Return value to ignore output for the rule

[Phrogz]

In some grammars there are separators that are important to the grammar but uninteresting in the output from the parse tree. Whitespace is the most obvious example.

It'd be cool if there was a way, from within a rule action, to indicate that the output of the rule should not be included in the parse tree.

Since null is already taken to indicate a match/predicate failure, and false is conceivable as a desired return value, perhaps a return value of undefined could be used to cull the rule's output.

[dmajda]

I want to concentrate on usability issues in version 0.7. I don't like the undefined idea much, but I agree that this is a problem that should be addressed in some way.

[dmajda]

I'll look into this sometime after 1.0.0.

[dmajda]

Deciding whether something belongs into the parse tree based on action return value seems to me as a wrong way to ignore some parts of the input. I won't implement this.

See also #11, which deals with a related issue.