Two of the functions of an NP are to refer (identify a particular entity) and to inform (provide new information about an entity). In most cases, a given NP may serve only one of these functions. However, in some cases, the writer/speaker may choose to conflate the functions, providing an NP which not only refers but also provides new information about the referent. For instance, this delicious apple indicates not only which apple the speaker is referring to, but also provides information as to the speaker's appreciation of the apple.
Most of the work on NP planning has considered only the referring function of the NP (e.g., Dale 1988, 1989; Reiter 1990; Reiter & Dale 1992; Horacek 1995). Appelt (e.g., Appelt 1985; Appelt & Kronfeld 1987) has considered the question of integrating referring and informing, although rather briefly, and without much detail. This paper will extend upon his discussion, and describe its role in ILEX, a text generation system which delivers descriptions of entities on-line from an underlying knowledge-base (see Mellish et al. 1998). ILEX is at present generating descriptions in the museum domain, in particular, that of 20th Century jewellery.
Our focus on this topic has grown out of the need to integrate two strands of research within ILEX. One strand involves the work on anaphora by Janet Hitzeman. She implemented a module to construct contextually appropriate referring expressions within ILEX, based on Centering Theory (Grosz et al. 1986). See Hitzeman et al. 1997.
The second strand involves the aggregation module (implemented by Hua Cheng, see Cheng et al. 1997). The task of this module is to re-package discrete informational units into single complex sentences. She is presently exploring the aggregation of information into the NP, for instance this gold and silver ring, designed by King.
These two functions, the referring and the informing, interfere with each other, to the extent that each wishes to control the construction of the NP form. These tasks thus need to pay regard to each other, and this paper, and the implementation it describes, are an attempt to answer this need.
Appelt's approach seems to be to build an NP for referring, then either modify the elements (e.g., substitution of the head noun) or fill unused structural slots with non-referring information. However, we have found that the two tasks of referring and informing can be more highly integrated, with each decision within the construction of the NP taking into account the needs of both tasks, rather than satisfying the referring function first, then looking to the informing function. In other words, we follow an integrated rather than pipeline approach.
Section 2 will describe how information is represented in ILEX. Section 3 describes the interface between the text-planner and the NP- planner, the input specification for the NP-planner. Section 4 discusses the syntactic structure of the NP, and which syntactic positions allow informing without interfering with referring. Section 5 details the referring expression generator which integrates referring and informing goals. An example of the generation process is given in section 6 and section 7 summarises.