Semantic Analysis

Syntax-Driven Semantic Analysis

Definition: Syntax-driven semantic analysis — assigning meaning representations based soley on static knowledge from the lexicon and the grammar. [JM]

This provides a representation that is "both context independent and inference free." [JM], presumably referring to semantic context.

Definition: principle of compositionality — the meaning of a sentence is the sum of the meanings of its constituent elements.
NB this includes the relationships among these elements (ordering, grouping, etc.) I.e., the meaning of a sentence is "partially based on its syntactic structure."

Pipeline Representation

Parser => Syntactic Representation => Semantic Analyzer => Meaning Representation

Notes:

• Syntactic representation could be parse tree (real or temporal);
  or feature structures; or etc.
• Possible Meaning representations have been discussed —
  Frames, FOPC, Semantic networks, etc.

Ambiguities arise from the syntax and lexicon and are thus woven into the output of the semantic analyzer. A major assumption on the handling of ambiguity in syntax-driven semantic analysis is that subsequent interpretation processes will be required to resolve these ambiguities. These processes will use domain knowledge and knowledge of context to select among competing representations [JM]

Parser => Syntactic Representation => Semantic Analyzer =>
Ambiguous Meaning Representations =>
Additional Disambiguating Stages [domain-specific knowledge; contextual knowledge] =>
Selection process

Example: Rudimentary analysis of:

  15.1 AyCaramba serves meat.

[JM Fig. 15.2] shows a simplified parse tree, annotated with meaning representation (lacking feature attachments.)

Notes:

• "first" subtree retrieved by the semantic analyzer (is supposed to) corresponds to the verb "to serve".
• the semantic analyzer discerns that VP requires
  • a partcular NP (viz., an actor) to precede it
  • a partcular NP (viz., a direct object) to complement the Verb

• this also demonstrates a method of encoding semantics using FOPC

In English, this says

• There exists an element e which satisfies the predicate of being an event of Serving [a serving event]
  ∃ e such that e Is a Serving instance
  
• And e, together with AyCaramba also satisfies the predicate Server, i.e., AyCaramba is the server in the event
  ∧ Server(e,AyCaramba)
• And e, together with Meat satisfies the predicate Served,
  i.e., meat is served during the event.
  ∧ Served(e,Meat)

Note the similarities to Logic Form's representation of meaning.

Problems to overcome when using the above schema:

The interpretator of the tree needs to know that the Verb is to be used to define the template for interpretation, where the Verb can be found in the tree, and where the NP arguments to the template occur in the tree.

Since there are potentially infinitely many trees generated by any reasonably sized grammar for NLP, this task needs some other processing aids.

To facilitate the processing of the parse tree by the semantic analyzer, grammar rules are augmented with semantic attachments."