12.6.4 Canned Text Output

There is nothing in the definition of the grammar that requires English input and the parse tree as output. A query of grammar rule with the meaning of the sentence bound and a free variable representing the sentence can produce a sentence that matches the meaning.

One such use of grammar rules is to provide canned text output from logic terms; the output is a sentence in English that matches the logic term. This is useful for producing English versions of atoms, rules, and questions that a user - who may not know the intended interpretation of the symbols, or even the syntax of the formal language - can easily understand.

% trans(Term,T0,T1) is true if Term translates into the words contained in the difference list T0-T1.

     trans(session(S),T1,[of | T3])∧
     trans(course(C),T3,[is,scheduled,at | T5])∧
     trans(time(L),T5,[in | T7])∧
trans(session(w11),[the,winter,2011,session | T],T).
trans(course(cs422),[the,advanced,artificial,intelligence,course | T],T).
trans(time(clock(0,M)),[12,:,M,am | T],T).
trans(time(clock(H,M)),[H,:,M,am | T],T)←
trans(time(clock(12,M)),[12,:,M,pm | T],T).
trans(time(clock(H,M)),[H1,:,M,pm | T],T)←
     H1 is H-12.
trans(room(above(R)),[the,room,above | T1],T2)←
trans(room(csci333),[the,computer,science,department,office | T],T).
Figure 12.8: Grammar for output of canned English

Example 12.36: Figure 12.8 shows a grammar for producing canned text on schedule information. For example, the query
ask trans(scheduled(w11,cs422,clock(15,30),above(csci333)),T,[]).

produces the answer T=[the, winter, 2011, session, of, the, advanced, artificial, intelligence, course, is, scheduled, at, 3, :, 30, pm, in, the, room, above, the, computer, science, department, office]. This list could be written as a sentence to the user.

This grammar would probably not be useful for understanding natural language, because it requires a very stylized form of English; the user would have to use the exact translation of a term to get a legal parse.