It's traverse

traverse.hs

-- Imagine you're trying to guess someone's PIN code, and you have limited
-- info on each digit, e.g. if the digit is odd or even.
type Predicate a = a -> Bool

-- This is the info we have about the PIN code.
info :: [Predicate Int]
info = [odd, even, even, odd]

-- A helper to produce all digits satisfying a predicate.
digits :: Predicate Int -> [Int]
digits f = filter f [0 .. 9]

-- It's traverse!
guesses :: [[Int]]
guesses = traverse digits info