zipWith question: flip in rotateUntil

[TysonMN]

This is a question about your zipWith article.

Is flip essential in

rotateUntil p xs = zipWith
    (flip const)
    xs
    (dropWhile (not . p) (cycle xs))

?

My guess is that this can be simplified to

rotateUntil p xs = zipWith
    const
    (dropWhile (not . p) (cycle xs))
    xs

Am I correct?

[quchen]

The answer is subtle, but the order of zip’s arguments matters:

zipWith (flip const) [] (cycle []) :: [(Int,Int)]
[]

zipWith const (cycle []) [] :: [(Int,Int)]
-- *** Exception: Prelude.cycle: empty list

The reason is that the first parameter is checked first, and when it’s empty, zip terminates without ever looking at the second.