What's the most standard/generic way to zip a

问题:

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.

回答1:

What about mapAccumL/mapAccumR?

tzipWith :: Traversable t => (a -> b -> c) -> [a] -> t b -> Maybe (t c)
tzipWith f xs = sequenceA . snd . mapAccumL pair xs
    where pair [] y = ([], Nothing)
          pair (x:xs) y = (xs, Just (f x y))

tzip :: Traversable t => [a] -> t b -> Maybe (t (a, b))
tzip = tzipWith (,)

ghci> tzip [1..] [4,5,6]
Just [(1,4),(2,5),(3,6)]

ghci> tzip [1,2] [4,5,6]
Nothing

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 the State monad (or ST), given only Traversable t.