[GHC] #8178: Need TypeRep for Symbol and numeric type literals; and Typeable instances

27 Aug 2013


      #8178: Need TypeRep for Symbol and numeric type literals; and Typeable instances
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       Reporter:  simonpj           |             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:                    |
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 Nicolas Trangez points out that we don't have a `TypeRep` for types
 involving literal strings or numerics.  E.g. what is `(typeRep (undefined
 :: Proxy "foo"))`?

 Here is Nicolas's example:
 {{{
 {-# LANGUAGE DataKinds,
              KindSignatures,
              DeriveFunctor,
              DeriveDataTypeable #-}
 module Main where

 import Data.Typeable
 import GHC.TypeLits

 data NoSymbol n a b = NoSymbol a b
   deriving (Typeable)

 data WithSymbol (n :: Symbol) a b = WithSymbol a b
   deriving (Typeable)

 data Sym
   deriving (Typeable)

 main :: IO ()
 main = do
     print $ typeOf (undefined :: NoSymbol Sym Int Int)

     let d = undefined :: WithSymbol "sym" Int Int
     {-
     print $ typeOf d

     No instance for (Typeable Symbol "sym")
       arising from a use of ‛typeOf’
     -}

     return ()
 }}}
 Just as types contain literal strings and natural numbers, so too must
 `TypeRep`, in some way.  Once we have a suitable `TypeRep` we can make
 built-in instance for `Typeable` that return the appropriate
 representation.

 At the moment we have (in `Data.Typeable.Internals`):
 {{{
 data TypeRep = TypeRep {-# UNPACK #-} !Fingerprint TyCon [TypeRep]

 data TyCon = TyCon {
    tyConHash    :: {-# UNPACK #-} !Fingerprint,
    tyConPackage :: String,
    tyConModule  :: String,
    tyConName    :: String
  }
 }}}
 with instances for Eq and Ord.  Perhaps we need
 {{{
 data TypeRep
   = TcApp {-# UNPACK #-} !Fingerprint TyCon [TypeRep]
   | TcSym String
   | TcNat Integer
 }}}
 or something like that?  I'm not certain. I think Iavor is going to think
 about it.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8178
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