[GHC] #8950: Typeable instances for promoted lists and tuples

2 Apr 2014

      #8950: Typeable instances for promoted lists and tuples
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       Reporter:  kosmikus          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  libraries/base    |           Version:  7.8.1-rc2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 Please have a look at the following sample module:

 {{{
 {-# LANGUAGE DataKinds, KindSignatures, PolyKinds, AutoDeriveTypeable #-}
 {-# LANGUAGE StandaloneDeriving, DeriveDataTypeable #-}

 import Data.Proxy
 import Data.Typeable

 data Foo (xs :: [*]) (p :: (*, *))

 main :: IO ()
 main = print (typeRep (Proxy :: Proxy (Foo '[Int] '(Bool, Char))))

 -- Code above checks only with these instances, but shouldn't they
 -- be predefined?
 --
 -- deriving instance Typeable '[]
 -- deriving instance Typeable '(:)
 -- deriving instance Typeable '(,)
 }}}

 The good news is that the code does in principle work, but as the comments
 say, I would have expected this to work without having to define
 additional instances.

 (A somewhat related question is whether concrete type literals such as `3
 :: Nat` or `"foo" :: Symbol` should be instances of `Typeable`. They don't
 seem to be, and `deriving` does not work in these cases.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8950
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