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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.

Defining a grammar

You can create freestanding Rule instances, however the recommended way of creating a grammar is to place each rule inside a class. The Calculator example follows this pattern, with a CalculatorGrammar class containing one field for each rule. This is recommended for two reasons:

  1. It allows lazy creation, rather than requiring the rules to be instantiated on program creation. This gives faster start-up times and means that memory is only used when the grammar is actually used.
  2. It improves encapsulation. Although it is completely safe to refer to rules in other grammars, placing them in a class makes it easy for people binding actions to the grammar to identify all of the rules that they must (or, might want to) provide actions for.

Simple character or string recognising expressions can be created using the _E custom literal suffix. For example "int"_E creates an expression that will match the literal string "int". This is useful for terminals.

You can define more complex operations from these by using the following operators (where a and b are expressions):

  • *a matches zero or more instances of a
  • +a matches one or more instances of a
  • -a matches zero or one instance of a
  • a >> b matches a and then b
  • a | b matches either a or b

Newline rules have no special meaning in parsing, but are used to increment the line counter for input ranges. If you declare a rule as matching a newline, it will increment the line counter every time it is successfully matched.

Whitespace rules allow implicit whitespace in between all non-terminal expressions (sequences).

Building an AST

The ASTContainer class is intended to be used as the superclass for most AST nodes. Any ASTPtr and ASTList fields of subclasses of this class will automatically be created from the AST stack. Each AST class that is constructed is pushed onto the stack in the order that it is constructed and then popped off by its parents.

AST nodes do not, by default, keep around the InputRange of the text that they matched. This is to save space for cases where it is not required. If you intend to do helpful error reporting after semantic analysis, then it is strongly recommended that you do keep such a reference.

After you have defined your grammar and AST, all that remains is to bind the two together. To do this, create a subclass of ASTParserDelegate, with one BindAST field for each AST node, initialised with the corresponding grammar rule. The following is the parser for the Calculator example:

class CalculatorParser : public ASTParserDelegate
	BindAST<AST::Number> num = CalculatorGrammar::get().num;
	BindAST<AST::AddExpression> add = CalculatorGrammar::get().add_op;
	BindAST<AST::SubtractExpression> sub = CalculatorGrammar::get().sub_op;
	BindAST<AST::MultiplyExpression> mul = CalculatorGrammar::get().mul_op;
	BindAST<AST::DivideExpression> div = CalculatorGrammar::get().div_op;
	const CalculatorGrammar &g = CalculatorGrammar::get();

Invoking the parse() method on this class will cause an AST::Number class to be created for every terminal matching the num rule in the grammar, and so on. Note that this parser is reentrant. It is safe to use it from multiple threads to parse different strings. It is therefore safe to also make the parser a singleton.

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 a Parsing Expression Grammar library that rocks!

To do

Not in order:

  • Write parameterised delegate for generating semantic HTML markup
  • Check whether the USE_RTTI setting for the headers and library had to match it would be nice if it didn't...


A library for reusable parsers







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