Author: Eric Nichols <underspecified@gmail.com>
This module contains python implementations of some scala-style monads.
It provides a generator-based syntax using a decompilation trick from
Pony [1] to translate generators into nested calls to a monad's
flat_map(), map(), and filter() functions, in a similar
fashion to scala's for comprehensions [2].
The idea was inspired by a comment by Shin no Noir [3] on a post on A Neighborhood of Infinity [4].
GenMonads supports syntax like scala for-comprehensions by using a
special function to evaluate a generator over monads (the functions is
named mfor(), short for "monadic for comprehension," as it is
modeled after scala's for comprehensions, but the synonym do() is
also available):
>>> from genmonads.monad import mfor
>>> from genmonads.option import *
>>> print(mfor(x + y
for x in option(2)
if x < 10
for y in option(5)
if y % 2 != 0))
Some(7)The above generator is automatically translated into the following at run-time:
>>> print(option(2) \
.filter(lambda x: x < 10) \
.flat_map(lambda x: option(5) \
.filter(lambda y: y % 2 != 0) \
.map(lambda y: x + y)))
Some(7)Both generator expressions and generator functions are supported, though variable assignment in generator function bodies is not currently implemented:
>>> def make_gen():
for x in option(4):
if x > 2:
for y in option(10):
if y % 2 == 0:
yield x - y
>>> print(mfor(make_gen()))
Some(-6)Monad chaining with the bind operator is also supported (>>= and
>> are combined into a single >> operator due to syntactic
limitations in overloading >>= in python):
>>> print(option(5) >> (lambda x: option(x * 2)))
Some(10)
>>> print(option(5) >> (lambda _: option(2)))
Some(2)
>>> print(option(5) >> Nothing())
NothingFollowing scala's monadic handling of NULL, the option()
function can be used to inject computations that can return None into
the Option monad:
>>> print(option(None))
Nothing
>>> pets = {'cat': 1, 'dog': 2, 'bird': 3}
>>> print(option(pets.get('dog')))
Some(2)
>>> print(option(pets.get('iguana')))
Nothing- Python 3: https://www.python.org
- pony: https://pypi.python.org/pypi/pony
GenMonads can be installed from the GitHub project page https://github.com/underspecified/GenMonads via pip:
> pip3 install git+https://git@github.com/underspecified/GenMonads.gitSee the project's Read the Docs page at https://underspecified.github.io/GenMonads/
- variable assignment in generator functions
- optional Haskell nomenclature
This project is licensed under the GNU Affero General Public License v3 (AGPLv3).