pyeither
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
Motivation
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!
Special Thanks
This library depends on the absolutely wonderful
attrs
library, without which there would
have been far more ugly boilerplate.
License
Copyright 2018 Michael-Keith Bernard
See LICENSE.txt for the full license.