Run-time Extensible Deterministic Top-Down Parsing

Abstract

When reading a text or listening to speech, words are processed on-line by humans in the order they come. Humans mainly use this kind of parsing even when they process deterministic text (programs). Intuitively there are some mental actions just after the morphological analysis of any newly recognized word. This mental action helps to understand the given word (or to position the word within the frame of the – still not complete – sentence). Within parsing of formal languages the concept closest to this idea is top-down parsing that is usually used only together with different classes of LL grammars. The advantage of top-down parsing of programming languages is the possibility to implement it by a recursive descent parser – i.e. by a system of procedures that may recursively call each other. Such a system may be `tuned' by handmade changes. The usage of LL grammars is not always possible, because the grammars of programming languages may have left recursive symbols. Programming language grammars are intuitively `close' to LL grammars. A good model for such grammars are the kind grammars studied in this paper. Kind grammars preserve all the important features of LL grammars that are advantageous for parsing.