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:

>>> cons(1, None)
[1]

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

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

>>> cdr([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
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

pip install cons