How to map over a Pair/Tuple?

问题

In Haskell I would do join (***). In Idris flatten (***) does not work ((***) is complicated).

回答1:

In Idris, there's no Functor/Applicative/Monad instances for r -> _ and no Arrow instance for ->, only via Morphism, so using flatten to do \f x -> f x x leads to horribly verbose code full of going from/to Morphism.

You can do it, of course, I'm just not sure it's worth it... compare this:

import Control.Arrow
import Data.Morphisms

both : (a -> b) -> (a, a) -> (b, b)
both = applyMor . applyMor (flatten (Mor (Mor . (***)))) . Mor

to this:

both : (a -> b) -> (a, a) -> (b, b)
both f (x, y) = (f x, f y)

来源：https://stackoverflow.com/questions/32043985/how-to-map-over-a-pair-tuple