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This project is superseded by Hymn(https://github.com/pyx/hymn).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 ๐), 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.
Monads in python, with some helpful functions.
>>> 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 not.
- CPython >= 2.7
Install from PyPI:
pip install monad
Install from source, download source package, decompress, then cd
into source directory, run:
make install
BSD New, see LICENSE for details.
- Documentation:
- http://monad.readthedocs.org/
- Issue Tracker:
- https://bitbucket.org/pyx/monad/issues/
- Source Package @ PyPI:
- https://pypi.python.org/pypi/monad/
- Mercurial Repository @ bitbucket:
- https://bitbucket.org/pyx/monad/
- Git Repository @ Github:
- https://github.com/pyx/monad/