Unifying Parsing and Reflective Printing for Fully Disambiguated Grammars

Abstract

Language designers usually need to implement parsers and printers. Despite being two closely related programs, in practice they are often designed separately, and then need to be revised and kept consistent as the language evolves. It will be more convenient if the parser and printer can be unified and developed in a single program, with their consistency guaranteed automatically. Furthermore, in certain scenarios (like showing compiler optimisation results to the programmer), it is desirable to have a more powerful reflective printer that, when an abstract syntax tree corresponding to a piece of program text is modified, can propagate the modification to the program text while preserving layouts, comments, and syntactic sugar. To address these needs, we propose a domain-specific language BIYACC, whose programs denote both a parser and a reflective printer for a fully disambiguated context-free grammar. BIYACC is based on the theory of bidirectional transformations, which helps to guarantee by construction that the generated pairs of parsers and reflective printers are consistent. Handling grammatical ambiguity is particularly challenging: we propose an approach based on generalised parsing and disambiguation filters, which produce all the parse results and (try to) select the only correct one in the parsing direction; the filters are carefully bidirectionalised so that they also work in the printing direction and do not break the consistency between the parsers and reflective printers. We show that BIYACC is capable of facilitating many tasks such as Pombrio and Krishnamurthi's 'resugaring', simple refactoring, and language evolution.

InDubio: A Combinator Library to Disambiguate Ambiguous Grammars

“…Thus, we are optimizing HaGLR so that it uses a better parse-table representation, and we are also considering to use shared packed parse forests [24] and data structure free compilation [18], which both use sharing to reduce runtime and memory consumption. We are also integrating the disambiguation filters with BiYacc [25], a tool for generating both a parser and a reflective printer [27,26] for an unambiguous context-free grammar. The use of disambiguation filters can be helpful in extending this tool to also support ambiguous context-free grammars, therefore increasing its expressiveness and allowing for more test cases to be supported by this tool.…”