[Template-haskell] reifyType function

7 Jul 2003

      Hi all,

Sean Seefried and I managed to implement a reifyType function, which 
we're calling rType since there's already that annoying built-in 
reifyType identifier ;).  It's defined in a ReifyType module which 
looks like thus:

--8<--
module ReifyType
where

import Data.Dynamic
import Language.Haskell.THSyntax

class ReifyType a where
    rType :: a -> Typ

instance (ReifyType a, ReifyType b) => ReifyType (a, b) where
    rType (_ :: (t, u)) = Tapp (Tapp (Tcon (Tuple 2)) (rType (undefined 
:: t)))
                          (rType (undefined :: u))

instance ReifyType a => ReifyType [a] where
    rType (_ :: [t]) = Tapp (Tcon List) (rType (undefined :: t))

instance Typeable a => ReifyType a where
    rType (_ :: t) = (Tcon (TconName typeAsString))
       where
          typeAsString = show $ typeOf (undefined :: t)

instance (ReifyType a, ReifyType b) => ReifyType (a->b) where
   rType (_ :: t -> u)
       = (Tapp (rType (undefined :: t)) (rType (undefined :: u)))
--8<--

An example of how to use it:

18:48 ~/Desktop % ghci -fglasgow-exts -fallow-overlapping-instances 
-fallow-undecidable-instances ReifyType.hs
Type.hs)
...
Loading package base ... linking ... done.
Compiling ReifyType        ( ReifyType.hs, interpreted )
Ok, modules loaded: ReifyType.
*ReifyType> rType Char.toLower
Loading package haskell98 ... linking ... done.
Loading package haskell-src ... linking ... done.
Tapp (Tcon (TconName "Char")) (Tcon (TconName "Char"))
*ReifyType> rType ((+) :: (Int -> Int -> Int))
Tapp (Tcon (TconName "Int")) (Tapp (Tcon (TconName "Int")) (Tcon 
(TconName "Int")))
*ReifyType> rType "foobar"
Tapp (Tcon List) (Tcon (TconName "Char"))
*ReifyType>

The major restriction is that you're limited to using it on monomorphic 
types (since the typeOf function only works on monomorphic types), but 
otherwise, it seems to work remarkably well.

-- 
% Andre Pang : just.your.average.bounty.hunter