Link grammar (LG) is a theory of syntax which builds simple, explicit relations between pairs of words, rather than constructing constituents in tree-like hierarchy. There are two basic parameters: directionality and distance.

For example, in an SVO language like English, the verb would look left to form a subject link, and right to form an object link. Nouns would look right to complete the subject link, or left to complete the object link.

In an SOV language like Persian, the verb would look left to form an object link, and a more distant left to form a subject link. Nouns would look to the right for both subject and object links.

Examples

Example 1

A basic rule file for an SVO language might look like:

<determiner>:   D+;

<noun-subject>:   {D-} & S+;

<noun-object>:     {D-} & O-;

<verb>:          S-    &    {O+};

Thus the English sentence, “The boy painted a picture” would appear as:

              +-----O-----+
 +-D--+---S---+     +--D--+
 |    |       |     |     |
The  boy   painted  a  picture   

Example 2

While a rule file for an SOV language might consist of the following links:

<noun-subject>:   S+;

<noun-object>:     O+;

<verb>:            {O-}   &   S-;

And a simple Persian sentence, "من نان خوردم" 'I ate bread' would look like:

 +-----S-----+
 |     +--O--+
 |     |     |
man   nAn xordam

Implementations

The Link grammar syntax parser is a library for natural language processing written in C. It is currently licensed under an Apache-style license.

There are also Perl and Ruby implementations available.

External links

• The Link grammar homepage

• Online English demonstration

• Persian LG-parser extension

• Online Persian demonstration

• Perl implementation
• Ruby implementation