MonadReader example

Lib.hs

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE NamedFieldPuns #-}

module Lib
  ( foo
  , someFunc
  ) where

import Data.Text (Text)
import Control.Monad.Reader (runReader)
import Control.Monad.Reader.Class (MonadReader, ask)

newtype MyEnv = MyEnv { token :: Text }

foo :: MonadReader MyEnv m => m Int
foo = do
  MyEnv { token } <- ask
  pure $ case token of
    "foo" -> 42
    "bar" -> 123
    _     -> (-1)

someFunc :: IO ()
someFunc = do
  putStrLn "hello from `someFunc`"
  let x = runReader foo (MyEnv "foo")
  let y = runReader foo (MyEnv "bar")
  let z = runReader foo (MyEnv "baz")
  putStrLn $ "runReader foo (MyEnv \"foo\"): " <> show x
  putStrLn $ "runReader foo (MyEnv \"bar\"): " <> show y
  putStrLn $ "runReader foo (MyEnv \"baz\"): " <> show z