HIGHER-ORDER ATTRIBUTE GRAMMARS

Higher Order Attribute Grammars (HAGs) are an extension of normal attribute grammars in the sense that the distinction between the domain of parse-trees and the domain of attributes has disappeared: parse trees may be computed in attributes and grafted to the parse tree at various places. As a result semantic functions may be described by attribute evaluation. We will present the basic definitions for HAGs, and compare them with attribute coupled grammars, extended affix grammars and functional programming languages. We will indicate how multi-pass compilers and a compiler for supercombinators can be described this way. It will be shown that, especially in the case of incremental evaluation, the conventional execution model has to be generalised. Such a model, based on function caching, hash-consing and combinator construction will be discussed. This model encompasses many of the more ad-hoc optimisations one finds in standard implementations of normal attribute grammars.