fastcons

Fastcons is a Python extension module that provides an efficient implementation of cons lists.

The fastcons module provides two types: nil and cons. The nil type represents the empty list, while the cons type represents a pair - an immutable cell containing two elements.

Currently requires Python 3.11.

Installation

You can install fastcons using pip:

pip install fastcons

Usage

The fastcons module provides two types:

• nil: represents the empty list; and
• cons: represents a pair.

You can create the nil object by calling nil().

You can create a cons object by calling cons(head, tail). cons can be used to create linked lists: a chain of cons objects is considered a list if it is terminated by nil(), e.g. cons(1, cons(2, cons(3, nil()))).

You can efficiently create cons lists from Python sequences using the cons.from_xs method, where xs is a sequence.

from fastcons import cons, nil

# Create a cons list using the cons function
lst = cons(1, cons(2, cons(3, nil())))

# Create a cons list from a Python sequence
lst2 = cons.from_xs([1, 2, 3])

# Access the head and tail of a cons list
assert lst.head == 1
assert lst.tail.head == 2

# Test for equality
assert lst == lst2

The cons objects are printed using Lisp-style notation, which makes it easier to read long lists.

>>> cons.from_xs(range(1, 4))
(1 2 3)
>>> cons("foo", "bar")
('foo' . 'bar')
>>> cons(cons(1, 2), cons(cons(3, 4), nil()))
((1 . 2) (3 . 4))

Pattern matching

PEP 622 pattern matching is supported for cons and nil:

>>> match nil():
...   case nil():
...     print(nil())
...
nil()
>>> match cons(1, nil()):
...   case cons(a, d):
...     print(f"{a = }, {d = }")
...
a = 1, d = nil()

API Reference

nil()

Returns the singleton nil object.

cons(head, tail)

Returns a cons object with the given head and tail.

cons.from_xs(xs)

Returns a cons object created from the Python sequence xs.

License

fastcons is released under the MIT license.