Categorial grammar
Categorial grammar is a term used for a family of formalisms in natural language syntax motivated by the principle of compositionality and organized according to the view that syntactic constituents should generally combine as functions or according to a function-argument relationship. Most versions of categorial grammar analyze sentence structure in terms of constituencies and are therefore phrase structure grammars.
Basics
A categorial grammar consists of two parts: a lexicon, which assigns a set of types to each basic symbol, and some type inference rules, which determine how the type of a string of symbols follows from the types of the constituent symbols. It has the advantage that the type inference rules can be fixed once and for all, so that the specification of a particular language grammar is entirely determined by the lexicon.A categorial grammar shares some features with the simply typed lambda calculus.
Whereas the lambda calculus has only one function type,
a categorial grammar typically has two function types, one type which is applied on the left,
and one on the right. For example, a simple categorial grammar might have two function types and.
The first,, is the type of a phrase that results in a phrase of type
when followed by a phrase of type.
The second,, is the type of a phrase that results
in a phrase of type when preceded by a phrase of type
.
The notation is based upon algebra. A fraction when multiplied by its denominator yields its numerator. As concatenation is not commutative, it makes a difference whether the denominator occurs to the left or right. The concatenation must be on the same side as the denominator for it to cancel out.
The first and simplest kind of categorial grammar is called a basic categorial grammar, or sometimes an AB-grammar.
Given a set of primitive types, let
be the set of types constructed from primitive types. In the basic case, this is the least set such that
and if
then.
Think of these as purely formal expressions freely generated from the primitive types; any semantics will be added later. Some authors assume a fixed infinite set of primitive types used by all grammars, but by making the primitive types part of the grammar, the whole construction is kept finite.
A basic categorial grammar is a tuple
where is a finite set of symbols,
is a finite set of primitive types, and.
The relation is the lexicon, which relates types to symbols.
Since the lexicon is finite, it can be specified by listing a set of pairs like.
Such a grammar for English might have three basic types, assigning count nouns the type, complete noun phrases the type
, and sentences the type.
Then an adjective could have the type, because if it is followed by a noun then the whole phrase is a noun.
Similarly, a determiner has the type,
because it forms a complete noun phrase when followed by a noun.
Intransitive verbs have the type, and transitive verbs the type.
Then a string of words is a sentence if it has overall type.
For example, take the string "the bad boy made that mess". Now "the" and "that" are determiners, "boy" and "mess" are nouns, "bad" is an adjective, and "made" is a transitive verb, so the lexicon is
and the sequence of types in the string is
now find functions and appropriate arguments and reduce them according to the two inference rules
and
The fact that the result is means that the string is a sentence, while the sequence of reductions shows that it must be parsed as.
Categorial grammars of this form are equivalent in generative capacity to context-free grammars and are thus often considered inadequate for theories of natural language syntax. Unlike CFGs, categorial grammars are lexicalized, meaning that only a small number of rules are employed, and all other syntactic phenomena derive from the lexical entries of specific words.
Another appealing aspect of categorial grammars is that it is often easy to assign them a compositional semantics, by first assigning interpretation types to all the basic categories, and then associating all the derived categories with appropriate function types. The interpretation of any constituent is then simply the value of a function at an argument. With some modifications to handle intensionality and quantification, this approach can be used to cover a wide variety of semantic phenomena.
Lambek calculus
A Lambek grammar is an elaboration of this idea that has aconcatenation operator for types, and several other inference rules.
Mati Pentus has shown that these still have the generative capacity of
context-free grammars.
For the Lambek calculus, there is a type concatenation
operator, so
that
and if
then.
The Lambek calculus consists of several deduction rules, which specify
how type inclusion assertions can be derived. In the following
rules, upper case roman letters stand for types, upper case Greek
letters stand for sequences of types. A sequent of the form
can be read: a string is of type if it consists of the concatenation
of strings of each of the types in. If a type is
interpreted as a set of strings, then the
may be interpreted as,
that is, "includes as a subset".
A horizontal line means that the inclusion above the line
implies the one below the line.
The process is begun by the Axiom rule, which has no antecedents and
just says that any type includes itself.
The Cut rule says that inclusions can be composed.
The other rules come in pairs, one pair for each type construction
operator, each pair consisting of one rule for the operator in the
target, one in the source, of the arrow.
The name of a rule consists of the operator and an arrow, with the
operator on the side of the arrow on which it occurs in the conclusion.
