opencog / unify

Atomese expression unifier

Unify

opencog	singnet

Unify, the AtomSpace unifier, is an expression unifier for the AtomSpace. Given two expressions containing variables, it finds all terms that provide groundings for those variables.

It is used as a foundation for the term rewriting engine (the Unified Rule Engine or URE for short) for OpenCog.

Building and Installing

Prerequisites

To build the Unifier you need to build and install the AtomSpace first, see Building-and-installing-the-AtomSpace for more information.

Building Unify

Be sure to install the pre-requisites first! Perform the following steps at the shell prompt:

    cd unify
    mkdir build
    cd build
    cmake ..
    make -j

Libraries will be built into subdirectories within build, mirroring the structure of the source directory root.

Unit tests

To build and run the unit tests, from the ./build directory enter (after building opencog as above):

    make -j test

Tests can be run in parallel as well:

    make -j test ARGS=-j4

Install

After building, you must install Unify

    sudo make install

Examples

Examples can be found in this repository in the examples directory.

More info

The primary documentation for the URE is here:

• URE wiki
• URE README.md

unify's Issues

Provide form of unifier that returns QueueValue

The UniferLink and the UnifyReduceLink return SetLinks. The classic problem with Set Links is that they pollute the AtomSpace. A form that uses QueueValues would see to be better.

See
https://wiki.opencog.org/w/UnifierLink
https://wiki.opencog.org/w/UnifyReduceLink
https://wiki.opencog.org/w/QueueValue

Provide Atomese wrapper

Provide an Atomese wrapper for Unify.

So, here's the deal: (Unify A B) is more or less the same thing as (Meet (Identical A B)) . However... There are some conceptual differences. Unify allows the form:

  (Unify
     (VariableList left-vars...)
     (VariableList right-vars..)
     left-expr
     right-expr)

where left-vars... are the things to be taken as variables in left-expr. There's nothing like this for IdenticalLink or MeetLink. We can almost get the same thing by writing

  (Meet
     (VariableList left-vars... right-vars..)
     (Identical left-expr right-expr))

except that this can fail if (Variable "$X") appears in left-vars, and also appears in right-expr, but not in right-vars. There is no way to handle this case in any kind of easy or natural way using MeetLink. And that is how Unify is different from (Meet (Identical ..))

The proposal is then to provide two forms: either a simple form, with no vardecls

  (Unify
     left-expr
     right-expr)

or the full form, with vardecls.

  (Unify
     (VariableList left-vars...)
     (VariableList right-vars..)
     left-expr
     right-expr)

Maybe also provide (VariableListAuto) which means "yank them out for me".

What about the output? (next post)