可変長引数つきの printf

printf.hs

{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE TypeSynonymInstances   #-}

format :: String -> String -> String
format []          _ = []
format ('%':'%':s) x = '%' : '%' : format s x
format ('%':'s':s) x = escape x ++ s
format (x:xs)      y = x : format xs y

escape :: String -> String
escape []       = []
escape ('%':s)  = '%' : '%' : escape s
escape (x:xs)   = x : escape xs

unescape :: String -> String
unescape []             = []
unescape ('%':'%':s)    = '%' : s
unescape (x:xs)         = x : unescape xs

class Printf a where
  printf :: String -> a

instance Printf (IO ()) where
  printf = putStrLn . unescape

-- sprintf
instance Printf String where
  printf = unescape

instance Printf a => Printf (String -> a) where
  printf fmt x = printf (format fmt x)

main :: IO ()
main = do
  printf "hello world" :: IO ()
  printf "hello, %s" "myuon" :: IO ()
  printf "%s,%s%%s,%s" "a" "b" "c" :: IO ()
  printf "testing a escape %%s" :: IO ()

  let { x :: String
      ; x = printf "hello, %s" "myuon"
      } in print x