Translation to DCG Semicontext not working

问题

Since this question uses list, I wanted to solve it using DCG. In the process I realized that semicontext could be used. (DCG Primer)

The original problem is to return count of items in a list but if two identical items are next to each other then don't increment the count.

While my code works for some of the test cases, it fails for others. It is just one clause that is failing. In looking at the code with a debugger it appears that the second state variable, the returned list, is bound upon a call to the clause when I am thinking that it should be unbound. EDIT Resolved this part of problem below.

I am using SWI-Prolog 8.0.

The clause causing the problem:

count_dcg(N0,N),[C2] -->
    [C1,C2],
    { N is N0 + 1 }.

Note: C1 is flagged as Singleton variables: [C1]

Normally I would change C1 to _ but in this case I need to indicate that the first and second items currently being processed are different. In other words it is using the failure of unification as a guard.

Looking at the DCG using listing/1 reveals the use _ which might be the problem but not sure.

count_dcg(C, B, A, E) :-
    A=[_, F|D],
    B is C+1,
    G=D,
    E=[F|G].

What is the correct way to solve the problem using DCG?

See follow-on question.

Current source code

% empty list
% No semicontext (push back) needed because last item in list.
count_dcg(N,N) --> [].

% single item in list
% No semicontext (push back) needed because only one item removed from list.
count_dcg(N0,N) -->
    [_],
    \+ [_],
    { N is N0 + 1 }.

% Semicontext (push back) needed because two items removed from list.
% Need second item to stay on list.
count_dcg(N,N),[C] -->
    [C,C].

% Semicontext (push back) needed because two items removed from list.
% Need second item to stay on list.
count_dcg(N0,N),[C2] -->
    [C1,C2],
    { N is N0 + 1 }.

count(L,N) :-
    DCG = count_dcg(0,N),
    phrase(DCG,L).

Test cases

:- begin_tests(count).

test(1,[nondet]) :-
    count([],N),
    assertion( N == 0 ).

test(2,[nondet]) :-
    count([a],N),
    assertion( N == 1 ).

test(3,[nondet]) :-
    count([a,a],N),
    assertion( N == 1 ).

test(4,[nondet]) :-
    count([a,b],N),
    assertion( N == 2 ).

test(5,[nondet]) :-
    count([b,a],N),
    assertion( N == 2 ).

test(6,[nondet]) :-
    count([a,a,b],N),
    assertion( N == 2 ).

test(7,[nondet]) :-
    count([a,b,a],N),
    assertion( N == 3 ).

test(8,[nondet]) :-
    count([b,a,a],N),
    assertion( N == 2 ).

:- end_tests(count).

Running of test

?- run_tests.
% PL-Unit: count ..
ERROR: c:/question_110.pl:80:
        test 3: failed

ERROR: c:/question_110.pl:84:
        test 4: failed

ERROR: c:/question_110.pl:88:
        test 5: failed

ERROR: c:/question_110.pl:92:
        test 6: failed

ERROR: c:/question_110.pl:96:
        test 7: failed

ERROR: c:/question_110.pl:100:
        test 8: failed

 done
% 6 tests failed
% 2 tests passed
false.

EDIT 1

Realized that need tail call for two of the predicates

% Semicontext (push back) needed because two items removed from list.
% Need second item to stay on list.
count_dcg(N0,N),[C] -->
    [C,C],
    count_dcg(N0,N).

% Semicontext (push back) needed because two items removed from list.
% Need second item to stay on list.
count_dcg(N0,N),[C2] -->
    [C1,C2],
    { 
        C1 \== C2,
        N1 is N0 + 1 
    },
    count_dcg(N1,N).

Code still not working, but this explains why state variable was bound when I expected it to be unbound.

EDIT 2

While not using DCG semicontext as I was hoping, using a variation of semicontext as a lookahead, the code works. Not posting this as an answer because I would like the answer to either show DCG code working with the semicontext on the clause header or explain why that is wrong.

lookahead(C),[C] -->
    [C].

% empty list
% No lookahead needed because last item in list.
count_3_dcg(N,N) --> [].

% single item in list
% No lookahead  needed because only one in list.
count_3_dcg(N0,N) -->
    [_],
    \+ [_],
    { N is N0 + 1 }.

% Lookahead needed because two items in list and
% only want to remove first item.
count_3_dcg(N0,N) -->
    [C1],
    lookahead(C2),
    { C1 == C2 },
    count_3_dcg(N0,N).

% Lookahead needed because two items in list and
% only want to remove first item.
count_3_dcg(N0,N) -->
    [C1],
    lookahead(C2),
    {
        C1 \== C2,
        N1 is N0 + 1
    },
    count_3_dcg(N1,N).

count(L,N) :-
    DCG = count_3_dcg(0,N),
    phrase(DCG,L).

Running of test

?- run_tests.
% PL-Unit: count ........ done
% All 8 tests passed
true.

回答1:

An alternative solution that doesn't require push-back lists or lookahead:

count_dcg(N0,N) -->
    [C], {N1 is N0 + 1}, count_dcg(N1,N,C).
count_dcg(N,N) -->
    [].

count_dcg(N0,N,C) -->
    [C],
    count_dcg(N0,N,C).
count_dcg(N0,N,C) -->
    [C1],
    {C \== C1, N1 is N0 + 1},
    count_dcg(N1,N,C1).
count_dcg(N,N,_) -->
    [].

count(L,N) :-
    phrase(count_dcg(0,N),L).

来源：https://stackoverflow.com/questions/54984486/translation-to-dcg-semicontext-not-working

标签

prolog

dcg

dcg-semicontext