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
Hi Robert,
The derive program [http://www.cs.york.ac.uk/~ndm/derive] works
extensively with Template Haskell, and has come across loads of
instances where you get either AppT or ListT, and similarly where you
get either ListE or AppE - and I seem to remember ListP as well. I
think the same also holds for TupT/TupE/TupP My advice is to treat
them all the same, and if it matters to you, normalise them before you
start work. I think the pretty printer may print them differently, but
otherwise they are equivalent. For example derive has:
peep (AppE (AppE cons x) nil)
| cons ~= ":" && nil ~= "[]" = ListE [x]
i.e. if you have a list using cons, change it to a list using the list notation.
Thanks
Neil
On 6/24/08, Robert Greayer
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
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Neil Mitchell -
Robert Greayer