Generation: How to Say It

Basic Approaches

• Information in the "message" vs. information "forced" on the speaker by the grammar
• Grammar-driven (top-down) vs. lexically-driven (bottom-up) approaches
• Incremental vs. pre-specified approaches
• Speech errors as evidence of how human generation works

A Sketch of a Bottom-Up (Lexically-Driven), Breadth-First Generation Algorithm

• Start with feature-value matrices specifying the semantics of the objects.
• Unify all of these matrices against the CONTENT or CONTEXT values of the SYNSEMS of lexical signs.
• As in bottom-up parsing, until no more such unifications are possible
  • Unify instantiated signs with the daughters of instantiated phrasal signs or with phrasal signs in the grammar.
• If
  • All instantiated signs but one saturated one (S) are associated with daughters of other instantiated signs and
  • the PHON value of all instantiated signs is completely specified
  return the complete S structure
• Else fail.

An Example

• We start with semantic specifications of the relevant objects.

  1 [RELN like
     LIKER l1
     LIKED l2]
  
  2 [INDEX l1
     RESTR {[RELN doctor
             INST l1]}]
  
  3 [INDEX l2
     RESTR {[RELN librarian
             INST l2]}]
  

• Unifying each of these with lexical entries gives the following. (Actually different forms of like would unify with the verb specification if the subject noun hadn't yet been found.)

  4 [PHON ((? ? likes))
     SYNSEM [CAT [HEAD [VFORM fin]
                  SUBCAT ([CAT [HEAD noun SUBCAT ()]
                           CONTENT [INDEX l1 [PER 3rd NUM sing]]]
                          [CAT [HEAD noun SUBCAT ()]
                           CONTENT [INDEX l2]])]
             CONTENT [RELN like LIKER l1 LIKER l2]]]
  
  5 [PHON ((? ? doctor))
     SYNSEM [CAT [HEAD noun
                  SUBCAT ([CAT [HEAD det SUBCAT ()]])]
             CONTENT [INDEX l1 [PER 3rd NUM sing]
                      RESTR {[RELN doctor
                              INST l1]}]]]
  
  6 [PHON ((? ? librarian))
     SYNSEM [CAT [HEAD noun
                  SUBCAT ([CAT [HEAD det SUBCAT ()]])]
             CONTENT [INDEX l2 [PER 3rd NUM sing]
                      RESTR {[RELN librarian
                              INST l2]}]]]
  

• Skipping lots of details, we unify 4, 5, and 6 with the HEAD daughters of grammar rules, giving the following:

  7 [PHON ((? ? likes the librarian))
     SYNSEM [CAT [HEAD [VFORM fin]
                  SUBCAT ([CAT [HEAD noun SUBCAT ()]
                           CONTENT [INDEX l1 [PER 3rd NUM sing]]])]
             CONTENT [RELN like LIKER l1 LIKER l2]]
     DTRS ...]
  
  8 [PHON ((? ? the doctor))
     SYNSEM [CAT [HEAD noun SUBCAT ()]
                  SUBCAT ([CAT [HEAD det SUBCAT ()]])]
             CONTENT [INDEX l1 [PER 3rd NUM sing]
                      RESTR {[RELN doctor
                              INST l1]}]]
     DTRS [HEAD-DTR ...
           COMP-DTR [SYNSEM [CAT [HEAD det SUBCAT ()]]]]]
  

• Unifying the VP and then the remaining NP with the S rule, we get the following:

  9 [PHON ((0 5 the doctor likes the librarian))
     SYNSEM [CAT [HEAD [VFORM fin] SUBCAT ()]
             CONTENT [RELN like LIKER l1 LIKER l2]]
     DTRS ...]
  

Issues in the Generation of Definite Noun Phrases (Appelt)

• Concept activation: describing (NPs), pointing
• Reasoning about mutual knowledge; copresence heuristics
• Providing enough information in the description so that the hearer can "find" the referent of the NP, but not so much that the hearer is bored or insulted
• The planning of a sentence with 3 NPs
  Remove the pump with the wrench in the toolboox.
  • S reasons that removing the pump requires unfastening. Precondition for H doing this is that H know what the right tool is. S plans to inform H that the tool is a particular wrench that H knows of.
  • S decides he does not need to mention that the pump is to be removed from the platform because it is mutually believed that the platform is the only object to which the pump is attached.
  • The pump can be referred to simply as the pump because there is only one object that is mutually believed to be a pump.
  • S realizes that the informing action (which wrench to use) can be subsumed by the request because the verb remove can take an instrument NP.
  • In order to perform the removing, H must have the wrench. For this to happen, she must know where it is, go there and get it. So S plans to inform H where the wrench is. Again subsumption is possible; the location is added to the description for the NP referring to the wrench.

[L645 Syllabus]