An implementation of Data.Either from Haskell in Python

either, haskell, data, monad, applicative, functor, functional-programming, functional-python, python
pip install pyeither==0.1.1



A functional Python library that exposes Data.Either from Haskell.

data Either e a = Left e | Right a

Example Usage

import os
import json
import either

def main():
    path = "~/myfile.json"
    # Lift a normal value into Either
    e_path = either.pure(path)

    # Ensure the path is valid, returning Right(path) if it is or Left("not a file") if it isn't
    e_valid = e_path.bind(either.predicate(os.path.isfile, "not a file"))

    # Load a file as json, returning Right(contents) if it works, or Left(exc) if it doesn't
    e_data = e_valid.bind(lambda p: either.attempt(json.load, p))

    # It's annoying to assign intermediate values to variables, so you can chain expressions
    # Equivalent to above:

    e_data2 = (either.pure(path)
        .bind(either.predicate(os.path.isfile, "not a file"))
        .bind(lambda p: either.attempt(json.load, p)))

    # Python lacks do-notation, so there's not a nice equivalent syntax

    # Build pipelines of actions using monadic composition

    ensure_path = either.predicate(os.path.isfile, "not a file")
    load_file = lambda p: either.attempt(json.load, p)

    process = either.kleisli(ensure_path, load_file)

    # Equivalent to above:
    e_data3 = either.pure(path).bind(process)

Getting pyeither

pip install pyeither


I've created an IPython Notebook that describes the motivation for this project as an interactive tutorial building an analogous system from scratch. If you're interested in learning more about the why of this project, please check it out!

Motivation Notebook

Special Thanks

This library depends on the absolutely wonderful attrs library, without which there would have been far more ugly boilerplate.


Copyright 2018 Michael-Keith Bernard

See LICENSE.txt for the full license.