[GHC] #10978: Anonymous type instances

[GHC]

#10978: Anonymous type instances
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        Reporter:  benjamin.hodgson  |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.8.3
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Replying to [comment:3 nomeata]:
 > > Because type families are not injective and generative, you cannot
 specify a type family in a class instance head.
 >
 > Would it be possible to allow type families in instance heads if they
 reduce to something that is allowed, akin to `TypeSynonymInstances`?

 This has been suggested before but was discarded as a bad idea. There's
 some useful conversation on the ticket... but I can't find the ticket!
 Short story: this would be akin to allowing term-level function
 definitions like `foo (not True) = 5`. If we reduce `not True`, we'll get
 `False`, and this definition is perfectly sound. But it's terrible,
 nonetheless.

 If you could find the ticket, I'd be grateful. But I really did try. :(

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10978#comment:4>
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