For an example, here is a derivation of "type raising", which says that
. The names of rules and the substitutions used are to the right.
Relation to context-free grammars
Recall that a context-free grammar is a 4-tuple:where
1. is a finite set of non-terminals or variables.
2. is a finite set of terminal symbols.
3. is a finite set of production rules, that is, a finite relation
4. is the start variable.
From the point of view of categorial grammars, a context-free grammar
can be seen as a calculus with a set of special purpose axioms for
each language, but with no type construction operators and no
inference rules except Cut.
Specifically, given a context-free grammar as above, define a categorial
grammar
where ,
and .
Let there be an axiom
for every symbol
an axiom
for every production rule,
a lexicon entry for every terminal symbol
and Cut for the only rule.
This categorial grammar generates the same language as the given CFG.
Of course, this is not a basic categorial grammar, since it has
special axioms that depend upon the language; i.e. it is not lexicalized.
Also, it makes no use at all of non-primitive types.
To show that any context-free language can be generated by
a basic categorial grammar, recall that
any context-free language can be generated by a context-free grammar
in Greibach normal form.
The grammar is in Greibach normal form if every production rule is
of the form
where capital letters are variables,,
and,
that is, the right side of the production is a single terminal symbol
followed by zero or more variables.
Now given a CFG in Greibach normal form,
define a basic categorial grammar with a primitive type
for each non-terminal variable
and with an entry in the lexicon
for each production rule
It is fairly easy to see that this basic categorial grammar
generates the same language as the original CFG.
Note that the lexicon of this grammar will generally
assign multiple types to each symbol.
The same construction works for Lambek grammars, since
they are an extension of basic categorial grammars.
It is necessary to verify that the extra inference rules
do not change the generated language. This can be done
and shows that every context-free language is generated
by some Lambek grammar.
To show the converse, that every language generated by a
Lambek grammar is context-free, is much more difficult.
It was an open problem for nearly thirty years, from
the early 1960s until about 1991 when it was proven
by Pentus.
The basic idea is, given a Lambek grammar,
construct a context-free grammar
with the same set of terminal symbols, the
same start symbol, with variables some types
and with a production rule
for each entry
in the lexicon,
and production rules
for certain sequents
that are derivable in the Lambek calculus.
Of course, there are infinitely many types
and infinitely many derivable sequents, so in
order to make a finite grammar it is necessary
put a bound on the size of the types and sequents
that are needed. The heart of Pentus's proof
is to show that there is such a finite bound.
Notation
The notation in this field is not standardized. The notations used informal language theory, logic, category theory, and linguistics, conflict
with each other. In logic, arrows point to the more general from the more particular,
that is, to the conclusion from the hypotheses. In this article,
this convention is followed, i.e. the target of the arrow is the more general type.
In logic, arrows usually point left to right. In this article this convention is
reversed for consistency with the notation of context-free grammars, where the
single non-terminal symbol is always on the left. We use the symbol
in a production rule as in Backus-Naur form. Some authors use an arrow, which
unfortunately may point in either direction, depending on whether the grammar is
thought of as generating or recognizing the language.
Some authors on categorial grammars write instead of
. The convention used here follows Lambek and algebra.
Historical notes
The basic ideas of categorial grammar date from work by Kazimierz Ajdukiewicz and Yehoshua Bar-Hillel. In 1958, Joachim Lambek introduced a syntactic calculus that formalized the function type constructors along with various rules for the combination of functions. This calculus is a forerunner oflinear logic in that it is a substructural logic. Montague grammar uses an ad hoc syntactic system for English that is based on the principles of categorial grammar. Although Montague's work is sometimes regarded as syntactically uninteresting, it helped to bolster interest in categorial grammar by associating it with a highly successful formal treatment of natural language semantics. More recent work in categorial grammar has focused on the improvement of syntactic coverage. One formalism which has received considerable attention in recent years is Steedman and Szabolcsi's combinatory categorial grammar which builds on combinatory logic invented by Moses Schönfinkel and Haskell Curry.
There are a number of related formalisms of this kind in linguistics, such as type logical grammar and abstract categorial grammar.
Some definitions
;Derivation: A derivation is a binary tree that encodes a proof.;Parse tree: A parse tree displays a derivation, showing the syntactic structure of a sentence.
;Functor and argument: In a right function application, the node of the type A\B is called the functor, and the node of the type A is called an argument.
;Functor–argument structure