A Simple Recipe for Writing Decent Recursive Descent Parsers (Pearl)
Abstract
Parsing well-designed computer languages should not be a hard problem, be it for humans or for machines. This is not a new idea: in 1973, Vaughan R. Pratt argued against formalistic grammar specifications and in favor of a more intuitive and meaningful approach to designing and parsing syntax. In this Pearl, we take the reader on a journey through handwritten recursive descent parsing, revisiting Pratt’s original philosophy in a modern, statically-typed functional programming language. Contrary to many existing tutorials on the subject, we do not stop at simple expression languages: we also discuss how to tackle the full syntax of a simple programming language while avoiding the pitfalls of ad-hoc implementations. Indeed, a downside of recursive descent parsing is that the specification of what the parser accepts is written in code, which may contain subtle bugs and is not easily accessible to end users. We describe a simple recipe for architecting extensible recursive descent parsers that can automatically produce a readable representation of the syntax specification. We illustrate our approach by implementing a parser for a variant of Caml Light. Overall, this paper serves both as a pedagogical introduction to Pratt parsing in a modern programming language and as a practical guide to programmers who just want to implement, without unnecessary headaches, a computer language that is easy to parse and easy to read.
