haskell-jp / questions #64

g::String->IO [Integer]
g = ...

f::(Eq a)=>String->(String->IO [a])->IO Bool
f x g = ..   -- "Integerだよ"というメッセージを表示したい。 

> import Data.Typeable
> import Data.Proxy
> funResultTy (typeRep (Proxy :: Proxy (String -> IO [Integer]))) (typeRep (Proxy :: Proxy String))
Just (IO [Integer])

{-# LANGUAGE ScopedTypeVariables #-}
...
f :: forall a. (Typeable a, Eq a) => String -> (String->IO [a]) -> IO Bool
f _ _ =
  do
    print $ typeOf (undefined :: a)
    return True

fnToProxy :: (String -> IO [a]) -> Proxy a
fnToProxy _ = Proxy

printType :: Proxy a -> IO ()
printType _ = putStrLn "<unknown>"
{-# NOINLINE printType #-}

printTypeT :: Typeable a => Proxy a -> IO ()
printTypeT p = print $ typeRep p

{-# RULES
"T-String" forall (p::Proxy String). printType p = printTypeT p
"T-Integer" forall (p::Proxy Integer). printType p = printTypeT p
#-}
{-以下来そうな型を列挙-}

f :: forall a. Eq a => String -> (String->IO [a]) -> IO Bool
f _ _ =
 do
   printType (Proxy :: Proxy a)
   return True

{-# COMPLETE Single :: Only #-}
{-# COMPLETE Single :: Identity #-}