Martin Kay (personal communication) has developed a parsing algorithm that seems to be the parsing correlate to the generation algorithm presented here. Its existence might point the way towards a uniform architecture.

Pereira and Warren [15] point out that Earley deduction is not restricted to a left-to-right expansion of goals, but this suggestion was not followed up with a specific algorithm addressing the problems discussed here.

We use the term ``analysis tree'' rather than the more familiar ``parse tree'' to make clear that the source of the tree is not necessarily a parsing process; rather the tree serves only to codify a particular analysis of the structure of the string.

In case there are two right-hand-side elements that are semantically identical to the left-hand side, there is some freedom in choosing the semantic head, although the choice is not without ramifications. For instance, in some analyses of NP structure, a rule such as

np/NP -> det/NP, nbar/NP.

is postulated. In general, a chain rule is used bottom-up from its semantic head and top-down on the non-semantic-head siblings. Thus, if a non-semantic-head subconstituent has the same semantics as the left-hand-side, a recursive top-down generation with the same semantics will be invoked. In theory, this can lead to nontermination, unless syntactic factors eliminate the recursion, as they would in the rule above regardless of which element is chosen as semantic head. In a rule for relative clause introduction such as the following (in highly abbreviated form)

nbar/N -> nbar/N, sbar/N.

we can (and must) choose the nominal as semantic head to effect termination. However, there are other problematic cases, such as verb-movement analyses of verb-second languages, whose detailed discussion is beyond the scope of this paper.

Further details of the use of shuffle in scoping are given by Pereira and Shieber [14].

Gertjan van Noord
Thu Nov 24 18:39:44 MET 1994