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Subsections


Language technological motivations

In order to build practical NLP systems, the use of reversible grammars can be motivated, both on methodological ground (as a means to obtain `better' systems) and practical grounds (as a means to obtain systems in a more efficient way).

Methodological.

An important motivation for reversible grammars in NLP is of a methodological nature. If we are to use grammars both for parsing and generation we are forced to write grammars declaratively. This in turn implies that a more abstract analysis of the linguistic facts is necessary in the general case. If we are to write a declarative grammar which is used only for, say, parsing, it is quite easy to `cheat' and `adapt' the grammar to the parsing algorithm that is being used. In a reversible grammar this is much harder because at any moment there are two algorithms for which the grammar must be applicable.

The claim is that the use of reversible grammars will eventually lead to better grammars. For example, a grammar that is written for parsing will typically over-generate quite a lot; i.e. it will assign logical forms to sentences that are in fact ungrammatical. Not only is such a state of affairs undesirable if we are interested in describing the relation between form and meaning, it can also be argued that over-generation of parsing is a problem, even if we are only interested in parsing well-formed utterances, because over-generation typically leads to `false ambiguities'.

An example may clarify this point (this example was actually encountered in the development of a working system). Consider a grammar for English that is intended to handle auxiliaries. Suppose that the English auxiliaries are analyzed as verbs that take an obligatory VP-complement. Moreover each auxiliary may restrict the vform (participle, infinite) of this complement. This allows the analysis of sentences such as

\begin{exam}
Graham {\em will have been traveling} with his aunt
\end{exam}
However, the possible order of English auxiliaries (eg. `have' should precede `be') is not accounted for and the analysis sketched above will for example allow sentences such as

\begin{exam}
*Graham {\em will be having traveled} with his aunt
\end{exam}
In the case of a reversible grammar such constructions should clearly not be generated, hence the analysis will be changed accordingly. However, even if the grammar is only used for parsing, this analysis runs into problems because it will assign two meanings to the sentence:

\begin{exam}
Graham is having grilled meat
\end{exam}
The meanings that are assigned, roughly correspond to the sentences:

\begin{exams}
\item Graham ordered grilled meat
\item Graham has been grilling the meat
\end{exams}

where only the first reading is acceptable. Thus, over-generation is not acceptable, even for grammars which are used only for parsing, because over-generation typically implies that `false ambiguities' are produced.

In some cases, the over-generation may also lead to an explosion of local possibilities during parsing. If the grammar is more constrained, then this may sometimes be good from an efficiency point of view, because in that case there are less local ambiguities the parser has to keep track of.1.1

Thus, a reversible architecture may be a useful methodology to obtain a good parsing system.

Similarly, a grammar that is built for generation will usually under-generate; i.e. it will only generate a canonical sentence for a given logical form, even if there are several possibilities. Again, from a theoretical perspective this is clearly undesirable. It can be argued that a reversible architecture also leads to a better generation system. It has often been argued that, in particular situations, a generation system should produce an un-ambiguous utterance. In other situations, however, ambiguous utterances are harmless because the hearer can easily disambiguate the utterance. In [62] we propose a model of language production in which a generator instructs its grammatical component whether or not it should check for ambiguity of its proposed utterance. The grammatical component, quite independently, computes an un-ambiguous utterance if so desired. For this model to be possible at all, it must be the case that the grammatical component has at its disposal several utterances for a given semantic structure in order to find in a given situation the most appropriate one. Note that `ambiguity' might be only one of several parameters that may or may not be instantiated in a given situation. Summarizing this point, the claim is that under-generation is undesirable from the point of view of extendability.

Consistency.

Consider an NLP system which is used both to convey messages to a user, and to understand the requests of the user. Necessarily, the sentences the system is able to produce and to understand are somewhat limited, given the state of the art. This may not be problematic for a user, as she might adapt herself to this restriction. However, a user will invariably assume that she can use the sort of sentences the system produces itself. That is, a reasonable constraint for such a system is that the sentences it produces is a subset of the sentences it is able to understand. In a reversible grammar the problem to check that the parser and generator are consistent in this respect (i.e. that the system should be able to understand those types of sentences, which it produces itself) is solved automatically; hence no special consistency checking device needs to be considered.

Practical.

From a practical point of view it may be argued that it is cheaper to construct one reversible grammar than two separate grammars. The same argument can then be applied to the costs of maintaining the grammars. These two arguments of course extend to the lexical entries in the grammar: in a reversible architecture only one lexicon needs to be built and maintained, although clearly non-reversible grammars may share their lexicon too.

Furthermore, a reversible grammar provides grammar writers with a very effective debugging tool. To check whether the grammar does accept ungrammatical sentences it is possible to use the generator to see whether such ungrammatical sentences are being produced. Clearly, this technique cannot be used to ensure that a grammar does not produce ungrammatical sentences, however in practice such a tool turns out to be quite useful, as many errors in the grammar are detected this way.


next up previous contents
Next: Psychological motivation Up: Motivation for reversible grammars Previous: Linguistic motivation
Noord G.J.M. van
1998-09-30