An implementation of Lisp/Scheme-like cons in Python.
cons
An implementation of cons
in Python.
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
.
list
, tuple
, Iterator
, OrderedDict
.>>> from collections import OrderedDict
>>> cons(('a', 1), OrderedDict())
OrderedDict([('a', 1)])
cons
behavior can be changed and support for new collections can be added through the generic functions cons.core._car
and cons.core._cdr
.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]
pip install cons
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:
$ pre-commit install --install-hooks