Hi Hans,

I think this code should work:

{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TupleSections #-}

import Control.Applicative
import Control.Lens

dependingOn :: Lens s t (x,a) (x,b) -> (x -> Lens a b c d) -> Lens s t c d
dependingOn l depending f = l (\ (x,a) -> (x,) <$> depending x f a)

The trick here is that Lens s t a b is just an type synonym for Functor f => (a -> f b) -> s -> f t.

I think it should also be a speed improvement on more complex lenses, only going over the structure once whereas the use of lens would examine the structure twice (first to get and then to set the value).

I hope this helps.

Felix