FlipOut.hs

{-# LANGUAGE ImplicitParams #-}
{-# LANGUAGE DeepSubsumption #-}

import Prelude

main :: IO ()
main = let ?implicit = "yeah" in flipOut printImplicit
  where
    printImplicit :: (?implicit :: String) => IO ()
    printImplicit = putStrLn ?implicit

f :: forall a b. (?implicit :: String) => b -> ((?implicit :: String) => a) -> a
f _wotevs i = let ?implicit = "nah" in i

-- Using `flip` produces a different result.
flipOut :: (?implicit :: String) => ((?implicit :: String) => a) -> a
flipOut a = flip f a undefined
flipOut a = f undefined a

https://ghc-proposals.readthedocs.io/en/latest/proposals/0511-deep-subsumption.html#id5