What is a clean way to call a function using several Getters from Control.Lens.

Question

Given some data structure with lenses defined, for example:

import Control.Lens

data Thing =
  Thing {
    _a :: String
  , _b :: String
  , _c :: Int
  , _d :: Int
  }

makeLenses ''Thing

And given some function that I want to call using several getters, for example:

fun :: Int -> String -> Int -> String -> Bool
fun = undefined

At the moment, I end up with a lot of ugliness with parens to access each field, for example:

thing = Thing "hello" "there" 5 1

answer = fun (thing^.c) (thing^.a) (thing^.d) (thing^.b)

Given the conciseness of the lens library in most other situations I was hoping for something a little more elegant, but I can't find any combinators that will help this specific case.

Answer 1

Since any lens could be used in either the viewing or the setting "mode", we'll need to at least specify view X for each lens X. But for any lens l :: Lens' a b, view l has a type like a -> b if you translate some of the MonadReader noise.

We can thus get rid of some of the repetition using the Applicative instance for ((->) a).

thing & fun <$> view c <*> view a <*> view d <*> view b