[GHC] #12384: Type family not reduced, again

[GHC]

#12384: Type family not reduced, again
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        Reporter:  kosmikus          |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  invalid           |             Keywords:
Operating System:  Linux             |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 Yes, I think this is something we simply don't support at all.

 Associated type declarations are just a way to remind you to add `type
 instance` decl for D whenever you make an `instance` decl for `C`.  It's
 always equivalent to doing it separately, thus:
 {{{
 type instance D a = True
 instance (F a ~ Bool) => C a where
 }}}
 And you can see that this isn't going to work.

 The point is that `(F a ~ Bool)` is has local evidence, bound by the
 dictionary lambda for the instance dfun.  That evidence can't be used in a
 `type instance` decl.

 So I'll close this as invalid, but you can re-open as a feature request if
 (a) you think it's important, (b) you can specify what it does, and (c)
 you can extend System FC to support it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12384#comment:3>
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