[GHC] #13006: Possible program should type check but does not using Implicit Parameters and Vectors

[GHC]

#13006: Possible program should type check but does not using Implicit Parameters
and Vectors
-------------------------------------+-------------------------------------
        Reporter:  clinton           |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2-rc2
      Resolution:  invalid           |             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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Changes (by simonpj):

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


Comment:

 In
 {{{
 f _ = let ?v = x in g
 }}}
 what type did you expect `f` to have?  GHC infers
 {{{
 f :: forall a n. (...) => a -> n ()
 }}}
 because `g :: forall m. (PrimMonad m, ?v :: T m) => m ()`.  (I've used `n`
 instead of `m` in `f`'s type to emphasise that it is different to `m`.

 What is the `(...)`?  The body of `f` has a call `g`, which gives rise to
 the constraints `(PrimMonad n, ?v :: T n)`. Alas the `?v` binding in f's
 RHS binds `?v :: T m` (since `x :: T m`), and that is no help in
 discharging `?v :: T n`.  So we end up with
 {{{
 f :: forall a n. (PrimMonad n, ?v :: T n) => n ()
 }}}
 and hence the error.

 By adding the signature you make `f` monomorphic (`-XMonoLocalBinds` has
 the same effect) which solves the problem.

 Perhaps we should be more aggressive in reporting an error when functional
 dependencies can't be satisfied, but I think the program should indeed be
 rejected.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13006#comment:1>
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