Re: [Haskell-cafe] Minor issue with mapAccumR

On 06/02/2008, Ross Paterson wrote:

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.

However, it's perhaps worth considering as well that ((,) s) is a Functor instance, something which the MTL ignores in its types, using less-convenient (a,s) pairs in Writer and State. - Cale