[GHC] #13463: Deriving Functor / Foldable / Traversable for co-pattern type

21 Mar 2017

      #13463: Deriving Functor / Foldable / Traversable for co-pattern type
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           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           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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 This is probably asking too much, but could GHC derive `Functor`,
 `Foldable` ... for this
 [https://www.reddit.com/r/haskell/comments/4aju8f/simple_example_of_emulating...
 co-pattern] data type?

 {{{#!hs
 data ListTag :: Type -> Type -> Type where
   HeadTag :: ListTag a a
   TailTag :: ListTag a (Stream a)

 newtype Stream a = S (forall res. ListTag a res -> res)

 instance Functor Stream where
   fmap :: (a -> a') -> (Stream a -> Stream a')
   fmap f (S stream) = S $ \case
     HeadTag ->      f (stream HeadTag)
     TailTag -> fmap f (stream TailTag)

 instance Foldable Stream where
   foldMap :: Monoid m => (a -> m) -> (Stream a -> m)
   foldMap f (S stream) = f (stream HeadTag) <> foldMap f (stream TailTag)
 }}}

 I'm guessing it's not worth it in any case, but is it uniquely determined?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13463
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