matts-majestic-monads 0.0.5

Monadic data structures

Matts Majestic Monads

This overblown library implements Functor, Applicative and Monad in Python.

Synopsis

You can use Either.convert or Maybe.convert as a decorator to convert return types. This example converts exceptions into Left values:

from monads import Either

@Either.convert
def parse_arguments():
    parser = argparse.ArgumentParser(description='Download a URL and print it')
    parser.add_argument('URL', help='URL to download')
    return parser.parse_args()

Once you have your Either you can use fmap and bind to operate over the contained value:

result = (
    make_parser()
        .fmap(parse)
        .fmap(to_url)
        .bind(read)
)

The methods will be passed the current value. You should use fmap when the function will return a normal value. When the function returns a monad, use bind.

You can use Function as a decorator to convert functions. Once converted they become composable:

from monads import Function

@Function
def parse(parser):
    return parser.parse_args()

@Function
def to_url(arguments):
    return arguments.URL

both = parse.compose(to_url)

Example

There is a simple example of using this library in example.py. This script reads a url that you provide and prints the length of the body:

➜ pipenv run python example.py 'http://python.org/'
Right(48817)

➜ pipenv run python example.py 'http://pythonasdasd.org/'
Left(<urlopen error [Errno -2] Name or service not known>)