Non-strict independence (examples)

• Quick-sort using append:

  qsort([],[]).
  qsort([X|Xs],R) :-           
          partition(Xs,X,L,R),    
          qsort(L,SL),      
          qsort(R,SR),
          append(SL,[X|SR],R).         
  
  partition([],_,[],[]).
  partition([E|R],C,[E|L1],R) :-  
          E=<C, !,                
          partition(R,C,L1,R).    
  partition([E|R],C,L,[E|R1]) :-  
          E>C, !,                 
          partition(R,C,L,R1).

• Using difference lists:

  qsort(I,O) :-                   
          qsort(I,O,[]).          
  qsort([],S,S).
  qsort([X|Xs],S,S2) :-           
          partition(Xs,X,L,R),    
          qsort(L,S,[X|S1]),      
          qsort(R,S1,S2).