[GHC] #8486: cannot derive new-typeable instances for promoted data types taking parameters

29 Oct 2013

      #8486: cannot derive new-typeable instances for promoted data types taking
parameters
------------------------------------+-------------------------------------
       Reporter:  aavogt            |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 Hi,

 The following code shows the issue:

 {{{
 {-# LANGUAGE DataKinds, DeriveDataTypeable, KindSignatures, PolyKinds,
 StandaloneDeriving #-}
 import Data.Typeable
 import GHC.TypeLits

 data T (a :: [*]) = T deriving (Typeable)

 t = T :: T [Int, Double]

 deriving instance Typeable '[]
 {-

 *Main> typeOf t

 <interactive>:24:1:
     No instance for (Typeable (* -> [*] -> [*]) ((':) *))
       arising from a use of ‛typeOf’
     In the expression: typeOf t
     In an equation for ‛it’: it = typeOf t
 -}
 }}}

 ghc7.7-20131004 doesn't allow either of the following:
 {{{
 deriving instance (Typeable a, Typeable b) => Typeable (a ': b)
 -- or
 deriving instance Typeable (':)
 }}}

 On ghc-7.6 I could write my own `instance Typeable (T a)`, but this has
 been disabled it seems.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8486
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler