writer.hs

newtype Writer w a = Writer { runWriter :: (a, w) }

instance Functor (Writer w) where
	fmap f x = let (Writer (a, w)) = x in Writer (f a, w)

instance (Monoid w) => Applicative (Writer w) where
	pure x = Writer (x, mempty)
	Writer (f, w2) <*> Writer (x, w1) = Writer (f x, w1 `mappend` w2)

instance (Monoid w) => Monad (Writer w) where
	return = pure
	Writer (a, w) >>= f = let (Writer (a', w')) = f a in Writer (a', w `mappend` w')

tell :: w -> Writer w ()
tell w = Writer ((), w)