In messing around with TH, I noticed (what I consider to be an) odd wrinkle in the handling of list types within TH's syntax meta-data. For example, given the program at the end of this email, which prints out the TH representation of the types 'Ints' and '[Int]', where 'Ints' is just a type-alias for '[Int]', the following results are generated: Ints is: AppT (ConT GHC.Base.[]) (ConT GHC.Base.Int) [Int] is: AppT ListT (ConT GHC.Base.Int) type0, 'Ints' resolves to the application of the named type constructor "GHC.Base.[]" to the named type constructor "GHC.Base.Int" (which makes perfect sense). type1, '[Int]' resolves to the application of the 'built-in' ListT type constructor to the named type constructor "GHC.Base.Int" (which also makes perfect sense). What's odd (to me) is that in one situation, the 'named' constructor 'GHC.Base.[]' appears, and in the other, the 'built-in' constructor appears. Does anyone have insight into why this might be? (These results were obtained with GHC 6.8.2, btw).
{-# OPTIONS_GHC -XTemplateHaskell #-} module Main where import Language.Haskell.TH
type Ints = [Int]
type0 = $( reify ''Ints >>= (\ (TyConI (TySynD _ _ t)) -> return $ show t) >>= \s -> [| s |]) type1 = $( [t| [Int] |] >>= return . show >>= \s -> [| s |])
main = do putStrLn ("Ints is: " ++ type0) putStrLn ("[Int] is: " ++ type1)
Thanks rcg