The Non-associativity of Polarized Tree-Based Grammars

Abstract. Polarities are used to sanction grammar fragment combination in high level tree-based formalisms such as eXtenssible MetaGrammar (XMG) and polarized unification grammars (PUG). We show that attaching polarities to tree nodes renders the combination operation non-associative, and in practice leads to overgeneration. We first provide some examples of non-associative combination operators in existing polarity-based formalisms. We then prove that there is no other nontrivial polarity system for which grammar combination is associative. This property of polarities casts doubt on the usability of polarity-based grammars for grammar engineering.

• Type: Conference
• Year: 2007
• Where: CICLING
• Authors: Yael Cohen-Sygal, Shuly Wintner
• Keywords: CICLING 2007, Grammar, Grammar Fragment Combination, Natural Language Processing, Polarities Casts Doubt