% Quicksort in Prolog.

% filter(List, Pivot, Lower, Higher) divides List up into items Lower & Higher than Pivot
filter( [],    _, [],    []).
filter( [X|Y], P, [X|L], H     ):- X =< P, filter(Y, P, L, H).
filter( [X|Y], P, L,     [X|H] ):- X >  P, filter(Y, P, L, H).

% Here is the sort 
qsort([], []):-!.
qsort([X], [X]):-!.
qsort([X|Y], SXY):- filter(Y, X, L, H), qsort(L,SL), qsort(H,SH), append(SL, [X], SLX), append(SLX, SH, SXY).