Link grammar

From Free net encyclopedia

Link grammar (LG) is a theory of syntax by Davy Temperley and Daniel Sleator which builds simple relations between pairs of words, rather than constructing constituents in tree-like hierarchy. There are two basic parameters: directionality and distance. Dependency grammar is similar to link grammar, but dependency grammar includes a head-dependent relationship, as well as lacking directionality in the relations between words.

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.

1 Syntax
2 Examples
- 2.1 Example 1
- 2.2 Example 2
3 Applications
4 Implementations
5 Further reading
6 External links

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Syntax

Rightward links are represented as a +, and leftward links with a -. Optional links are contained in curly brackets {...}. Undesirable links are contained in any number of square brackets [...]. Multiple links are joined either by a conjunction & or a disjunction or. Each rule ends with a semicolon ;.

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Examples

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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

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Example 2

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

<noun-subject>: S+;

<noun-object>: O+;

<verb>: {O-} & {S-};

And a simple Persian sentence, man nAn xordam (من نان خوردم) 'I ate bread' would look like:

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

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Applications

Image:Abiword grammar.jpg AbiWord, a free word processing program, uses Link Grammar for on-the-fly grammar checking. Words which cannot be linked anywhere receive a green line underneath.

Link Grammar has also been employed for information extraction of biomedical texts, as well as experimental machine translation systems from English to German and Turkish.

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Implementations

The Link grammar syntax parser is a library for natural language processing written in C. It is currently licensed under a GPL compatible free software license.

There are also Perl and Ruby implementations available.

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