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Logic Programming in Python
kanren enables the expression of relations and the search for values which satisfy them. The following code is the
Hello, world! of logic programming. It asks for
x, such that
x == 5
>>> from kanren import run, eq, membero, var, conde >>> x = var() >>> run(1, x, eq(x, 5)) (5,)
Multiple variables and multiple goals can be used simultaneously. The
following code asks for a number x such that
x == z and
z == 3
>>> z = var() >>> run(1, x, eq(x, z), eq(z, 3)) (3,)
kanren uses unification, an advanced form of pattern matching, to match within expression trees.
The following code asks for a number, x, such that
(1, 2) == (1, x) holds.
>>> run(1, x, eq((1, 2), (1, x))) (2,)
The above examples use
eq, a goal constructor to state that two expressions
are equal. Other goal constructors exist such as
membero(item, coll) which
item is a member of
coll, a collection.
The following example uses
membero twice to ask for 2 values of x,
such that x is a member of
(1, 2, 3) and that x is a member of
(2, 3, 4).
>>> run(2, x, membero(x, (1, 2, 3)), # x is a member of (1, 2, 3) membero(x, (2, 3, 4))) # x is a member of (2, 3, 4) (2, 3)
As in the above examples,
z = var() creates a logic variable. You may also, optionally, pass a token name for a variable to aid in debugging:
>>> z = var('test') >>> z ~test
Lastly, you may also use
vars() with an integer parameter to create multiple logic variables at once:
>>> a, b, c = vars(3) >>> a ~_1 >>> b ~_2 >>> c ~_3
kanren stores data as facts that state relationships between terms.
The following code creates a parent relationship and uses it to state facts about who is a parent of whom within the Simpsons family.
>>> from kanren import Relation, facts >>> parent = Relation() >>> facts(parent, ("Homer", "Bart"), ... ("Homer", "Lisa"), ... ("Abe", "Homer")) >>> run(1, x, parent(x, "Bart")) ('Homer',) >>> run(2, x, parent("Homer", x)) ('Lisa', 'Bart')
We can use intermediate variables for more complex queries. Who is Bart's grandfather?
>>> y = var() >>> run(1, x, parent(x, y), parent(y, 'Bart')) ('Abe',)
We can express the grandfather relationship separately. In this example we use
conde, a goal constructor for logical and and or.
>>> def grandparent(x, z): ... y = var() ... return conde((parent(x, y), parent(y, z))) >>> run(1, x, grandparent(x, 'Bart')) ('Abe,')
kanren depends on functions, tuples, dicts, and generators. There are almost no new data structures/classes in kanren so it should be simple to integrate into preexisting code.
kanren uses Multiple Dispatch and the unification library to support pattern matching on user defined types. Also see unification (wikipedia). Types which can be unified can be used for logic programming. See the project examples for how to extend the collection of unifiable types to your use case.
pip install kanren
git clone firstname.lastname@example.org:logpy/logpy.git cd logpy python setup.py install
Run tests with tox
It is, in short, a light weight dependency.
New BSD license. See LICENSE.txt
Logic programming is a general programming paradigm. This implementation however came about specifically to serve as an algorithmic core for Computer Algebra Systems in Python and for the automated generation and optimization of numeric software. Domain specific languages, code generation, and compilers have recently been a hot topic in the Scientific Python community. kanren aims to be a low-level core for these projects.