Lascarides, A. and A. Copestake [1999] Default Representation in Constraint-based Frameworks, Computational Linguistics, 25(1), pp55-105, MIT Press.
Default unification has been used in several linguistic applications. Most of them have utilised defaults at a meta-level, as part of an extended description language. We propose that allowing default unification to be a fully integrated part of a typed feature structure system requires default unification to be a binary, order independent function, so that it acquires the perspicuity and declarativity familiar from normal unification-based frameworks. Furthermore, in order to respect the behaviour of defaults, default unification should allow default reentrancies and values on more general types to be overridden by conflicting default information on more specific types. We define what we believe is the first definition of default unification to satisfy these criteria, and argue that it can improve the declarativity of existing uses of default inheritance within the lexicon (because it doesn't require one to pre-specify the order in which information is accumulated) without loss of expressivity (because it validates the overriding of general defaults by conflicting more specific ones). We also argue that some linguistic phenomena suggest that there are conventional default generalisations that persist as default beyond the lexicon, and are potentially overridden by more open-ended pragmatic reasoning. We demonstrate that our version of default unification can be used to model this. Finally, we discuss the complexity of the operation, and argue that the overhead of using it in practical systems need not be large.
@article{lascarides:copestake:1999,
author = {Alex Lascarides and Ann Copestake},
year = {1999},
title = {Default Representation in Constraint-based Frameworks},
journal = {Computational Linguistics},
volume = {128},
number = {1},
pages = {55--105}
}