Comment by itishappy

Here's a minimal python translation of the important bits:

    class Reader:
        def __init__(self, func):
            self.run = func
        def pure(x):
            return Reader(lambda _: x)
        def bind(self, f):
            return Reader(lambda env: f(self.run(env)).run(env))

    ask = Reader(lambda env: env)

    def calc():
        return ask.bind(lambda input_str:
            Reader.pure(len(input_str)))

    test = calc().run("test")
    print(test)

Admittedly this is a bit unwieldy in Python. Haskell's `do` notation desugars to repeated binds (and therefore requires something to be a Monad), and does a lot of handiwork.

    -- this:
    calc :: Reader String Int
    calc = do
      input <- ask
      pure $ length input

    -- translates to:
    calc' :: Reader String Int
    calc' = ask >>= (\input -> pure $ length input)