[GHC] #14835: QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b"

21 Feb 2018


      #14835: QuantifiedConstraints: Can't deduce "(a, b)" from "a" and "b"
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
  QuantifiedConstraints wipT2893     |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# Language QuantifiedConstraints, RankNTypes, PolyKinds,
 ConstraintKinds, UndecidableInstances, GADTs, LiberalTypeSynonyms,
 TypeOperators, MultiParamTypeClasses, FlexibleInstances #-}

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

 infixr |-
 class    (a => b) => (a |- b)
 instance (a => b) => (a |- b)

 type a :- b = Dict (a |- b)

 class    ((a, b) => c) => Curried a b c
 instance ((a, b) => c) => Curried a b c

 curryC :: forall a b c. ((a, b) |- c) :- (a |- b |- c)
 curryC = Dict
 }}}

 This fails with

 {{{
 GHCi, version 8.5.20180128: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( /tmp/Test.hs, interpreted )

 /tmp/Test.hs:16:10: error:
     • Could not deduce (a, b) arising from a use of ‘Dict’
       from the context: (a, b) |- c
         bound by a quantified context at /tmp/Test.hs:1:1
       or from: a bound by a quantified context at /tmp/Test.hs:1:1
       or from: b bound by a quantified context at /tmp/Test.hs:1:1
       Possible fix:
         add (a, b) to the context of
           the type signature for:
             curryC :: forall (a :: Constraint) (b :: Constraint) (c ::
 Constraint).
                       ((a, b) |- c) :- (a |- (b |- c))
     • In the expression: Dict
       In an equation for ‘curryC’: curryC = Dict
    |
 16 | curryC = Dict
    |          ^^^^
 Failed, no modules loaded.
 }}}

 but it works by changing the type of `curryC`:

 {{{#!hs
 curryC :: forall a b c. Curried a b c :- (a |- b |- c)
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14835
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