[GHC] #9725: Constraint deduction failure

[GHC]

#9725: Constraint deduction failure
-------------------------------------+-------------------------------------
              Reporter:  heisenbug   |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by simonpj):

 Richard, why do you think this is a bug?

 Looking at the simplified reproduction test case, we see
 {{{
 -- Restricted kind signature:
 --test :: forall (ent :: Bool -> En) . (forall i . Kn (ent i) => Fm (Fac
 (ent i))) -> Co Fm (O ent)

 test :: forall ent. (forall i . Kn (ent i) => Fm (Fac (ent i))) -> Co Fm
 (O ent)
 }}}
 The program typechecks fine with "restricted kind signature" (agreed?),
 with GHC 7.8.2 or HEAD.  So decomposition of type applications is working
 fine (i.e. comment:1 is not right; although in previous versions of GHC
 Reid was correct).

 The actual problem is that the type signature (the one not in comments)
 kind-checks like this:
 {{{
 test :: forall k. forall (ent :: k -> En).
         (forall (i::k). Kn (ent i) => Fm (Fac (ent i)))
      -> Co Fm (O ent)
 }}}
 The pattern-match on `Mb` indeed yields the given equality `ent::k ~
 M::Bool`.  But we can't ''use'' that unless we know that `k~Bool` and,
 until Richard implements kind-level equalities, we don't know that.

 The "restricted kind signature" de-polymorphises" it, which is all you
 need to make it work.

 Richard: you should add this to your test suite for kind-level equalities!
 Is there a wiki page for this?  Is it [wiki:DependentHaskell]?  I suspect
 not.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9725#comment:6>
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