{-# LANGUAGE ScopedTypeVariables #-}

import Control.Monad (void)

f1 :: forall m. MsgsRead m => m ()
f1 = void (readMsgPayloads :: m [FooMsgPayload])

f2 :: MsgsWrite m => m ()
f2 = writeMsgs [initMsg ZzzMsgA]

f3 :: forall m. (MsgsRead m, MsgsWrite m) => m ()
f3 = do
    void (readMsgPayloads :: m [BarMsgPayload])
    writeMsgs [initMsg ZzzMsgC]

main :: IO ()
main = runAppEnv $ f1 >> f2 >> f3  -- pretend this is doing something useful