Need TypeRep for Symbol and numeric type literals; and Typeable instances

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.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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Need TypeRep for Symbol and numeric type literals; and Typeable instances

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