Re: [GHC] #14863: QuantifiedConstraints: Can't deduce `c' from `(a, b)' and `a |- b |- c'

27 Feb 2018

      #14863: QuantifiedConstraints: Can't deduce `c' from `(a, b)' and `a |- b |- c'
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.5
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints wipT2893
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Once again, I think this `:-` thing is a red herring. Here's what I
 believe to be the essence of the bug:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE ImpredicativeTypes #-}
 {-# LANGUAGE MultiParamTypeClasses #-}
 {-# LANGUAGE QuantifiedConstraints #-}
 {-# LANGUAGE UndecidableInstances #-}
 module Bug where

 data Dict c where
   Dict :: c => Dict c

 class    (a => b) => Implies a b
 instance (a => b) => Implies a b

 uncurryCImpredic1 :: forall a b c. Implies a (b => c) => Dict (Implies (a,
 b) c)
 uncurryCImpredic1 = Dict

 uncurryCImpredic2 :: forall a b c. a => Implies b c => Dict (Implies (a,
 b) c)
 uncurryCImpredic2 = Dict

 uncurryC1 :: forall a b c. (a => b => c) => Dict (Implies (a, b) c)
 uncurryC1 = Dict

 uncurryC2 :: forall a b c. Implies a (Implies b c) => Dict (Implies (a, b)
 c)
 uncurryC2 = Dict
 }}}

 {{{
 $ ghc-cq/inplace/bin/ghc-stage2 --interactive Bug.hs
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Bug              ( Bug.hs, interpreted )

 Bug.hs:27:13: error:
     • Could not deduce: c arising from a use of ‘Dict’
       from the context: Implies a (Implies b c)
         bound by the type signature for:
                    uncurryC2 :: forall (a :: Constraint) (b :: Constraint)
 (c :: Constraint).
                                 Implies a (Implies b c) =>
                                 Dict (Implies (a, b) c)
         at Bug.hs:26:1-77
       or from: (a, b) bound by a quantified context at Bug.hs:1:1
     • In the expression: Dict
       In an equation for ‘uncurryC2’: uncurryC2 = Dict
     • Relevant bindings include
         uncurryC2 :: Dict (Implies (a, b) c) (bound at Bug.hs:27:1)
    |
 27 | uncurryC2 = Dict
    |             ^^^^
 }}}

 All of these `uncurryC`-functions typecheck except for `uncurryC2`. (Note
 that the first two require `ImpredicativeTypes`, so I'm not sure if this
 is intended to be supported or not.)

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14863#comment:1
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