Possible Elm lens, based on https://gist.github.com/evancz/78293dc6a4ac2547676c

Lens.elm

-- Based on https://gist.github.com/evancz/78293dc6a4ac2547676c

type alias Lens s a =
    { get : s -> a
    , set : a -> s -> s
    }

modify : Lens s a -> (a -> a) -> s -> s
modify l f s =
    l.set (f (l.get s)) s

assignTo : Lens s a -> s -> a -> s
assignTo l = flip l.set

(=>) : Lens s a -> Lens a b -> Lens s b
(=>) sa ab =
    { get s = ab.get (sa.get s)
    , set b s = sa.set (ab.set b (sa.get s)) s
    }


fst' : Lens (a,b) a
fst' =
    { get = fst
    , set a (_,b) = (a,b)
    }

snd' : Lens (a,b) b
snd' =
    { get = snd
    , set b (a,_) = (a,b)
    }