The diagnosis and cure of garden paths

Abstract

We propose that, for the human parser, recovery from garden paths consists in repairing the structure built so far, rather than reparsing the input. The difficulty of a repair is attributable not to the cost of effecting the structural alterations but to the cost of deducing which alterations are needed. The parser must diagnose its error in order to correct it. The error is signaled by an input word that is incompatible with the current structure; this is the symptom from which the diagnosis must be made. If the error is transparently clear from the nature of the symptom, recovery is easy; but sometimes the necessary reasoning is obscure, and then the diagnosis is unsuccessful and the garden path persists. Unlike other repair models, the diagnosis model needs no special mechanism for revising garden path analyses. The garden path recovery device is the same machine as the first-pass parser, merely set into emergency mode. When faced with a breakdown the parser does not stop its normal activities and enter a new mode of reasoning to detect what went wrong. It simply continues to parse, attaching the problematic input item in the least ungrammatical way it can, despite the conflict with previously built structure. This conflict is productive; it provokes adjustments to the existing structure. In successful cases, one adjustment leads to another until a stable state is reached, at which point the original error will have been eliminated. Examples suggest that the parser gives more weight to syntatctic than to pragmatic acceptability; only a syntactic clash between the input and the existing structure sets the adjustment process in motion.