Scheme-like cons in Python (original) (raw)

Python `cons`

An implementation of cons in Python.

Usage and Design

The cons package attempts to emulate the semantics of Lisp/Scheme's cons as closely as possible while incorporating all the built-in Python sequence types:

from cons import cons, car, cdr cons(1, []) [1]

cons(1, ()) (1,)

cons(1, [2, 3]) [1, 2, 3]

In general, cons is designed to work with collections.abc.Sequence types.

According to the cons package, None corresponds to the empty built-in list, as nil does in some Lisps:

The cons package follows Scheme-like semantics for empty sequences:

car([]) Traceback (most recent call last): File "", line 1, in ConsError: Not a cons pair

cdr([]) Traceback (most recent call last): File "", line 1, in ConsError: Not a cons pair

By default, str types are not considered cons-pairs, although they are sequences:

cons("a", "string") ConsPair('a' 'a string')

This setting can be overridden and other types can be similarly excluded from consideration by registering classes with the abc-based classes MaybeCons and NonCons.

Features

Built-in support for the standard Python ordered sequence types: i.e. list, tuple, Iterator, OrderedDict.

from collections import OrderedDict cons(('a', 1), OrderedDict()) OrderedDict([('a', 1)])

Existing cons behavior can be changed and support for new collections can be added through the generic functions cons.core._car and cons.core._cdr.
Built-in support for unification.

from unification import unify, reify, var unify([1, 2], cons(var('car'), var('cdr')), {}) {~car: 1, ~cdr: [2]}

reify(cons(1, var('cdr')), {var('cdr'): [2, 3]}) [1, 2, 3]

reify(cons(1, var('cdr')), {var('cdr'): None}) [1]

Installation

Development

First obtain the project source:

git clone git@github.com:pythological/python-cons.git

Create a virtual environment and install the development dependencies:

$ pip install -r requirements.txt

Set up pre-commit hooks: