[GHC] #13404: Derive instances for classes with associated types

9 Mar 2017


      #13404: Derive instances for classes with associated types
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When trying to derive an instance of a class with associated types

 {{{#!hs
 import Data.Distributive
 import Data.Functor.Rep

 data Pair a = a :# a
   deriving Functor

 instance Distributive Pair where
   distribute :: Functor f => f (Pair a) -> Pair (f a)
   distribute = distributeRep

 instance Representable Pair where
   type Rep Pair = Bool

   index :: Pair a -> (Bool -> a)
   index (a :# _) False = a
   index (_ :# b) True  = b

 newtype PAIR a = PAIR (Pair a)
   deriving (Functor, Representable)

 instance Distributive PAIR where
   distribute = {- distributeRep -}
 }}}

 {{{
     • Can't make a derived instance of ‘Representable PAIR’
         (even with cunning GeneralizedNewtypeDeriving):
         the class has associated types
     • In the newtype declaration for ‘PAIR’
 }}}

 But it could create an instance

 {{{#!hs
 instance Representable PAIR where
   type Rep PAIR = Bool

   index :: forall a. PAIR a -> (Bool -> a)
   index = coerce (index :: Pair a -> (Bool -> a))
 }}}

 Same with complicated expressions like

 {{{#!hs
 infixr 9 ·
 type (·) = Compose

 newtype P f g h a = P (Product (f·g·f) (h·f·g) a)
   deriving (Functor, Foldable, Traversable, Applicative, Alternative,
 Distributive)

 instance (Functor       f, Functor       g, Functor       h,
           Representable f, Representable g, Representable h)
        => Distributive  (P f g h) where
   distribute = distributeRep

 instance (Functor       f, Functor       g, Functor       h,
           Representable f, Representable g, Representable h)
        => Representable (P f g h) where
   type Rep (P f g h) = Either (Rep f, (Rep g, Rep f)) (Rep h, (Rep f, Rep
 g))

   index :: (P f g h) a -> (Either (Rep f, (Rep g, Rep f)) (Rep h, (Rep f,
 Rep g)) -> a)
   index = coerce (index @(Product (f·g·f) (h·f·g)))

   tabulate :: (Either (Rep f, (Rep g, Rep f)) (Rep h, (Rep f, Rep g)) ->
 a) -> (P f g h) a
   tabulate = coerce (tabulate @(Product (f·g·f) (h·f·g)))
 }}}

 Is this too limited to work on general

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13404
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