Pegmatite design overview

This is a fork and extensive rewrite of Achilleas Margaritis’s ParserLib. It has the following goals:

  • Idiomatic C++11
  • Simple use
  • Reuseable, reentrant grammars with multiple action delegates
  • No dependency on RTTI / exceptions (usable in embedded contexts)

It has the following explicit non-goals:

  • High performance (ease of use or modification should not be sacrificed in the name of performance)
  • Compatibility with the old ParserLib

Design outline

All rules should be immutable after creation. Ideally they’d be constexpr, but this is likely not to be possible. They should have no state associated with them, however, and so parsing should be entirely reentrant. State, for a grammar, is in two categories:

  • The current parsing state
  • The actions to be performed when parsing

The actions can also be immutable (they don’t change over a parse, at least), but should not be tied to the grammar. It should be possible to write one singleton class encapsulating the grammar, with members for the rules, and singleton subclasses (or, ideally, delegates) providing parser actions. The parsing state should be entirely contained within a context object and so the same grammar can be used from multiple threads and can be used for compilation, syntax highlighting, and so on.

RTTI Usage

ParserLib requires RTTI for one specific purpose: down-casting from ast_node to a subclass (and checking that the result really is of that class). If you are using RTTI in the rest of your application, then you can instruct ParserLib to use RTTI for these casts by defining the USE_RTTI macro before including the ParserLib headers and when building ParserLib.

If you do not wish to depend on RTTI, then ParserLib provides a macro that you can use in your own AST classes that will provide the required virtual functions to implement ad-hoc RTTI for this specific use. You use them like this:

	class MyASTClass : parserlib::ast_node
		/* Your methods go here. */
		PARSELIB_RTTI(MyASTClass, parserlib::ast_node)

This macro will be compiled away if you do define USE_RTTI, so you can provide grammars built with ParserLib that don’t force consumers to use or not-use RTTI. It is also completely safe to build without USE_RTTI, but still compile with RTTI.

What is Pegmatite

Pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size.

It is also, in the Computer Lab’s tradition of using bad puns for naming, a Parsing Expression Grammar library that rocks!