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   32
   33:- module(sets,
   34	  [ add_element/3,		% +Element, +Set1, -Set2
   35	    del_element/3,		% +Element, +Set1, -Set2
   36	    disjoint/2,			% +Set1, +Set2
   37	    intersect/2,		% +Set1, +Set2
   38	    set_order/3,		% +Xs, +Ys, -R
   39	    seteq/2,			% +Set1, +Set2
   40	    list_to_set/2,		% +List, -Set
   41	    disjoint_union/3,		% +Set1, +Set2, -Union
   42	    union/4			% +Set1, +Set2, -Union, -Difference
   43	  ]).   44:- reexport(library(lists),
   45	    [ is_set/1,
   46	      subset/2,
   47	      intersection/3,
   48	      subtract/3,
   49	      union/3
   50	    ]).   51:- use_module(library(lists), [selectchk/3]).

   68add_element(Element, Set1, Set2) :-
   69	(   memberchk(Element, Set1)
   70	->  Set2 = Set1
   71	;   Set2 = [Element|Set1]
   72	).
   73
   74del_element(Element, Set1, Set2) :-
   75	(   selectchk(Element, Set1, Set2)
   76	->  true
   77	;   Set2 = Set1
   78	).
   79
   80disjoint(Set1, Set2) :-
   81	intersection(Set1, Set2, []).
   82
   83intersect(Set1, Set2) :-
   84	intersection(Set1, Set2, Intersection),
   85	Intersection \= [].
   86
   87set_order(Xs, Ys, R) :-
   88	(   subset(Xs, Ys)
   89	->  (   subset(Ys, Xs)
   90	    ->  R = (=)
   91	    ;   R = (<)
   92	    )
   93	;   (   subset(Ys, Xs)
   94	    ->  R = (>)
   95	    ;   fail
   96	    )
   97	).
   98
   99seteq(Set1, Set2) :- set_order(Set1, Set2, =).

 list_to_set(+List, -Set) is det

Set is List with all duplicates removed. Duplicates are removed by unification. This is not the same as SWI-Prolog's list_to_set/2 in library(lists), which finds duplicates based on term equality (==).

Compatibility

- SICStus 4

  110list_to_set([], []).
  111list_to_set([X|Tail], Set) :-
  112	(   memberchk(X, Tail)
  113	->  Set = SetTail
  114	;   Set = [X|SetTail]
  115	),
  116	list_to_set(Tail, SetTail).
  117
  118disjoint_union(Set1, Set2, Union) :-
  119	disjoint(Set1, Set2),
  120	append(Set1, Set2, Union).
  121
  122union(Set1, Set2, Union, Difference) :-
  123	union(Set1, Set2, Union),
  124	subtract(Set1, Set2, Difference)