[FR] alternative, resumable, lazy, parser interface

[nsajko]

The only currently implemented parser interface takes an iterator of tokens, producing a (parse tree, error status) pair.

An alternative interface would take an incomplete parse tree (to be mutated) and a single token. Thus there would be a single call into the parser for each token with this interface.

The advantage is that the user might be able to extract useful information from the incomplete parse tree without having to process the entire token stream.