working example of "phantom constraint pattern"

a.hs

#!/usr/bin/env nix-script-haskell
#!haskellPackages transformer

{-# LANGUAGE FlexibleInstances #-}

import B
import qualified Control.Monad.Reader as Reader

instance Reader a

instance State a

main :: IO ()
main = do
  let initialGlobalEnv = Environment 123
  Reader.runReaderT (runApp myApp) initialGlobalEnv

b.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}

module B (Reader, State, Environment (..), runApp, myApp) where

import qualified Control.Monad.IO.Class as MonadIO
import qualified Control.Monad.Reader as Reader
import Prelude

class Reader r

class State s

askA :: Reader String => AppM String
askA = pure "hi"

getB :: State Int => AppM Int
getB = pure 1

putB :: State Int => Int -> AppM ()
putB _ = pure ()

-- Environment type
data Environment = Environment {someValue :: Int} -- mutable state, read-only values, etc. go in this record

newtype AppM a = UnsafeApp {runApp :: Reader.ReaderT Environment IO a} -- UnsafeApp not exported
  deriving (Functor, Applicative, Monad, MonadIO.MonadIO, Reader.MonadReader Environment)

myApp :: (Reader String, State Int) => AppM ()
myApp = do
  _ <- f
  pure ()

f :: Reader String => AppM String
f = askA