This is exactly where you might like to use the writer monad. Check out LYAH http://learnyouahaskell.com/for-a-few-monads-more http://learnyouahaskell.com/for-

This is exactly where you might like to use the writer monad. Check out LYAH http://learnyouahaskell.com/for-a-few-monads-more http://learnyouahaskell.com/for-a-few-monads-more It's just a fancy name for composing functions of the form `a -> (b, String)`, via (>>=) :: (a, String) -> (a -> (b, String)) -> (b, String) (a, log1) >>= f = let (b, log2) = f a in (b, log1 ++ log2).