问类Printf函数

EN

{-# LANGUAGE FlexibleInstances #-}
import Data.Typeable

data PackArgTypes = PackInt Int | PackString String
                              deriving Show

class PackArg t where
  toPackArg :: t -> PackArgTypes

instance PackArg Int where
  toPackArg = PackInt . fromIntegral
instance PackArg Integer where
  toPackArg = PackInt . fromIntegral

instance PackArg String where
  toPackArg = PackString

class PackType t where
  pack' :: String -> [PackArgTypes] -> t

instance PackType (IO a) where
  pack' fmt acc = do
    print fmt
    mapM_ print acc
    return undefined

instance (PackArg a, PackType r) => PackType (a -> r) where
    pack' fmt acc = \x -> pack' fmt $ acc ++ [toPackArg x]

pack :: (PackType t) => String -> t
pack fmt = pack' fmt []

main :: IO ()
main = do
  pack "asd" "qwe" "asd"(1 :: Int) -- Ok
  pack "asd" "qwe" "asd" 1         -- Sad

test1.hs:33:3-6: No instance for (PackArg a0) arising from a use of ‘pack’ …
The type variable ‘a0’ is ambiguous
Note: there are several potential instances:
  instance PackArg String
    -- Defined at /home/knesterov/test1.hs:13:10
  instance PackArg Int -- Defined at /home/knesterov/test1.hs:10:10
In a stmt of a 'do' block: pack "asd" "qwe" "asd" 1
In the expression: do { pack "asd" "qwe" "asd" 1 }
In an equation for ‘main’: main = do { pack "asd" "qwe" "asd" 1 }
test1.hs:33:26: No instance for (Num a0) arising from the literal ‘1’ …
The type variable ‘a0’ is ambiguous
Note: there are several potential instances:
  instance Num Double -- Defined in ‘GHC.Float’
  instance Num Float -- Defined in ‘GHC.Float’
  instance Integral a => Num (GHC.Real.Ratio a)
    -- Defined in ‘GHC.Real’
  ...plus 7 others
In the fourth argument of ‘pack’, namely ‘1’
In a stmt of a 'do' block: pack "asd" "qwe" "asd" 1
In the expression: do { pack "asd" "qwe" "asd" 1 }
Compilation failed.

{-# LANGUAGE ExtendedDefaultRules #-}
instance PackArg Integer where
  toPackArg = PackInt . fromIntegral