On Tue, Feb 05, 2008 at 12:03:38AM -0500, Cale Gibbard wrote:
Are many people using mapAccumR? How much would it hurt to change it?
It's specified in the Haskell 98 Report, so changing it is a big deal. Personally, I think the types should have been mapAccumL :: (s -> a -> (b,s)) -> s -> [a] -> ([b],s) mapAccumR :: (a -> s -> (s,b)) -> [a] -> s -> (s,[b]) to show which direction the state flows. I can't resist observing that these functions generalize to Traversable, so they can be used for numbering elements, zipping with a Stream, etc: -- left-to-right state transformer newtype StateL s a = StateL { runStateL :: s -> (s, a) } instance Functor (StateL s) where fmap f (StateL k) = StateL ((id *** f) . k) instance Applicative (StateL s) where pure x = StateL (\ s -> (s, x)) StateL kf <*> StateL kv = StateL $ \ s -> let (s', f) = kf s (s'', v) = kv s' in (s'', f v) mapAccumL :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c) mapAccumL f s t = runStateL (traverse (StateL . flip f) t) s -- right-to-left state transformer newtype StateR s a = StateR { runStateR :: s -> (s, a) } instance Functor (StateR s) where fmap f (StateR k) = StateR ((id *** f) . k) instance Applicative (StateR s) where pure x = StateR (\ s -> (s, x)) StateR kf <*> StateR kv = StateR $ \ s -> let (s', v) = kv s (s'', f) = kf s' in (s'', f v) mapAccumR :: Traversable t => (a -> b -> (a, c)) -> a -> t b -> (a, t c) mapAccumR f s t = runStateR (traverse (StateR . flip f) t) s