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copied!<p>Your problem (superficially) is that member/2 get called with an uninstantiated variable, and then 'builds' the partial list that you see in the output</p> <pre><code>[trace] ?- setify([1],X). Call: (6) setify([1], _G340) ? creep Call: (7) appendHlp(_G340, _G416, 1) ? creep ^ Call: (8) not(member(1, _G340)) ? creep ^ Fail: (8) not(user:member(1, _G340)) ? creep Redo: (7) appendHlp(_G340, _G416, 1) ? creep Call: (8) lists:member(1, _G340) ? creep Exit: (8) lists:member(1, [1|_G409]) ? creep Call: (8) _G418=[1|_G409] ? creep Exit: (8) [1|_G409]=[1|_G409] ? creep Exit: (7) appendHlp([1|_G409], [1|_G409], 1) ? creep Call: (7) setify([], [1|_G409]) ? creep Exit: (7) setify([], [1|_G409]) ? creep Exit: (6) setify([1], [1|_G409]) ? creep X = [1|_G409] . </code></pre> <p>It's not easy to correct, because you use a <strong>very</strong> convoluted way to do a <strong>very</strong> simple task. If you replace, for instance, member with memberchk, or <code>member(Addme,ListOrg)</code> with <code>[Addme]=ListOrg</code>, your program will fail.</p> <p>I would advise to take some programming habit accepted by Prolog community. For instance, attempt to place <em>input</em> parameters <em>before</em> output. It's not always possible, because Prolog is <em>relational</em>, but will help to write better code.</p> <p>Of course, sort/2 would produce the output efficiently and without hassle.</p> <p><em>edit</em> the solution can be quite compact, in basic Prolog:</p> <pre><code>setify([E|R], U) :- memberchk(E, R), !, setify(R, U). setify([E|R], [E|U]) :- setify(R, U). setify([], []). 3 ?- setify([1,1,1,2,1,1],L). L = [2, 1]. </code></pre>

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