[GHC] #13179: Levity-polymorphic data types

[GHC]

#13179: Levity-polymorphic data types
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Core           |           Version:  8.0.1
  Libraries                          |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #12708
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I don't know if this belongs as a part of [https://github.com/ghc-
 proposals/ghc-proposals/pull/30 levity-polymorphic type classes] (#12708)
 but many data types can be made levity-polymorphic, some have just been
 added to base:

 {{{#!hs

 -- Sum Array# Array# :: Type -> Type
 data Product :: forall rep. (k -> TYPE rep) -> (k -> TYPE rep) -> (k ->
 Type) where
   Pair :: f a -> g a -> (Product f g) a

 -- Sum Array# Array# :: Type -> Type
 data Sum :: forall rep. (k -> TYPE rep) -> (k -> TYPE rep) -> (k -> Type)
 where
   InL :: f a -> (Sum f g) a
   InR :: g a -> (Sum f g) a

 -- Compose Array# [] :: Type -> Type
 data Compose :: forall rep. (b -> TYPE rep) -> (a -> b) -> (a -> Type)
 where
   Compose :: f (g a) -> (Compose f g) a
 }}}

 Can you think of other examples?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13179>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler