{-# LANGUAGE OverloadedStrings #-}
import Control.Monad.Writer
import Data.String
data TM = TMempty
| TMappend TM TM
| TMfromString String
instance IsString TM where
fromString = TMfromString
instance Monoid TM where
mempty = TMempty
mappend = TMappend
instance Show TM where
showsPrec d TMempty = showString "\"\""
showsPrec d (TMfromString s) = showString $ show s
showsPrec d (TMappend a b) = showParen (d > 0) $
showsPrec 1 a .
showString " ++ " .
showsPrec 0 b
theWriter :: Writer TM ()
theWriter = do
tell "Hello"
replicateM_ 2 $ tell "World"
tell "!"
main = print $ execWriter theWriter
Выдает:
"Hello" ++ ("World" ++ "World" ++ "") ++ "!"