Applicative record building from maps

Main.hs

module Main where

import Data.Foldable (traverse_)
import Data.Functor.Compose (Compose (Compose, getCompose))
import Data.Map (Map)
import qualified Data.Map as Map

data Person = Person
  { id :: Int,
    name :: String,
    age :: Int
  }
  deriving (Show)

names :: Map Int String
names = Map.fromList [(1, "Alice"), (2, "Bob")]

ages :: Map Int Int
ages = Map.fromList [(1, 32), (2, 23)]

person :: Int -> (Map Int String, Map Int Int) -> Maybe Person
person i =
  getCompose
    ( Person i
        <$> Compose (Map.lookup i . fst)
        <*> Compose (Map.lookup i . snd)
    )

-- >>> main
-- Just (Person {id = 1, name = "Alice", age = 32})
-- Just (Person {id = 2, name = "Bob", age = 23})
-- Just (Person {id = 1, name = "Alice", age = 32})
-- Just (Person {id = 2, name = "Bob", age = 23})

main :: IO ()
main = do
  print $ person 1 (names, ages)
  print $ person 2 (names, ages)
  traverse_ (print . flip person (names, ages)) [1, 2]