Typed Constituents: The FSET Construct

A natural extension of the notion of typed features is to type constituents: typing a feature restricts its possible values; typing a constituent restricts the possible features it can have.

          
Type declarations (in the grammar):
(determiner ((fset (definite distance demonstrative possessive))))


Input FD describing a determiner:
(determiner ((definite yes)
	     (distance far)
	     (demonstrative no)
	     (possessive no)))
          
          

The fset feature specifies that only the four features listed can appear under the constituent determiner. This statement declares what the grammar knows about determiners. Fset expresses a completeness constraint as defined in LFGs [#!Kaplan&Bresnan!#]; it says what the grammar needs in order to consider a constituent complete. Without this construct, FDs can only express partial information. The exact syntax of fset is given in Section .  

Note that expressing such a constraint (a limit on the arity of a constituent) is impossible in the traditional FU formalism. It would be the equivalent of putting a NONE in the attribute field of a pair as in NONE:NONE.

          


Without  FSET:
((cat clause)
 (alt (((process-type action)
        (inherent-roles ((carrier  NONE)
                         (attribute  NONE)
                         (processor  NONE)
                         (phenomenon  NONE))))
       ((process-type attributive)
        (inherent-roles ((agent  NONE)
                         (medium  NONE)
                         (benef  NONE)
                         (processor  NONE)
                         (phenomenon  NONE))))
       ((process-type mental)
        (inherent-roles ((agent  NONE)
                         (medium  NONE)
                         (benef  NONE)
                         (carrier  NONE)
                         (attribute  NONE)))))))


With  FSET:
((cat clause)
 (alt (((process-type action)
        (inherent-roles (( FSET (agent medium benef)))))
       ((process-type attributive)
        (inherent-roles (( FSET (carrier attribute)))))
       ((process-type mental)
        (inherent-roles (( FSET (processor phenomenon))))))))


          
          

In general, the set of features that are allowed under a certain constituent depends on the value of another feature. Figure illustrates the problem. The fragment of grammar shown defines what inherent roles are defined for different types of processes (it follows the classification provided in [#!Halliday85!#]). We also want to enforce the constraint that the set of inherent roles is ``closed'': for an action, the inherent roles are agent, medium and benef and nothing else. This constraint cannot be expressed by the standard FUG formalism. fset makes it possible.

Note also that the set of possible features under the constituent inherent-roles depends on the value of the feature process-type. The first part of the Figure above shows how the constraint can be implemented without fset: we need to exclude all the roles that are not defined for the process-type.¹¹ Note that the problems are very similar to those encountered on the pronoun system: explosion of none branches, interdependent branches, long and inefficient grammar.

The fset (feature set) attribute solves this problem: fset specifies the complete set of legal features at a given level of an FD. fset adds constraints on the definition of the domain of admissible paths D of a grammar. The syntax is the same as cset. Note that all the features specified in fset do not need to appear in an FD: only a subset of those can appear. For example, to define the class of middle verbs (e.g., ``to shine'' which accepts only a medium as inherent role and no agent), the following statement can be unified with the fragment of grammar shown in the previous figure:

          
((verb ((lex `shine')))
 (process-type action)
 (voice-class middle)
 (inherent-roles (( FSET (medium)))))
          
          

The feature ( FSET (medium)) can be unified with ( FSET (agent medium benef)) and the result is ( FSET (medium)).

Typing constituents is necessary to implement the theoretical claim of LFG that the number of syntactic functions is limited. It also has practical advantages. An important advantage is good documentation of the grammar. Typing also allows checking the validity of inputs as defined by the type declarations.