Combining lenses - haskell

Question

Using a lens library I can apply a modification function to individual targets, like so:
Prelude Control.Lens> (1, 'a', 2) & _1 %~ (*3)
(3,'a',2)
Prelude Control.Lens> (1, 'a', 2) & _3 %~ (*3)
(1,'a',6)
How can I combine those individual lenses (_1 and _3) to be able to perform this update to both of the targets at once? I expect something in the spirit of the following:
Prelude Control.Lens> (1, 'a', 2) & ??? %~ (*3)
(3,'a',6)

Answer 1

Using untainted from the Settable type class in Control.Lens.Internal.Setter, it is possible to combine two setters, but the result will also only be a setter and not a getter.
import Control.Lens.Internal.Setter
-- (&&&) is already taken by Control.Arrow
(~&~) :: (Settable f) => (c -> d -> f a) -> (c -> a -> t) -> c -> d -> t
(~&~) a b f = b f . untainted . a f
You can test this:
>>> import Control.Lens
>>> (1, 'a', 2) & (_1 ~&~ _3) %~ (*3)
(3,'a',6)
EDIT
You don't actually need to use internal functions. You can use the fact that Mutator is a monad:
{-# LANGUAGE NoMonomorphismRestriction #-}
import Control.Monad
import Control.Applicative
(~&~) = liftA2 (>=>)
-- This works too, and is maybe easier to understand:
(~&~) a b f x = a f x >>= b f

Answer 2

There is a variation on what you are asking for which is more general:
(/\)
:: (Functor f)
=> ((a -> (a, a)) -> (c -> (a, c)))
-- ^ Lens' c a
-> ((b -> (b, b)) -> (c -> (b, c)))
-- ^ Lens' c b
-> (((a, b) -> f (a, b)) -> (c -> f c))
-- ^ Lens' c (a, b)
(lens1 /\ lens2) f c0 =
let (a, _) = lens1 (\a_ -> (a_, a_)) c0
(b, _) = lens2 (\b_ -> (b_, b_)) c0
fab = f (a, b)
in fmap (\(a, b) ->
let (_, c1) = lens1 (\a_ -> (a_, a)) c0
(_, c2) = lens2 (\b_ -> (b_, b)) c1
in c2
) fab
infixl 7 /\
Just focus on the type signature with lens type synonyms:
Lens' c a -> Lens' c b -> Lens' c (a, b)
It takes two lenses and combines them into a lens to a pair of fields. This is slightly more general and works for combining lenses that point to fields of different types. However, then you'd have to mutate the two fields separately.
I just wanted to throw this solution out there in case people were looking for something like this.