Table driven prediction for recursive descent LL(k) parsers

Programming languages are typically described in BNF or some extension of BNF, and the process of converting these descriptions into parsers is performed by parser generators. Some of the parser generators that convert LL grammars into parsers construct them to use recursive descent that gives them context during execution. The context is provided by the execution stack as the parser descends into the grammar and this is what allows the full expressiveness of LL grammars. Table driven parsers can be generated instead but restrictions are placed on the LL grammars that can be accepted. The benefit of tables is that they facilitate a separation of syntax analysis and semantic code written by a language designer. They are also faster and they simplify language implementation and modification. This paper proposes the possibility of a hybrid system that makes decisions using tables but once decisions are made recursive descent is employed to maintain context. The benefits of each system are maintained, and the drawbacks are mitigated. Also discussed are the modifications made to an existing parser generator, oops (version 2), so that it accepts LL(k) grammars and builds parsers using this system as proof-of-concept

Left Recursion in Parsing Expression Grammars

Parsing Expression Grammars (PEGs) are a formalism that can describe all deterministic context-free languages through a set of rules that specify a top-down parser for some language. PEGs are easy to use, and there are efficient implementations of PEG libraries in several programming languages. A frequently missed feature of PEGs is left recursion, which is commonly used in Context-Free Grammars (CFGs) to encode left-associative operations. We present a simple conservative extension to the semantics of PEGs that gives useful meaning to direct and indirect left-recursive rules, and show that our extensions make it easy to express left-recursive idioms from CFGs in PEGs, with similar results. We prove the conservativeness of these extensions, and also prove that they work with any left-recursive PEG. PEGs can also be compiled to programs in a low-level parsing machine. We present an extension to the semantics of the operations of this parsing machine that let it interpret left-recursive PEGs, and prove that this extension is correct with regards to our semantics for left-recursive PEGs.Comment: Extended version of the paper "Left Recursion in Parsing Expression Grammars", that was published on 2012 Brazilian Symposium on Programming Language

CHR Grammars

A grammar formalism based upon CHR is proposed analogously to the way Definite Clause Grammars are defined and implemented on top of Prolog. These grammars execute as robust bottom-up parsers with an inherent treatment of ambiguity and a high flexibility to model various linguistic phenomena. The formalism extends previous logic programming based grammars with a form of context-sensitive rules and the possibility to include extra-grammatical hypotheses in both head and body of grammar rules. Among the applications are straightforward implementations of Assumption Grammars and abduction under integrity constraints for language analysis. CHR grammars appear as a powerful tool for specification and implementation of language processors and may be proposed as a new standard for bottom-up grammars in logic programming. To appear in Theory and Practice of Logic Programming (TPLP), 2005Comment: 36 pp. To appear in TPLP, 200