If the parser fails
Ralph Weischedel, John E. Black · 1980
The unforgiving nature of natural language components... This paper presents heuristics for responding to inputs that cannot be parsed even using the techniques referenced in the last paragraph for relaxing syntactic and semantic constraints. The paper concentrates on the results of an experiment testing our heuristics. We assume only that the parser is written in the ATN formalism. In this method, the parser writer must assign a sequence of condition-action pairs for each state of the ATN. If no parse can be found, the condition-action pairs of the last state of the path that progressed furthest through the input string are used to generate a message about the nature of the problem, the interpretation being followed, and what was expected next. The conditions may refer to any ATN register, the input string, or any computation upon them (even semantic ones). The actions can include any computation (even restarting the parse after altering the unparsed portion} and can generate any responses to the user. These heuristics were tested on a granmar which uses only syntactic information. We constructed test data such that one sentence would block at each of the 39 states of the ATN where blockage could occur. In only 3 of the 39 cases did the parser continue beyond the point that was the true source of the parse failing. From the tests, it was clear that the heuristics frequently pinpointed the exact cause of the block. However, the response...