fastcons

Fastcons is a Python extension module that aims to provide an efficient implementation of cons.

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 3.12, and Linux or MacOS.

• Installation
• Usage
  • Pattern matching
• API Reference
• License

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
xs = cons(1, cons(2, cons(3, nil())))

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

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

# Test for equality
assert xs == ys

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))

cons.lift can be used to recursively transform dicts, lists, tuples and generators to cons objects. dicts will be transformed to cons lists of pairs (association lists or alists), the rest will be transformed to alists.

>>> cons.lift({'a': 2, 'b': 3})
(('a' . 2) ('b' . 3))
>>> cons.lift((1, 2, 3))
(1 2 3)
>>> cons.lift(x for x in ('a', 'b', 'c'))
('a' 'b' 'c')
>>> cons.lift({x: {x: y}} for x, y in zip(['a', 'b', 'c'], range(1, 4)))
((('a' ('a' . 1))) (('b' ('b' . 2))) (('c' ('c' . 3))))

The nil object provides a to_list() method that returns an empty Python list:

>>> nil().to_list()
[]

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. The nil object is falsy.

nil.to_list()

Returns an empty Python list.

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.

cons.lift(xs)

Recursively create a cons structure by converting:

• lists, tuples, and generators to cons lists; and
• dicts to cons lists of pairs (association lists).

assoc(object, alist)

Find the first pair in alist whose car is equal to object, and return that pair. If no pair is found, or alist is nil(), return nil().

assp(predicate, alist)

Return the first pair in alist for which the result of calling 'predicate' on its car is truthy. 'predicate' will be called with a single positional argument.

License

fastcons is released under the MIT license.