PEG matching/parsing lib in progress

[spir]

On Mon, 8 Nov 2010 11:48:20 +0100
spir <denis.spir at gmail.com> wrote:
[...]

A third point to solve is the one of recursive patterns (self-, mutual, circular, recursion). The issue is indeed that patterns must be defined in order, which is not possible in case of recursion. A side issue is that patterns are identified by ref (pointer): we must keep this ref unchanged, a point that eliminates some possible solutions.

What I usually do is to define a kind of fake Recursion pattern type, that delegates matching to the actual one, recorded as attribute. Recursion is used to predefine (one of) the recursive pattern(s); then the rest of the grammar can be defined normally; finally, the recursive one gets its actual pattern. Example:
	operation = new Recursion();			// placeholder
	group = new Tuple(L_PAREN, operation, R_PAREN);	// ( operation )
	operand = new Choice(group, number, variable);	// group|number|variable
	operation.define(new Tuple(operand, operator, operand)); // operand operator operand
(We can use operator overloading to replace of the call to define().)

What do you think of this solution? (In addition to matching indirection, it raises a few side-issues in the implementation of some features, but I know how to solve them.)

Can you find other solutions?

Denis
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vit esse estrany ☣

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