This is a Reader Monad implementation without introduction Monad Transformer

Reader.purs

module Main where

import Prelude

import Control.Monad.Reader.Class
import Control.Monad.Eff.Console (logShow)
import TryPureScript 

newtype Reader r a = Reader (r -> a)

runReader :: forall r a. Reader r a -> r -> a

runReader (Reader f) = f

instance functorReader :: Functor (Reader r) where
  map g (Reader f) = Reader (g <<< f)

instance applyReader :: Apply (Reader r) where
  apply (Reader g) (Reader f) = Reader \r -> (g r) (f r) 
  
instance applicativeReader :: Applicative (Reader r) where
  pure a = Reader \_ -> a

instance bindReader :: Bind (Reader r) where
  bind (Reader f) h = Reader \r -> runReader (h (f r)) r 

instance monadReader :: Monad (Reader r)

instance monadAskReader :: MonadAsk r (Reader r) where
  ask = Reader \r -> r

main = do
  logShow $ flip runReader "@" $ (\r -> r <> "1") <$> (Reader \r -> r <>"a")
  logShow $ flip runReader "@" $ (Reader \r r2 ->  r <> r2 <> "2") <*> (Reader \r -> r <>"b")
  logShow $ flip runReader "@" $ do
    a <- (Reader \r -> r <> "c")
    b <- (Reader \r -> r <> "4")
    pure (a <> b)
  logShow $ flip runReader "@" $ do
    r <- ask
    pure (r <> "d")