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
consbehavior can be changed and support for new collections can be added through the generic functionscons.core._carandcons.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
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