Hat Categories: Representing Form and Function Simultaneously in Combinatory Categorial Grammar

Combinatory Categorial Grammar (CCG, Steedman, 2000) is a lexicalised grammar formalism, which means that each word is associated with a category that specifies its argument structure and grammatical function. This accounts for the fact that different words have different argument structures, and allows the grammar to generate semantic analyses, instead of skeletal brackets. CCG’s lexicalisation has been found to improve parsing speed and accuracy (Clark and Curran, 2007b), and constitutes a strong theoretical claim about what aspects of language are universal. However, wide-coverage CCG parsing and generation systems currently use a set of typechanging rules that make their grammars less lexicalised. The rules were included to address sparse data problems that wide-coverage categorial grammars otherwise encounter. Without typechanging rules, categorial grammars require a proliferation of modifier categories, because the grammar is not always able to abstract away irrelevant detail about derivational context when constructing modifier categories. We analyse this modifier category proliferation problem in detail, and conclude that the CCG formalism makes it difficult to write efficient grammars for these constructs. The inefficiencies are introduced because the formalism has difficulty exploiting generalisations about syntactic form. This motivates some system of type-changing rules, in order to have more words receive categories that reflect their syntactic form. However, the existing system of type-changing rules reduces the level of lexicalisation in CCG, weakening some of the formalism’s most important properties. We propose a solution that allows type-changing rules to be lexicalised. Our proposal involves a modification to the category objects. CCG categories are composed of a number of attributes, including a type, a directional slash, and a feature structure. We propose an additional attribute, that represents the grammatical function that the category will later perform in the derivation. This allows the immediate type to represent the constituent’s syntactic form, enabling better analyses of form/function discrepancies. To avoid overusing function, we dub this new attribute a hat. To investigate the modifier category proliferation problem and evaluate our solution, we create and compare two versions of CCGbank. In addition to our HAT solution, we create a NO UNARY version of the corpus, which uses no type-changing rules. We evaluate these corpora by training and evaluating a state-of-the-art CCG parser on the different versions of the corpus. We find that the NO UNARY corpus makes parsing more difficult: accuracy decreased by 2.2%. The HAT version, however, achieved accuracy within 0.3% of the original parser, and increased in speed by 48%. In language processing, efficiency has often been contrasted with linguistic desirability or depth, but this distinction is artificial. Linguistic theories are keenly interested in how the human parser works as quickly as it does, and lexicalised grammars may be part of the answer. We have identified a key problem in the CCG theory, one aspect of which has caused CCG parsers to adopt a lower level of lexicalisation. By restoring that lost lexicalisation, we bring CCG parsing practice back in line with the core claims of the theory, to its immediate practical advantage.