dialog.py

from utils import singleton
from monads import monadic, MonadRunner

class Text(object):
    def __init__(self, text):
        self.text = text

class Input(object):
    def __init__(self, label):
        self.label = label

@singleton
class TerminalDialogRunner(MonadRunner):
    def bind_Text(self, obj):
        print obj.text
    def bind_Input(self, obj):
        print obj.label
        return raw_input()

@monadic
def bar(name):
    yield Text("so %s, i hope you like this" % (name,))
    yield Text("cuz this is some impressive shit")

@monadic
def foo():
    yield Text("hi man")
    name = yield Input("what's your name?")
    yield bar(name)
    yield Text("okay, m'bye")

TerminalDialogRunner.run(foo())

monads.py

import functools

class MonadReturn(Exception):
    def __init__(self, value):
        self.value = value

class UnknownMonadicAction(Exception):
    pass

class Monad(object):
    def __init__(self, gen):
        self.gen = gen
    def run(self, bind):
        inp = None
        ret = None
        while True:
            try:
                out = self.gen.send(inp)
            except StopIteration:
                break
            except MonadReturn as ex:
                ret = ex.value
                break
            inp = bind(out)
        return ret

def monadic(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        return Monad(func(*args, **kwargs))
    return wrapper

class MonadRunner(object):
    def bind(self, obj):
        return getattr(self, "bind_%s" % (type(obj).__name__,), self.unknown)(obj)
    def bind_Monad(self, mnd):
        return mnd.run(self.bind)
    run = bind_Monad
    def unknown(self, obj):
        raise UnknownMonadicAction(obj)

def mreturn(value):
    raise MonadReturn(value)