Reader monad example in Haskell.

reader-example.hs

-- https://www.stackage.org/package/transformers
import qualified Control.Monad.Trans.Reader as Reader

main :: IO ()
main = do
    let result = Reader.runReader computationToRun initialValue
    putStrLn result

{- Output:
There are 3 thing(s) in the environment.
x = Just "hello", y = Just "world"
-}

type Environment = [(String, String)]

initialValue :: Environment
initialValue =
    [ ("key", "value")
    , ("x", "hello")
    , ("y", "world")
    ]

computationToRun :: Reader.Reader Environment String
computationToRun = do
    count <- Reader.asks length
    let message = "There are " ++ show count ++ " thing(s) in the environment."
    rest <- nestedComputation
    return (message ++ "\n" ++ rest)

nestedComputation :: Reader.Reader Environment String
nestedComputation = do
    x <- Reader.asks (lookup "x")
    y <- Reader.asks (lookup "y")
    return ("x = " ++ show x ++ ", y = " ++ show y)