Reader monad in Elm for fun and profit

Reader.elm

module Reader where


type Reader ctx a = Reader (ctx -> a)


unit: a -> Reader any a
unit x =
  Reader (\_ -> x)


runReader: Reader ctx a -> ctx -> a
runReader r ctx = case r of
  (Reader f) -> f ctx


withContext: ctx -> Reader ctx a -> a
withContext = flip runReader


andThen: Reader ctx a -> (a -> Reader ctx b) -> Reader ctx b
andThen reader bindFn =
  Reader (\ctx -> 
    let a = runReader reader ctx
        newReader = bindFn a
    in runReader newReader ctx
  )


(>>=): Reader ctx a -> (a -> Reader ctx b) -> Reader ctx b
(>>=) = andThen


ask: Reader ctx ctx
ask = Reader identity

Nice!