/* 

  Test of "DCG" in Picat.

  This is some ways of handling grammars in Picat, inspired
  by Prolog's DCG.

  Note: Picat v3.* supports DCGs.

  This Picat model was created by Hakan Kjellerstrand, hakank@gmail.com
  See also my Picat page: http://www.hakank.org/picat/

*/


% import util.
% import cp.


main => go.

% From http://en.wikipedia.org/wiki/Definite_clause_grammar
% """
% A basic example of DCGs helps to illustrate what they are and what they look like.
% 
% sentence --> noun_phrase, verb_phrase.
% noun_phrase --> det, noun.
% verb_phrase --> verb, noun_phrase.
% det --> [the].
% det --> [a].
% noun --> [cat].
% noun --> [bat].
% verb --> [eats].
%
% This generates sentences such as "the cat eats the bat", "a bat eats the cat". 
% One can generate all of the valid expressions in the language generated by 
% this grammar at a Prolog interpreter by typing sentence(X,[]). Similarly, 
% one can test whether a sentence is valid in the language by typing something 
% like sentence([the,bat,eats,the,bat],[]).
%
% Translation into definite clauses
% 
% DCG notation is just syntactic sugar for normal definite clauses in Prolog. 
% For example, the previous example could be translated into the following:
% 
% sentence(S1,S3) :- noun_phrase(S1,S2), verb_phrase(S2,S3).
% noun_phrase(S1,S3) :- det(S1,S2), noun(S2,S3).
% verb_phrase(S1,S3) :- verb(S1,S2), noun_phrase(S2,S3).
% det([the|X], X).
% det([a|X], X).
% noun([cat|X], X).
% noun([bat|X], X).
% verb([eats|X], X).
% """

% Check a sentence
go ?=>
   S1 = [the,cat,eats,the,bat],
   sentence(S1,S3),
   writeln([s1=S1,s3=S3]),
   fail,
   nl.
go => true.

% Check a sentence
go2 =>
   S1 = [a,cat,eats,the,cat],
   sentence(S1,S3),
   writeln([s1=S1,s3=S3]),
   nl.

% Note: When using the grammar for generate sentences, one cannot use
%       pattern matching in the head, e.g. 
%       as in det/2, noun/2, and verb/2.
% If we split the parameter in the body, it works, see version 1
%
go3 => 
   All=findall(S1,$sentence1(S1,[])),
   writeln(All),
   writeln(len=All.length),
   nl.

go4 => 
   All=findall(S1,$sentence2(S1,[])),
   writeln(All),
   writeln(len=All.length),
   nl.

go5 => 
   All=findall(S1,$sentence3(S1,[])),
   writeln(All),
   writeln(len=All.length),
   nl.


%
% Note: This works only to check a sentence,
%       not for generate them. See below for variants to
%       do this
%
sentence(S1,S3) => 
  noun_phrase(S1,S2), 
  verb_phrase(S2,S3).

noun_phrase(S1,S3) => 
  det(S1,S2),
  noun(S2,S3).

verb_phrase(S1,S3) => 
  verb(S1,S2), 
  noun_phrase(S2,S3).

% Note: All but the last must be backtrackable (using ?=>)
det([the|X], X2) ?=> X2 = X.
det([a|X], X2)    => X2 = X.

noun([cat|X], X2) ?=> X2 = X.
noun([bat|X], X2)  => X2 = X.

verb([eats|X], X2) => X2=X.


%
% Generate sentences, version 1, "DCG"
%
sentence1(S1,S3) => 
  noun_phrase1(S1,S2), 
  verb_phrase1(S2,S3).

noun_phrase1(S1,S3) => 
  det1(S1,S2),
  noun1(S2,S3).

verb_phrase1(S1,S3) => 
  verb1(S1,S2), 
  noun_phrase1(S2,S3).

% Note: All but the last must be backtrackable (using ?=>)
det1(X1, X2) ?=> X1 = [the|X2].
det1(X1, X2)  => X1 = [a|X2].

noun1(X1, X2) ?=> X1=[cat|X2].
noun1(X1, X2)  => X1=[bat|X2].

verb1(X1, X2) ?=> X1=[eats|X2].


%
% Generate sentences, version 2, using member
%
% Here we use member to generate the different alternatives
% in det2/2, noun2/2, and verb2/2.
sentence2(S1,S3) => 
  noun_phrase2(S1,S2), 
  verb_phrase2(S2,S3).

noun_phrase2(S1,S3) => 
  det2(S1,S2),
  noun2(S2,S3).

verb_phrase2(S1,S3) => 
  verb2(S1,S2), 
  noun_phrase2(S2,S3).

det2(X1, X2) => 
   Dets = [the,a],
   member(Det,Dets),
   X1 = [Det|X2].

noun2(X1, X2) => 
   Nouns = [cat,bat],
   member(Noun, Nouns),
   X1=[Noun|X2].

verb2(X1, X2) => 
   Verbs = [eats,spots],
   member(Verb, Verbs),
   X1=[Verb|X2].


%
% Generate sentences, version 3, using facts.
%
% Using facts, e.g. det3/1, noun3/1, and verb3/1 
% for generating the alternatives
sentence3(S1,S3) => 
  noun_phrase3(S1,S2), 
  verb_phrase3(S2,S3).

noun_phrase3(S1,S3) => 
   det3(S1,S2),
   noun3(S2,S3).

verb_phrase3(S1,S3) => 
   verb3(S1,S2), 
   noun_phrase3(S2,S3).

det3(X1, X2) => 
   det3(Det),
   X1 = [Det|X2].
index(-)
det3(the).
det3(a).

noun3(X1, X2) => 
  noun3(Noun),
  X1=[Noun|X2].

index(-)
noun3(cat).
noun3(bat).
noun3(dog).

verb3(X1, X2) => 
  verb3(Verb),
  X1=[Verb|X2].
index(-)
verb3(eats).
verb3(spots).