Logic Programming in Python

LogPy 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 1 number, x, such that x == 5

>>> from logpy 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, 5))
(3,)

LogPy 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 states that 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)

LogPy supports relations and facts. This is best demonstrated by example.

The following code creates a parent relationship and uses it to state facts about who is a parent of whom.

>>> from logpy 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,')

With pip or easy_install

pip install logic

From source

git clone git@github.com:logpy/logpy.git
cd logpy
python setup.py install

Run tests with nose

nosetests

LogPy is pure Python

Matthew Rocklin

New BSD license. See LICENSE.txt

Logic Programming was first popularized through the Prolog language.

This implementation closely follows the design of miniKanren, a Scheme library for relational programming. More information can be found in the thesis of William Byrd.

miniKanren came to our attention through the core.logic Clojure library.