Lexers and parsers are typically defined separately and connected by a token stream. This separate definition is important for modularity and reduces the potential for parsing ambiguity. However, materializing tokens as data structures and case-switching on tokens comes with a cost. We show how to fuse separately-defined lexers and parsers, drastically improving performance without compromising modularity or increasing ambiguity. We propose a deterministic variant of Greibach Normal Form that ensures deterministic parsing with a single token of lookahead and makes fusion strikingly simple, and prove that normalizing context free expressions into the deterministic normal form is semantics-preserving. Our staged parser combinator library, flap, provides a standard interface, but generates specialized token-free code that runs two to six times faster than ocamlyacc on a range of benchmarks.

翻译：词法分析器和解析器通常分开定义，并通过词元流相连。这种分离定义对模块性至关重要，并能减少解析歧义的可能性。然而，将词元具体化为数据结构并对其执行分支选择会带来性能开销。我们展示了如何融合分别定义的词法分析器和解析器，在不损害模块性或增加歧义的前提下大幅提升性能。我们提出了一种确定性格雷巴赫范式变体，它通过单词元前瞻实现确定性解析，并使得融合过程异常简洁；同时证明了将上下文无关表达式归一化为此确定性范式具有语义保持性。我们的阶段化解析器组合子库flap提供标准接口，但生成专门的免词元代码，在多个基准测试中其运行速度比ocamlyacc快两到六倍。