Overview

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monad - a functional python package

Note

This project is superseded by Hymn(https://github.com/pyx/hymn).

Limited by Python's syntax, there is no way to have a clean implementation of do notation, the closest thing is a do decorator on generator functions using yield as <-, which feels like black magic.

That's why I stopped shoehorning this into Python, and did a complete rewrite in Hy (https://github.com/hylang/hy) a few years ago.

Being a lisp, or as they say, Homoiconic Python, Hy has the most flexible syntax (or lack thereof 😄), with it, I finally can write do notations, check this out (for added fun, a Lazy monad is being demonstrated here, we can never have such clean way to write thunk in pure python):

=> (import [hymn.types.lazy [force]])
=> (require [hymn.types.lazy [lazy]])
=> ;; lazy computation implemented as monad
=> ;; macro lazy creates deferred computation
=> (setv a (lazy (print "evaluate a") 42))
=> ;; the computation is deferred, notice the value is shown as '_'
=> a
Lazy(_)
=> ;; evaluate it
=> (.evaluate a)
evaluate a
42
=> ;; now the value is cached
=> a
Lazy(42)
=> ;; calling evaluate again will not trigger the computation
=> (.evaluate a)
42
=> (setv b (lazy (print "evaluate b") 21))
=> b
Lazy(_)
=> ;; force evaluate the computation, same as calling .evaluate on the monad
=> (force b)
evaluate b
21
=> ;; force on values other than lazy return the value unchanged
=> (force 42)
42
=> (require [hymn.macros [do-monad]])
=> ;; do notation with lazy monad
=> (setv c (do-monad [x (lazy (print "get x") 1) y (lazy (print "get y") 2)] (+ x y)))
=> ;; the computation is deferred
=> c
Lazy(_)
=> ;; do it!
=> (force c)
get x
get y
3
=> ;; again
=> (force c)
3

So, if you are interested in this package, please try Hymn(https://github.com/pyx/hymn) instead.

Introduction

What?

Monads in python, with some helpful functions.

How?

>>> from monad.decorators import maybe
>>> parse_int = maybe(int)
>>> parse_int(42)
Just(42)
>>> parse_int('42')
Just(42)
>>> parse_int('42.2')
Nothing

>>> parse_float = maybe(float)
>>> parse_float('42.2')
Just(42.2)

>>> from monad.actions import tryout
>>> parse_number = tryout(parse_int, parse_float)
>>> tokens = [2, '0', '4', 'eight', '10.0']
>>> [parse_number(token) for token in tokens]
[Just(2), Just(0), Just(4), Nothing, Just(10.0)]

>>> @maybe
... def reciprocal(n):
...     return 1. / n
>>> reciprocal(2)
Just(0.5)
>>> reciprocal(0)
Nothing

>>> process = parse_number >> reciprocal
>>> process('4')
Just(0.25)
>>> process('0')
Nothing
>>> [process(token) for token in tokens]
[Just(0.5), Nothing, Just(0.25), Nothing, Just(0.1)]
>>> [parse_number(token) >> reciprocal for token in tokens]
[Just(0.5), Nothing, Just(0.25), Nothing, Just(0.1)]
>>> [parse_number(token) >> reciprocal >> reciprocal for token in tokens]
[Just(2.0), Nothing, Just(4.0), Nothing, Just(10.0)]

Why?

Why not.

Requirements

  • CPython >= 2.7

Installation

Install from PyPI:

pip install monad

Install from source, download source package, decompress, then cd into source directory, run:

make install

License

BSD New, see LICENSE for details.