[GHC] #7543: Constraint synonym instances

[GHC]

#7543: Constraint synonym instances
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        Reporter:  monoidal          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.6.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
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Comment (by goldfire):

 The other thing we could do is support this feature at synonym-definition
 time. When defining a type synonym such that the head of the RHS is a
 class, record this fact in the `TyCon` and corresponding iface info. If
 such a synonym appears as the head of an instance declaration, we use this
 tidbit to do the name lookup.

 This would be easier to explain, at least, and wouldn't require much
 upheaval either. Actually, this is probably simpler than my idea in
 comment:7.

 About looking up instances: we really need to move beyond `grep` for this
 sort of thing! Of course, I exclusively use `grep` for this sort of
 thing... because I don't have a better option to hand. But that's a story
 for another day. In the meantime, worrying about obfuscation via synonyms
 is valid, but also not something that (I believe) should hold this up.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7543#comment:9>
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