module Writer where

type Writer a = (a, String -> String)

unitW :: a -> Writer a   
unitW a = (a, \ s -> s)

bindW :: Writer a -> (a -> Writer b) -> Writer b
m `bindW` k = let (a, f1) = m; (b, f2) = k a in (b, \ s -> f1 (f2 s))

writeW :: String -> Writer ()
writeW s = ((), (s++))