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An implementation of Data.Either from Haskell in Python

Project description

# 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

[**Motivation Notebook**](

## 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.

Project details

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