong type in instance
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11449, #11451    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"8136a5cbfcd24647f897a2fae9fcbda0b1624035/ghc" 8136a5c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="8136a5cbfcd24647f897a2fae9fcbda0b1624035"
 Tighten checking for associated type instances

 This patch finishes off Trac #11450.  Following debate on that ticket,
 the patch tightens up the rules for what the instances of an
 associated type can look like.  Now they must match the instance
 header exactly. Eg

    class C a b where
     type T a x b

 With this class decl, if we have an instance decl
   instance C ty1 ty2 where ...
 then the type instance must look like
      type T ty1 v ty2 = ...
 with exactly
   - 'ty1' for 'a'
   -  'ty2' for 'b', and
   - a variable for 'x'

 For example:

   instance C [p] Int
     type T [p] y Int = (p,y,y)

 Previously we allowed multiple instance equations and now, in effect,
 we don't since they would all overlap.  If you want multiple cases,
 use an auxiliary type family.

 This is consistent with the treatment of generic-default instances,
 and the user manual always said "WARNING: this facility (multiple
 instance equations may be withdrawn in the future".

 I also improved error messages, and did other minor refactoring.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:23>
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