[GHC] #15710: Should accept a type that needs coercion quantification

5 Oct 2018


      #15710: Should accept a type that needs coercion quantification
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider
 {{{
 f :: forall k (f :: k) (x :: k1). (k ~ (k1 -> *)) => f x
 f = error "uk"
 }}}
 Should we accept it?  Now that we have coercion quantification (Trac
 #15497), I think the answer should be yes, with the elaborated signature
 being
 {{{
 f :: forall k (f::k) (x::k1). forall (co :: k ~# (k1->*)). (f |> co) x
 }}}
 But there is a problem: the user wrote `k ~ (k1 -> *)`, and that's a boxed
 value that  we can't take apart in types.  I'm not sure what to do here.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15710
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