Eckhard Bick - VISL

3.7.3 Local vs. global rules: Constraint typology
In this section I shall as far as possible detach myself the CG grammar critic from
myself the CG rule writer, inspecting and quantifying the types of rule architecture used
in the system, and trying to map and interpret possible system immanent structural
regularities or tendencies. The point of this exercise is:
(a) to provide other CG-grammar writers with some standard for comparison and
CG novices with some guidelines as to how a CG may be expected to develop, what
grammar size and complexity to expect, which pitfalls to avoid etc., and
(b) to facilitate cross-system comparison, like when the author of a probabilistic
HMM tagger/parser wants to decide on the possibility to match or "emulate" a CG rule
set (a problem the relevance of which I have personally been confronted with when
discussing with NLP-researchers outside the CG camp).
What a CG grammar architecture looks like, may, of course, depends not only on
general linguistic and analytic factors, but also on the individual grammarian’s approach
to grammatical problem solving in general, and the technical limitations imposed by the
few presently available CG rule compilers in particular, - and with very few Constraint
Grammars around (and even fewer published), real proof of any structural universality
claim must therefore await future research. Still, even regularities found within one
system (and with one type of compiler), may help other researchers understand why CG
rules look the way they do, and how best to learn from their not so bad performance.
One of the ways to assess a given Constraint Grammar in a typological way is to
quantify rule types with regard to their contextual scope and complexity, as suggested
by Anttila in his discussion of the Helsinki group's English CG (Karlsson et. al., 1995,
p.352). Contextual scope is what ordinarily distinguishes probabilistic grammars
(narrow scope) from generative grammars (wide scope). Within Constraint Grammar,
bounded context conditions, especially of low order (i.e. close to the target), are natural
narrow scope tools, whereas unbounded context conditions are characteristic of a wide
scope approach. Thus, a CG rule set can be typologically located between probabilistic
and generative grammars, mimicking the first for part-of-speech discrimination, and the
second for syntactic parsing.
In table (1), a rule count is given for rules with unbounded context conditions
(henceforth "global" rules) or without ("local" rules 119 ), for all three operations
supported by the cg2-compiler. The columns containing numbers for non-heuristic rules
119 The concept of "local" rules is not to be confused with that of "local disambiguation" - the first term is used to describe
rules without unbounded context conditions (i.e. rules where all contexts conditions are bounded), while the second concerns
word-internal disambiguation (the minimal derivational complexity rule, or "Karlsson's law")
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