Using a Lens to read multiple fields

Question

Given the types

data Prisoner = P { _name   :: String
                  , _rank   :: Int
                  , _cereal :: Cereal }

data Cereal = C { _number           


        

Answer 1

Fleshing out danidiaz's answer above, I was able to construct a Getter Prisoner (String, Int, Int) using ReifiedGetter:

getNameRankAndCerealNumber_2 :: Prisoner -> (String, Int, Int)
getNameRankAndCerealNumber_2 = p ^. nameRankAndCerealNumber_2

nameRankAndCerealNumber_2 :: Getter Prisoner (String, Int, Int)
nameRankAndCerealNumber_2 = runGetter ((,,) <$> Getter name <*> Getter rank <*> Getter (cereal.number))

And a Lens' Prisoner (String, Int, Int) using alongside, though I had to manually construct Iso's between Prisoner and an HList [String, Int, Int] and between HList [a,b,c] and (a,b,c).

getNameRankAndCerealNumber_3 :: Prisoner -> (String, Int, Int)
getNameRankAndCerealNumber_3 p = p ^. nameRankAndCerealNumber_3

setNameRankAndCerealNumber_3 :: (String, Int, Int) -> Prisoner -> Prisoner
setNameRankAndCerealNumber_3 t p = p & nameRankAndCerealNumber_3 .~ t

nameRankAndCerealNumber_3 :: Lens' Prisoner (String, Int, Int)
nameRankAndCerealNumber_3 = hlist . alongside id (alongside id number) . triple
  where triple :: Iso' (a,(b,c)) (a,b,c)
        triple = dimap (\(a,(b,c)) -> (a,b,c)) (fmap $ \(a,b,c) -> (a,(b,c)))
        hlist :: Iso' Prisoner (String, (Int, Cereal))
        hlist = dimap (\(P n r c) -> (n,(r,c))) (fmap $ \(n,(r,c)) -> P n r c)

There may be an easier way to do this, but that's another question.

Answer 2

We can use the Applicative instance of the ReifiedGetter newtype from Control.Lens.Reified:

runGetter $ (,) <$> Getter number <*> Getter mascot

In general, the newtypes in Control.Lens.Reified offer a lot of very useful instances for getters and folds.

Note#1: Notice that we are combining the lenses as getters, and getting a getter in return. You can't obtain a composite lens in this way, as there would be problems if their "focuses" overlap. What could be the proper setter behaviour in that case?

Note#2: The alongside function lets you combine two lenses, getting a bona-fide lens that works on the two halves of a product. This is different form the previous case because we can be sure the lenses don't overlap. alongside comes in handy when your type is a tuple or has an isomorphism to a tuple.