Traversable
is in a sense the class of containers whose structure has a “path” (that can correspond to a list), the elements on which can be modified without dissolving the structure. Hence
zipTrav :: Traversable t => t a -> [b] -> Maybe (t (a,b))
zipTrav = evalStateT . traverse zp
where zp a = do
bs <- get
case bs of
[] -> lift Nothing
(b:bs') -> put bs' >> return (a,b)
However, that list-state traversal seems a bit hackish and likely not the most efficient way to do it. I'd suppose there would be a standard function that accomplished the above or a more general task, but I can't figure out what it would be.
What about
mapAccumL
/mapAccumR
?On the question of efficiency - under the hood the
mapAccum
functions use the state monad, so all I've really done is capture the imperative part of your code in a higher-order function. I wouldn't expect this code to perform better than yours. But I don't think you can do much better than theState
monad (orST
), given onlyTraversable t
.