Some doubts about how work this DCG grammar for subset of natural language in Prolog -

i studying dcg grammar natural language processing using prolog , have doubts if have understand correctly or if missing something.

this dcg grammar:

sentence2(vp) --> noun_phrase2(actor),               verb_phrase2(actor, vp).  /* noun phrase proper name of person (that unified in name variable) */ noun_phrase2(name) --> propername(name).  /* verb_phrase can intransitive verb */ verb_phrase2(actor, vp) --> intrans_verb(actor, vp).  /* verb_phrase can transitive verb followed object complement verb_phrase2(somebody, vp) --> trans_verb(somebody, something, vp),                                noun_phrase2(something).  /* meaning of proper name john john    meaning of proper name mary mary */ propername(john) --> [john]. propername(mary) --> [mary].  intrans_verb(actor, paints(actor)) --> [paints].  trans_verb(somebody, something, likes(somebody, something)) --> [likes]. 

so grammar can accept phrase like: [john, paints] have meaning: paints(john)

i see if idea how reach meaning correct.

so think happen when execute following query:

?- sentence2(meaning, [john, paints], []). meaning = paints(john)  

[john paints] final sentence have evaluate , if belong language or not.

a sentence have formed in following way:

sentence2(vp) --> noun_phrase2(actor),                   verb_phrase2(actor, vp). 

(by noun phrase followed verb phrase.

a noun phrase composed in way:

noun_phrase2(name) --> propername(name). 

so noun phrase proper name

the meaning of proper name simple because line:

propername(john) --> [john]. 

i john proper name , adding parameter dcg grammar specify meaning. so: the semantic meaning of proper name john john

so, meaning of noun phrase same meaning of proper name (because variables name unify)

so in previous case meaning ot noun_phrase2 predicate john , first evalutation step of original sentence end.

now have evaluate second part verbal phrase predicate: verb_phrase2(actor, vp)

a verbal phrase be, in case, intransitive verb:

verb_phrase2(actor, vp) --> intrans_verb(actor, vp). 

an intransitive verb definied in way:

intrans_verb(actor, paints(actor)) --> [paints]. 

so word paints intransitive verb , it's meaning paints(actor) actor variable dependnds context (in case actor rappresent who action, paints)

so, backtrack verb_phrase2(actor, vp) verify verb_phrase2(actor, vp)

now actor still remain not yet unified variable , meaning vp = paints(actor)

so, verified paints intransitive verb , meaning paints(actor)

so execute backtrack original sentence2(vp) predicate have verified noun_phrase2(actor) predicate , in actor = john

so have situation:

sentence2(vp) --> noun_phrase2(john),                   verb_phrase2(john, paints(john)). 

so final vp unified paints(john)

is reasoning correct or missing something? way reasoning in prolog way?

please ask specific, short question. include relevant code without excessive comments.

here's how.

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given dcg rules

sentence2(vp) --> noun_phrase2(actor),verb_phrase2(actor, vp). noun_phrase2(name) --> propername(name). verb_phrase2(actor, vp) --> intrans_verb(actor, vp). verb_phrase2(somebody, vp) --> trans_verb(somebody, something, vp),                                  noun_phrase2(something). propername(john) --> [john]. propername(mary) --> [mary]. intrans_verb(actor, paints(actor)) --> [paints]. trans_verb(somebody, something, likes(somebody, something)) --> [likes]. 

how following achieve result?

?- sentence2(meaning, [john, paints], []). meaning = paints(john)  

---

answer:

the above rules equivalent

sentence2(vp, l, z):- noun_phrase2(actor, l, l2),                        verb_phrase2(actor, vp, l2, z). noun_phrase2(name, l, z):- propername(name, l, z).     verb_phrase2(actor, vp, l, z):- intrans_verb(actor, vp, l, z).  verb_phrase2(somebody, vp, l, z):- trans_verb(somebody, something, vp, l, l2),                                    noun_phrase2(something, l2, z).     propername(john, l, z):-      l = [john | z].         %// 'john' present in input stream propername(mary, l, z):-      l = [mary | z].         %// 'mary' present in input stream /* alternative definition  propername(x) --> [x], { member(x, [john, mary]) }.    %% translated propername(x, l, z):- l = [x | z], member(x, [john, mary]).  */     intrans_verb(actor, paints(actor), l, z):-      l = [paints | z].       %// 'paints' present in input stream     trans_verb(somebody, something, likes(somebody, something), l, z):-     l = [likes | z].        %// 'likes' present in input stream 

in particular,

?- sentence2(meaning, [john, paints], []).      ?- noun_phrase2(actor, [john, paints], l2),                        verb_phrase2(actor, meaning, l2, []).    ?- noun_phrase2(actor, [john, paints], l2).   ?- propername(actor, [john, paints], l2).      ?- propername(john, [john, paints], l2).            { actor=john }   !- [john, paints] = [john | [paints]]               { l2=[paints] }    ?- verb_phrase2(john, meaning, [paints], []).   ?- intrans_verb(john, meaning, [paints], []).   ?- intrans_verb(john, paints(john), [paints], []).  { meaning=paints(john) }   !- [paints] = [paints | []]  !-