如何使用函子或应用程序在元组列表上重写此Haskell函数

Question

Is there a nicer way to write the following function fs' with functors or applicatives?

fncnB = (* 2)
fncnA = (* 3)
fs' fs = zip (map (fncnA . fst) fs) $ map (fncnB . snd) fs

I see from this question that I could rely on the functor instance of lists to map a single function acting over both elements of each tuple, or eg on the applicative instance of tuples to apply functions to just the second-half of a two-tuple, but I'm curious how functors and applicatives fit into the picture of operating over lists of multi-component data types.

Answer 1

A tuple is a bifunctor, so bimap is available.

import Data.Bifunctor

fncnB = (* 2)
fncnA = (* 3)
fs' = map (bimap fncnA fncnB)

No third-party libraries required.

Answer 2

what you need is the mapPair function, it's defined in the utility-ht package but it's just

mapPair :: (a -> c, b -> d) -> (a,b) -> (c,d)
mapPair (f,g) (a,b) = (f a, g b) 

use

Prelude> mapPair ((* 2), (* 3)) (2,3)
(4,9)

or

Prelude> map (mapPair ((* 2), (* 3))) [(1,2),(2,3)]
[(2,6),(4,9)]