[GHC] #14010: UndecidableSuperClasses - Could not deduce (Category d)

22 Jul 2017

      #14010: UndecidableSuperClasses - Could not deduce (Category d)
-------------------------------------+-------------------------------------
           Reporter:  int-index      |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  UndecidableSuperClasses            |
       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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 Consider this well-typed code:

 {{{#!hs
 {-# LANGUAGE NoImplicitPrelude, TypeInType, TypeFamilies,
              UndecidableSuperClasses, RankNTypes, TypeOperators,
              FlexibleContexts, TypeSynonymInstances, FlexibleInstances,
              UndecidableInstances #-}
 module Monolith where

 import Data.Kind (Type)
 import GHC.Exts (Constraint)

 type family (~>) :: c -> c -> Type

 type instance (~>) = (->)
 type instance (~>) = ArrPair

 type family Fst (p :: (a, b)) :: a where
   Fst '(x, _) = x

 type family Snd (p :: (a, b)) :: b where
   Snd '(_, y) = y

 data ArrPair a b = ArrPair (Fst a ~> Fst b) (Snd a ~> Snd b)

 type family Super c :: Constraint where
   Super Type = ()
   Super (c, d) = (Category c, Category d)

 class Super cat => Category cat where
   id :: forall (a :: cat). a ~> a

 instance Category Type where
   id = \x -> x

 instance (Category c, Category d) => Category (c, d) where
   id = ArrPair id id

 class Category (c, d) => Functor (f :: c -> d) where
 --class (Category c, Category d) => Functor (f :: c -> d) where
   map :: (a ~> b) -> (f a ~> f b)

 data OnSnd f a b = OnSnd (f '(a, b))

 instance Functor (f :: (c, d) -> Type) => Functor (OnSnd f a) where
   map f (OnSnd x) = OnSnd (map (ArrPair id f) x)
 }}}

 The compiler accepts it. But if I change the definition of 'Functor' for
 the commented one, I get an error:

 {{{
 super.hs:39:10: error:
     • Could not deduce (Category d)
         arising from the superclasses of an instance declaration
       from the context: Functor f
         bound by the instance declaration at super.hs:39:10-61
       Possible fix:
         add (Category d) to the context of the instance declaration
     • In the instance declaration for ‘Functor (OnSnd f a)’
    |
 39 | instance Functor (f :: (c, d) -> Type) => Functor (OnSnd f a) where
    |
 }}}

 I can reproduce this on 8.0.1, 8.2.0.20170704 and HEAD.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14010
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