Binary map in Haskell

Question

Is there a two-operand equivalent to map built into Haskell with a type signature of:

map2 :: (a -> b -> c) -> [a] -> [b] -> [c]

The following equivalence should hold:

map2 operator as bs === [operator a b | (a, b) <- zip as bs]

Example:

ghci> map2 (*) [2,3] [4,5]
[8,15]

Answer 1

A more general solution is to use the ZipList instance of Applicative for lists:

let z = (+) <$> ZipList [2,3] <*> ZipList [4,5]
 in runZipList z

The nice thing here is that it works with operators of arbitrary arity, so instead of zipWith3 .. zipWith7, you just tack on one more <*> e

Answer 2

It's called zipWith.

Here's its type: zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]

Example:

zipWith (*) [2,3] [4,5]

Produces:

[8,15]

I might also recommend that you take a look at Hoogle for such questions in the future. A search for "(a->a->a)->[a]->[a]->[a]" turns up what you were looking for. :)

Answer 3

The function zipWith does what you need. There are also zipWith3 .. zipWith7 for cases with more than two lists.