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

[GHC]

#13463: Deriving Functor / Foldable / Traversable for co-pattern type
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by Iceland_jack:

Old description:

> 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_copattern_matching_in/
> 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?

New description:

 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_copattern_matching_in/
 co-pattern] data type?

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

 newtype Stream a = S (forall res. StreamTag 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#comment:1>
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