[GHC] #13179: Levity-polymorphic data types

[GHC]

#13179: Levity-polymorphic data types
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        Reporter:  Iceland_jack      |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              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:  #12708            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I'm afraid your definitions may not be what you think they are. In
 particular, the way you've written your GADT constructors forces `TYPE rep
 ~ Type`, so in effect they're not much different from `Product`, `Sum`,
 and `Compose` and they exist today in `base`.

 If you tried to write truly levity polymorphic definitions for, say,
 `Product`, it'll actually be rejected!

 {{{#!hs
 data Product :: forall rep. (k -> TYPE rep) -> (k -> TYPE rep) -> (k ->
 Type) where
   Pair :: forall rep k (f :: k -> TYPE rep) (g :: k -> TYPE rep) (a :: k).
 f a -> g a -> (Product f g) a
 }}}

 {{{
 <interactive>:9:85: error:
     • A levity-polymorphic type is not allowed here:
         Type: f a
         Kind: TYPE rep
     • In the definition of data constructor ‘Pair’
       In the data type declaration for ‘Product’

 <interactive>:9:85: error:
     • A levity-polymorphic type is not allowed here:
         Type: g a
         Kind: TYPE rep
     • In the definition of data constructor ‘Pair’
       In the data type declaration for ‘Product’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13179#comment:2>
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