stateless 0.1.0

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stateless

Statically typed, purely functional algebraic effects for Python.

Concepts

Actions, Effects and Handlers

stateless is a purely functional effect system, that enables you to program without side-effects, even when writing code that needs to do IO, such as reading or writing from files, accessing the network etc.

When programming with stateless you will describe your programs side-effects using the stateless.Effect type. This is in fact just a type alias, roughly equivalent to:

from typing import Generator, Never, Any
from stateless import Action


Effect = Generator[stateless.Action | Exception, Never, Any]

(In reality its a bit more complicated. But for now, think of effects in stateless like this)

In words: effects are generators of stateless.Action instances or exceptions, that can return anything. A stateless.Action instance in turn, describes a side-effect you want your program to perform.

Lets begin with a simple example:

from typing import Never

from stateless.console import Input, Print
from stateless import Effect


def say_hello() -> Effect[Print, Never, None]:
    name = yield from Input("What is your name?")
    yield from Print(f"Hello, {name}!")

stateless.console.Print is a subtype of stateless.Action

Error Handling

TODO

Scheduling and Retrying

TODO

Async Actions

TODO

Background

• Do be do be do (Lindley, McBride and McLaughlin)

• Handlers of Algebraic Effects (Plotkin and Pretnar)

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