It appears that it is possible to differentiate a failed match vs. an exhausted input string. If the recognizer specifically rejects the string as unable to match, it will throw an error. If the input string is exhausted, but has conformed to the grammar requirements up till that point, the recognizer instead pauses the parse. This allows for the opportunity to add to the input string and resume parsing.

However, if the full grammar has not been matched and the recognizer is paused, the 'value' method will result in an undef return. An undef return from the 'value' method means there was no parse result, or a failure.

This is pretty much exactly what Amon was telling me above but I didn't quite put things together until I understood that you could pause the parsing, add to your input, and resume parsing.

Short story is don't rely on an error from the recognizer to determine whether the grammar was matched. An error clearly indicates a failed match, but no error can mean 'input string was exhausted, but does not yet match fully' or 'grammar was matched successfully'. To differentiate these, ensure the value returned from the 'value' method is defined. Thanks to the Marpa author Jeffrey for the insight on this point https://groups.google.com/forum/#!topic/marpa-parser/fZzhxdBDbGk