可変数引数の実験, format (可変String -> IO())

format.hs

{-# LANGUAGE FlexibleInstances #-}

class PType r where
  hoge :: [String] -> r

instance PType (IO ()) where
  hoge = putStr . concat . reverse

instance PType String where
  hoge = concat . reverse

instance PType r => PType (String -> r) where
  hoge xs = \x -> hoge (x : xs)


format :: PType r => r
format = hoge []

(|>) x f = f x
(||>) mx f = mx >>= (return.f)

main = do
  format "hello" ", world" "!\n" :: IO ()
  format "sum of " (show ls) " is " (show$sum ls) "." |> putStrLn
  where
    ls = [1 .. 10]

hello, world!
sum of [1,2,3,4,5,6,7,8,9,10] is 55.