1. Quick Introduction

1.1. Explanation by Example

Effect starts with a very simple idea: instead of having a function which performs side-effects (such as IO):

def get_user_name():
    return raw_input("Enter User Name> ") # or 'input' in Python 3

you instead have a function which returns a representation of the side-effect:

def get_user_name():
    return Effect(ReadLine("Enter User Name> "))

We call objects like ReadLine an intent – that is, the intent of this effect is to read a line of input from the user. Ideally, intents are very simple objects with public attributes and no behavior, only data.

class ReadLine(object):
    def __init__(self, prompt):
        self.prompt = prompt

To perform the ReadLine intent, we must implement a performer function:

def perform_read_line(dispatcher, readline):
    return raw_input(readline.prompt)

To do something with the result of the effect, we must attach callbacks with the on method:

def greet():
    return get_user_name().on(
        success=lambda r: Effect(Print("Hello,", r)),
        error=lambda exc_info: Effect(Print("There was an error!", exc_info[1])))

(Here we assume another intent, Print, which shows some text to the user.)

A (sometimes) nicer syntax is provided for adding callbacks, with the effect.do.do() decorator.

from effect.do import do

def greet():
        name = yield get_user_name()
    except Exception as e:
        yield Effect(Print("There was an error!", e))
        yield Effect(Print("Hello,", name))

Finally, to actually perform these effects, they can be passed to effect.sync_perform(), along with a dispatcher which looks up the performer based on the intent.

from effect import sync_perform

def main():
    eff = greet()
    dispatcher = ComposedDispatcher([
        TypeDispatcher({ReadLine: perform_read_line}),
    sync_perform(dispatcher, eff)

This has a number of advantages. First, your unit tests for get_user_name become simpler. You don’t need to mock out or parameterize the raw_input function - you just call get_user_name and assert that it returns a ReadLine object with the correct ‘prompt’ value.

Second, you can implement ReadLine in a number of different ways - it’s possible to override the way an intent is performed to do whatever you want. For example, you could implement an HTTPRequest client either using the popular requests package, or using the Twisted-based treq package – without needing to change any of your application code, since it’s all in terms of the Effect API.

1.2. A quick tour, with definitions

  • Intent: An object which describes a desired action, ideally with simple inert data in public attributes. For example, ReadLine(prompt='> ') could be an intent that describes the desire to read a line from the user after showing a prompt.
  • effect.Effect: An object which binds callbacks to receive the result of performing an intent.
  • Performer: A callable that takes the Dispatcher, an Intent, and a Box. It executes the Intent and puts the result in the Box. For example, the performer for ReadLine() could call raw_input(intent.prompt).
  • Dispatcher: A callable that takes an Intent and finds the Performer that can execute it (or None). See TypeDispatcher and ComposedDispatcher for handy pre-built dispatchers.
  • Box: An object that has succeed and fail methods for providing the result of an effect (potentially asynchronously). Usually you don’t need to care about this, if you define your performers with effect.sync_performer() or txeffect.deferred_performer from the txeffect package.

There’s a few main things you need to do to use Effect.

  • Define some intents to describe your side-effects (or use a library containing intents that already exist). For example, an HTTPRequest intent that has method, url, etc attributes.
  • Write your application code to create effects like Effect(HTTPRequest(...)) and attach callbacks to them with Effect.on().
  • As close as possible to the top-level of your application, perform your effect(s) with effect.sync_perform().
  • You will need to pass a dispatcher to effect.sync_perform(). You should create one by creating a effect.TypeDispatcher with your own performers (e.g. for HTTPRequest), and composing it with effect.base_dispatcher (which has performers for built-in effects) using effect.ComposedDispatcher.

1.3. Callback chains

Effect allows you to build up chains of callbacks that process data in turn. That is, if you attach a callback a and then a callback b to an Effect, a will be called with the original result, and b will be called with the result of a. This is exactly how Twisted’s Deferreds work, and similar to the monadic bind (>>=) function from Haskell.

This is a great way to build abstractions, compared to non-chaining callback systems like Python’s Futures. You can easily build abstractions like the following:

def request_url(method, url, str_body):
    """Perform an HTTP request."""
    return Effect(HTTPRequest(method, url, str_body))

def request_200_url(method, url, str_body):
    Perform an HTTP request, and raise an error if the response is not 200.
    def check_status(response):
        if response.code != 200:
            raise HTTPError(response.code)
        return response
    return request_url(method, url, str_body).on(success=check_status)

def json_request(method, url, dict_body):
    Perform an HTTP request where the body is sent as JSON and the response
    is automatically decoded as JSON if the Content-type is
    str_body = json.dumps(dict_body)
    return request_200_url(method, url, str_body).on(success=decode_json)