Implicit params instead of ReaderT

Implicits.hs

{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE RankNTypes     #-}

module Lib where

data Env = Env { envFoo :: Int, envBar :: String }

type App a = (?env :: Env) => IO a

someFunc :: IO ()
someFunc =
    let ?env = Env 3 "hello"
     in printFoo

-- λ ~/Projects/implicits/ stack build && stack exec implicits-exe
-- 3
-- 7
printFoo :: App ()
printFoo = do
    foo <- asksIO envFoo
    print foo
    localIO (\e -> e { envFoo = 7 }) $ do
        foo' <- asksIO envFoo
        print foo'

askIO :: App Env
askIO = return ?env

asksIO :: (Env -> r) -> App r
asksIO f = fmap f askIO

localIO :: (Env -> Env) -> App a -> App a
localIO f action = do
    env <- asksIO f
    let ?env = env in action