README.md

Darser: A LL(1) to Recursive Decent Parser/AST/Visitor Generator

Given a language BNF, as e.yaml, darser will generate a recursive decent parser, a set of classes making up the AST, a visitor class and a AST printer class. The parser, AST, and visitor can be extended by hand written extensions, see $ darser --help for more information.

e.yaml

Expression:
    Postfix: [PostfixExpression#post]

PostfixExpression:
    Primary: [PrimaryExpression#prim]
    Array: [PrimaryExpression#prim, lbrack, Expression#expr, rbrack]
    Call: [PrimaryExpression#prim, lparen, Expression#expr, rparen]

PrimaryExpression:
    Identifier: [ identifier#value ]
    Float: [ float64#value ]
    Integer: [ integer#value ]
    Parenthesis: [lparen, Expression#expr, rparen]

The parser for PrimaryExpression looks like this:

PrimaryExpression parsePrimaryExpressionImpl() {
	string[] subRules;
	subRules = ["Identifier"];
	if(this.lex.front.type == TokenType.identifier) {
		Token value = this.lex.front;
		this.lex.popFront();

		return new PrimaryExpression(PrimaryExpressionEnum.Identifier
			, value
		);
	} else if(this.lex.front.type == TokenType.float64) {
		Token value = this.lex.front;
		this.lex.popFront();

		return new PrimaryExpression(PrimaryExpressionEnum.Float
			, value
		);
	} else if(this.lex.front.type == TokenType.integer) {
		Token value = this.lex.front;
		this.lex.popFront();

		return new PrimaryExpression(PrimaryExpressionEnum.Integer
			, value
		);
	} else if(this.lex.front.type == TokenType.lparen) {
		this.lex.popFront();
		subRules = ["Parenthesis"];
		if(this.firstExpression()) {
			Expression expr = this.parseExpression();
			subRules = ["Parenthesis"];
			if(this.lex.front.type == TokenType.rparen) {
				this.lex.popFront();

				return new PrimaryExpression(PrimaryExpressionEnum.Parenthesis
					, expr
				);
			}
			auto app = appender!string();
			formattedWrite(app,
				"Found a '%s' while looking for",
				this.lex.front
			);
			throw new ParseException(app.data,
				__FILE__, __LINE__,
				subRules,
				["rparen"]
			);

		}
		auto app = appender!string();
		formattedWrite(app,
			"Found a '%s' while looking for",
			this.lex.front
		);
		throw new ParseException(app.data,
			__FILE__, __LINE__,
			subRules,
			["float64 -> PostfixExpression",
			 "identifier -> PostfixExpression",
			 "integer -> PostfixExpression",
			 "lparen -> PostfixExpression"]
		);

	}
	auto app = appender!string();
	formattedWrite(app,
		"Found a '%s' while looking for",
		this.lex.front
	);
	throw new ParseException(app.data,
		__FILE__, __LINE__,
		subRules,
		["identifier","float64","integer","lparen"]
	);

}

Expression, PostfixExpression, and PrimaryExpression are rules. One level of indentation gives the subrules. All subrules for one rule must the representable by a trie. The entries in the square brackets are the rule elements. Rule elements are either tokens or rules. Rules start with upper case letters token with lower case letters. If a rule element is followed by a # it is stored in the AST node. Rule elements that appear in multiple sub rules and are stored need to have the name.

The Lexer passed to the ctor of Parser needs to be an input range.

About Kaleidic Associates

We are a boutique consultancy that advises a small number of hedge fund clients. We are not accepting new clients currently, but if you are interested in working either remotely or locally in London or Hong Kong, and if you are a talented hacker with a moral compass who aspires to excellence then feel free to drop me a line: laeeth at kaleidic.io

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http://symmetryinvestments.com/about-us/