This is a cross-platform library for parsing boolean arithmetic expressions like
a && b && (!c || !d) and evaluating them by assigning values to the variables.
Boolean expressions are parsed into an abstract syntax tree (AST).
This is done with a generated lexer and parser. The generators used are:
- Java: Ragel and Bison
- Ruby: (C extension): Ragel and Bison
- JRuby: Ragel and Bison
This choice of generators makes it easy to port the implementation to other languages since they are ported to most programming languages.
Evaluation of the boolean expressions is done by traversing the AST with a visitor. (This is obviously overkill for something as simple as boolean expressions, keep reading to understand why).
The Ruby gem uses a C extension (for speed) and the JRuby gem uses a Java extension (also for speed). Support for e.g. Python could be added easily by using the same C code as the Ruby gem. Java programs (or any other JVM-based program such as Scala or Clojure) can also use the Java library as-is.
The purpose of this library is twofold.
First, it serves as a simple example of how to build a custom interpreted language with a fast lexer/parser that builds a visitor-traversable AST, and that runs on many different platforms. People who want to build a bigger cross-platform language could leverage the structure and build files in this project. For example, the Gherkin 3.0 project will use this project as a template.
Second, this project will be used by Cucumber to evaluate tag expressions in a more elegant way.