A DEFINITION OF PARIKH CONTROLLED CONTEXT-FREE GRAMMARS AND SOME PROPERTIES OF THEM

2001 ◽  
pp. 357-367
Author(s):  
TAISHIN Y. NISHIDA ◽  
SHIGEKO SEKI
Keyword(s):  
Definition Of ◽  
Context Free ◽  
Context Free Grammars

Nonminimal Derivations in Unification-based Parsing

2001 ◽  
Vol 27 (2) ◽  
pp. 277-285
Author(s):  
Noriko Tomuro ◽  
Steven L. Lytinen
Keyword(s):  
Computational Cost ◽  
Precise Definition ◽  
Parse Tree ◽  
Feature Structures ◽  
Unification Grammars ◽  
Parsing Algorithm ◽  
Definition Of ◽  
Context Free ◽  
Context Free Grammars

Shieber's abstract parsing algorithm (Shieber 1992) for unification grammars is an extension of Earley's algorithm (Earley 1970) for context-free grammars to feature structures. In this paper, we show that, under certain conditions, Shieber's algorithm produces what we call a nonminimal derivation: a parse tree which contains additional features that are not in the licensing productions. While Shieber's definition of parse tree allows for such nonminimal derivations, we claim that they should be viewed as invalid. We describe the sources of the nonminimal derivation problem, and propose a precise definition of minimal parse tree, as well as a modification to Shieber's algorithm which ensures minimality, although at some computational cost.

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