Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
6.6 Assessment of the GLC (“stack based”) parsers
6.6.1 Termination
(50) We have not found any recognition method that is guaranteed to terminate (i.e. has a finite search space)
on any input, even when the grammar has left recursion and empty categories. In fact, it is obvious that
we do not want to do this, since a context free grammar can have infinitely ambiguous strings.
6.6.2 Coverage
(51) The GLC recognition methods are designed for CFGs. Human languages have structures that are only
very inelegantly handled by CFGs, and structures that seem beyond the power of CFGs, as we mentioned
earlier (Savitch et al., 1987).
6.6.3 Ambiguity (local and global) vs. glcola(k) parsing
(52) Ambiguity is good.
If you know which Clinton I am talking about, then I do not need to say “William Jefferson Clinton.”
Doing so violates normal conventions about being brief and to-the-point in conversation (Grice, 1975),
and consequently calls for some special explanation (e.g. pomposity, or a desire to signal formality).
A full name is needed when one Clinton needs to be distinguished from another. For most of us
non-Clintons, in most contexts, using just “Clinton” is enough, even though the name is semantically
ambiguous.
(53) For the same reason, it is no surprise that standard Prolog uses the list constructor “.” both as a function
to build lists and as a predicate whose “proof” triggers loading a file. Some dialects of Prolog also use
“/” in some contexts to separate a predicate name from its arity, and in other contexts for division. This
kind of multiple use of an expression is harmless in context, and allows us to use shorter expressions.
(54) There is a price to pay in parsing, since structural ambiguities must be resolved. Some of these ambiguities
are resolved definitively by the structure of the sentence; other ambiguities persist throughout a
whole sentence and are resolved by discourse context. It is natural to assume that these various types
of ambiguity are resolved by similar mechanisms in human language understanding, but of course this
is an empirical question.
(55) Global ambiguity (unresolved by local structure) How much structural ambiguity do sentences of
human languages really have? 28 We can get a first impression of how serious the structural ambiguity
problem is by looking at simple artificial grammars for these constructions.
a. PP attachment in [ VP V D N PP1 PP2 ...]
Consider the grammar:
VP → VNPPP ∗
NP → DNPP ∗
A grammar like this cannot be directly expressed in standard context free form. It defines a context free
language, but it is equivalent to the following infinite grammar:
np → dn vp→ vnp
np → dnpp vp→ vnppp
np → d n pp pp vp → vnppppp
np → d n pp pp pp vp → vnppppppp
np → d n pp pp pp pp vp → vnppppppppp
np → d n pp pp pp pp pp vp → vnppppppppppp
… …
28 Classic discussions of this point appear in, Church and Patil (1982) and Langendoen, McDaniel, and Langsam (1989).
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