[GHC] #13295: Failure to resolve type parameter determined by type family

[GHC]

#13295: Failure to resolve type parameter determined by type family
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        Reporter:  alexvieth         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         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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Comment (by simonpj):

 It's not just that GHC doesn't do this kind of reasoning; it's also that
 system FC does not (yet, anyway) have a way to express the evidence.

 What you want to say is "for any type `t`, `F t` must be equal to `'D s`
 for some type `s`".   But

 * If `F` has no instances that statement isn't true; `F t` isn't equal to
 any type other than `F t`.

 * Even if it does, we'd need an axiom of form
 {{{
    forall t.  F t ~ (exists s. 'D s)
 }}}
 and we have no such form.

 So I think this is well out of reach just at the moment.  Not wrong; just
 out of reach.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13295#comment:2>
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