Parsing with First-Class Derivatives Jonathan Immanuel Brachthauser¨ Tillmann Rendel Klaus Ostermann rtifact A Comple * t * te n * A * te is W E s A e n C l l L o D C S University of Tubingen,¨ Germany o * * c P u e m s E u O e e n v R t e O o d t a y * s E a * fbrachthaeuser, rendel,
[email protected] l u d a e t Abstract often reducing the modularity of their parser because these ad- Brzozowski derivatives, well known in the context of reg- hoc additions are cross-cutting with respect to the otherwise ular expressions, have recently been rediscovered to give a context-free syntactic structure of the language. simplified explanation to parsers of context-free languages. Parser combinators [10, 10, 15, 21, 22] are a parsing We add derivatives as a novel first-class feature to a standard approach that is well-suited for such ad-hoc additions. With parser combinator language. First-class derivatives enable an parser combinators, a parser is described as a first-class inversion of the control flow, allowing to implement modular entity in some host language, and parsers for smaller subsets parsers for languages that previously required separate pre- of a language are combined into parsers of larger subsets processing steps or cross-cutting modifications of the parsers. using built-in or user-defined combinators such as sequence, We show that our framework offers new opportunities for alternative, or iteration. The fact that parsers are first-class reuse and supports a modular definition of interesting use entities at runtime allows an elegant way of structuring cases of layout-sensitive parsing.