We describe the development of space-efficient implementations of GLL parsers, and the process by which we refine a set-theoretic model of the algorithm into a practical parser generator that creates practical parsers. GLL parsers are recursive descent-like, in that the structure of the parser’s code closely mirrors the grammar rules, and so grammars (and their parsers) may be debugged by tracing the running parser in a debugger. While GLL
are straightforward to describe, full GLL parsers present technical traps and challenges for the unwary. In particular, naïve implementations based closely on the theoretical description of GLL can result in data structures that are not practical for grammars for real programming language grammars such as ANSI-C. We develop an equivalent formulation of the algorithm as a high-level set-theoretic model supported by table-based indices, in order to then explore a set of alternative implementations which trade space for time in ways which preserve the cubic bound.