Bratko, Chapter 17: Language Processing with Grammar Rules

See 'grammar?.pl' files.

17.1 Grammar Rules in Prolog
- Backus-Naur Form (BNF)
  - nonterminals enclosed in <>, otherwise terminal
  - e.g., <s> ::= a b       OR     <s> ::= a b | a <s> b
          <s> ::= a <s> b
    - generates sentences of the form a^n b^n
- Definite Clause Grammar (DCG)
  - nonterminals are Prolog atoms, terminals are one-element lists
  - e.g., s --> [a], [b].
          s --> [a], s, [b].
- can query sentences (difference-list representation)
  - e.g., ?- s([a,a,b,b],[])  or  s([a,a,b,b,c],[c])  or  s(X,[])
- internally
  - s(A,B) :- 'C'(A,a,C), 'C'(C,b,B).
    s(A,B) :- 'C'(A,a,C), s(C,D), 'C'(D,b,B).
  - where 'C'([H|T],H,T).
  - for s --> [a], [b] | [a], s, [b].
    s(A,B) :- ( 'C'(A,a,C), 'C'(C,b,B) ;
                'C'(A,a,D), s(D,E), 'C'(E,b,B) ).
- simple English grammar [grammar1.pl]
- augmenting grammar
  - problem with number "the cats hates the mouse"
  - add Number argument
  - e.g., s(Number) --> np(Number), vp(Number).
          s(A,B,C) :- np(A,B,D), vp(A,D,C).
  - see [grammar1.pl]

17.2 Handling Meaning
- parse trees
  - "the cat scares the mice" [Fig17.2, p440]
  - sample grammar [grammar1.pl]
- extracting meaning from parse trees
  - augment grammar to construct parse tree
  - decompose parse tree to compute result
  - e.g., arithmetic expression parsing
    - see expr in [grammar2.pl]
- adding semantics to DCGs
  - enclose Prolog statments in curly braces
  - place anywhere in grammar
  - e.g., expr(R) --> expr(R1), [plus], expr(R2), {R is R1 + R2}.
  - see expr1 [grammar2.pl]

17.3 Defining the Meaning of Natural Language
- meaning in logic
  - "the wumpus is at 1 1" -> location(wumpus,[1,1])
                           -> wumpus_location(1,1)
  - simple grammar in [grammar3.pl]
  - attach meaning via augmentation
    - e.g., transverb(Actor,Object,ate(Actor,Object)) --> [ate].
            prep(What,Where,at(What,Where)) --> [at].
    - see wumpus grammar in [grammar3.pl]
- quantifiers
  - existential ("a" "an" ["the"])
    - "a wumpus" --> exists(X,wumpus(X))
    - "a wumpus is at 1 1" --> exists(X,wumpus(X) and location(X,[1,1]))
  - universal ("every")
    - "every wumpus is dead" --> forall(X,wumpus(X) => dead(X))
  - see determiner in [Fig17.6, p455]
- relative clauses
  - "the wumpus THAT MOVES ate the agent"
  - grammar syntax
    - np --> det, noun, rel_clause.
      rel_clause --> [that], vp | [].
  - exists(X,wumpus(X) and moves(X) and ate(X,agent))
  - semantics
    - see rel_clause [Fig17.6, p455]