Haskell - zip 替代品

Question

I have written the following function:

f :: String -> [(Int, Int)] 
f s = zip x y where
  x = g s 
  y = h s

The function g has the following signature:

g :: String -> [Int]

and h :

h :: String -> [Int]

I want to modify f so it that it returns [(Int, Int, String)] , where String is its input s . I think the problem here is that zip strictly works on two lists. How can I do this?

Answer 1

You can make use of zipWith :: (a -> b -> c) -> [a] -> [b] -> [c] and thus use an arbitrary function that takes elements from the two lists, and produces a value.

In this specific case, we can for example use a lambda expression:

f :: String -> [(Int, Int, String)]
f s =  (g s) (h s)

Or you can make use of the TupleSections extension [ghc-doc] , and implement this as:

{-# LANGUAGE  #-}

f :: String -> [(Int, Int, String)]
f s = zipWith  (g s) (h s)