Noah Easterly wrote:
Somebody suggested I post this here if I wanted feedback.
So I was thinking about the ReverseState monad I saw mentioned on r/haskell a couple days ago, and playing around with the concept of information flowing two directions when I came up with this function:
bifold :: (l -> a -> r -> (r,l)) -> (l,r) -> [a] -> (r,l) bifold _ (l,r) [] = (r,l) bifold f (l,r) (a:as) = (ra,las) where (ras,las) = bifold f (la,r) as (ra,la) = f l a ras
(I'm sure someone else has come up with this before, so I'll just say I discovered it, not invented it).
Basically, it's a simultaneous left and right fold, passing one value from the start of the list toward the end, and one from the end toward the start.
I also needed a bidirectional fold in the past. See foldl'r in http://code.haskell.org/~thielema/utility/src/Data/List/HT/Private.hs You can express it using foldr alone, since 'foldl' can be written as 'foldr': http://www.haskell.org/haskellwiki/Foldl_as_foldr You may add your example to the Wiki.