reader-monad in python

reader_m.py

class Monad:
    """provides generic methods like fmap, map, reduce, chain which use polymorphic
    implementations of unit and bind"""
    pass

class _Reader_m(Monad):
    def unit(self, v): return lambda env: v
    def bind(self, mv, mf):
        def _(env):
            val = mv(env)
            return mf(val)(env)
        return _

reader_m = _Reader_m()


from collections import namedtuple
def _test_reader_m():
    Env = namedtuple('Env', ['hostname', 'port', 'outfile'])

    def doSomething(hostname, port, outfile):
        return [outfile, port, hostname]

    # getters describe how to read a value from an environment,
    # but do not have access to the environment itself
    hostname = lambda env: env.hostname
    port = lambda env: env.port
    outfile = lambda env: env.outfile

    def bootstrap():
        """no dependencies passed as parameters. could have very long list of
        dependencies"""
        r = reader_m.bind( hostname, lambda h:
            reader_m.bind( port,     lambda p:
            reader_m.bind( outfile,  lambda o:
            reader_m.unit( doSomething(h,p,o) ))))
        return r

    env = Env("localhost", 80, "/etc/passwd")
    assert bootstrap()(env) == ["/etc/passwd", 80, "localhost"]

_test_reader_m()

hi tony,

I heard your talk "dependency injection without the gymnastics" at PhillyETE. i was the most active questioner.

I've implemented a bunch of monads in python to better understand them. Scroll up for my impl of reader-monad. I'm not concerned with writing out the macroexpansions of a monad comprehension.

you wrote: "OK then, pass arguments through explicitly — bit clumsy innit?" - I'm confused. In my example, I see no value add to the monadic style, we end up passing dependencies as arguments anyway. is the entire bulk of the business logic supposed to be a monad comprehension?

i work in enterprise and understand the pain of DI, but I don't see how this helps. if the dependency is a logger or database connection, its cool that an application can't reach out and grab a singleton--you have to be in the monad--but once the monad injects the dependency, you are still reduced to passing it around in the business logic, which is no different than a runtime passing these dependencies to the business logic entry point. a reader-writer monad stack could guarantee that resource access is logged, but i can provide that guarantee without monads.

thanks for your time.
Dustin Getz