Friday, March 23, 2012

PyCon: Parsing Sentences with the OTHER Natural Language Tool: Link Grammar

See the website.

When it comes to NLTK (the Natural Language Toolkit), some assembly is definitely required. If you're not a linguist, it's not so easy.

Link Grammar is a theory of parsing sentences. It is also a specific implementation written in C. It's from CMU. It started in 1991. The latest release was in 2004.

Link Grammar does not use "explicit constituents" (e.g. noun phrases).

It puts an emphasis on lexicality.

Sometimes, specific words have a large and important meaning in the language. For instance, consider the word "on" in "I went to work on Friday."

pylinkgrammar is a Python wrapper for Link Grammar. (Make sure you use the version of pylinkgrammar on BitBucket.)

Often, there are multiple valid linkages for a specific sentence.

It can produce a sentence tree. It can even generate Postscript containing the syntax tree. (The demo was impressive.)

A link grammar is a set of rules defining how words can be linked together to form sentences.

Words have connectors to the left and/or the right.

General rules:
  • Connectivity: All words must be connected (with some exceptions).
  • Exclusion: No two links can connect the same pair of words.
  • Planarity: If links are drawn above words, no two links can cross.
  • Ordering: When satisfying multiple connectors on one side of a disjunct, the ordering of the words must match the ordering of the connectors.
The software does some post processing in order to handle a lot of things.

The system has knowledge of 60,000 words and 170 (?) link types.

It's amazing how it can recognize the invalidity of certain sentences that are only very subtly invalid.

It can guess the purpose of unknown words.

It's pretty robust against garbage thrown into the sentence.

It's used as the grammar checker in Abiword.

Link Grammar is a powerful theory for parsing sentences. It has a comprehensive implementation. It's easy to use with Python.

The distinction between syntax and semantics is pretty blurry.

2 comments:

Hesham A. Amin said...

Thanks for the good introduction to link grammar.
I have a question: is it possible to traverse the parse tree using pylinkgrammar? because it seems that Linkage and Link objects contain links and words as strings not as pointers or references to other sentences.

thanks

Shannon Behrens said...

Hesham, sorry, I don't know. I've never used it myself.