Question about difference lists

Hi.

I am teaching myself prolog. As an exercise, I wrote a predicate
"unique" to find all the unique elements in a list. I wrote a version
with an accumulator and made an attempt to write an equivalent
predicate using difference lists. I was motivated to use difference
lists to learn them better and to avoid having to reverse the list at
the end of the computation.

The difficulty is that I use member/2 to decide if I can safely discard
an element by testing if it is already part of the output. With
difference lists, the member/2 predicate will find a variable in the
difference list and bind to it. This causes the evaluation to fail.

Below is some sample code that demonstrates what I am trying to explain
here.

Thanks for any suggestions.

John

% uniq_acc works
uniq_acc(List,Set) :- uniq_acc(List,[],Set1), reverse(Set1, Set).
uniq_acc([],A,A).
uniq_acc([Head|Tail],Acc,Result) :-
member(Head,Acc),
!,
uniq_acc(Tail,Acc,Result).
uniq_acc([Head|Tail],Acc,Result) :-
uniq_acc(Tail,[Head|Acc],Result).

% uniq_dl fails in the call to member below
uniq_dl(List,Set) :- uniq_dl(List,Set,[]).
uniq_dl([],Slot,Slot).
uniq_dl([Head|Tail],Set,Slot) :-
member(Head,Set), % trouble is here.
!,
uniq_dl(Tail,Set,Slot).
uniq_dl([Head|Tail],[Head|Slot1],Slot) :-
uniq_dl(Tail,Slot1,Slot).

.

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