暗黙の型変換

Implicit.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeSynonymInstances  #-}
{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE FlexibleContexts      #-}

module Control.Implicit where

import qualified Data.Text as T
import qualified Data.ByteString.Char8 as BS
import qualified Data.ByteString.Lazy.Char8 as BSL

class Implicit a b where
  convert :: b -> a

implicit :: Implicit a b => (a -> c) -> b -> c
implicit = (. convert)

instance Implicit a a where
  convert = id

instance Implicit String T.Text where
  convert = T.unpack

instance Implicit T.Text String where
  convert = T.pack

instance Implicit String BS.ByteString where
  convert = BS.unpack

instance Implicit BS.ByteString String where
  convert = BS.pack

instance Implicit String BSL.ByteString where
  convert = BSL.unpack

instance Implicit BSL.ByteString String where
  convert = BSL.pack


-- implicitPutStr :: Implicit String a => a -> IO ()
-- implicitPutStr = implicit putStr
--
-- implicitPutStr "hello" -- OK
-- implicitPutStr $ BS.pack "hello" -- OK
-- implicitPutStr $ T.pack "hello" -- OK