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Problems with Bottom-up Generators
The bottom-up Earley-deduction generator does not fall prey to these problems of nontermination in the face of recursion because lexical information is available immediately. However, several important frailties of the Earley generation method were noted, even in the earlier work.
For efficiency, generation using this Earley deduction method requires an incomplete search strategy, filtering the search space using semantic information. The semantic filter makes generation from a logical form computationally feasible, but preserves completeness of the generation process only in the case of semantically monotonic grammars, those grammars in which the semantic component of each right-hand-side nonterminal subsumes some portion of the semantic component of the left-hand-side. The semantic monotonicity constraint itself is quite restrictive. Although it is intuitively plausible that the semantic content of subconstituents ought to play a role in the semantics of their combination-this is just a kind of compositionality claim-there are certain cases in which reasonable linguistic analyses might violate this intuition. In general, these cases arise when a particular lexical item is stipulated to occur, the stipulation being either lexical (as in the case of particles or idioms) or grammatical (as in the case of expletive expressions).
Second, the left-to-right scheduling of Earley parsing,
geared as it is toward the structure of the string rather than that of
its meaning, is inherently more appropriate for parsing than
generation.
This manifests itself in an overly high degree of nondeterminism in the
generation process. For instance, various nondeterministic
possibilities for generating a noun phrase (using different cases, say)
might be entertained merely because the NP occurs before the verb which
would more fully specify, and therefore limit, the options. This
nondeterminism has been observed in practice.
Gertjan van Noord
Thu Nov 24 18:39:44 MET 1994