[Haskell-cafe] Template Haskell ListT wrinkle

[Neil Mitchell]

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 <robgreayer at yahoo.com> wrote:
>
>  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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