A solution to the Dutch flag problem

Source: The Art of Prolog

Program source code: dutch_flag.pl

/*	
    dutch(Xs,RedsWhitesBlues) :-
	RedsWhitesBlues is a list of elements of Xs ordered
	by color: red, then white, then blue.
*/
 
	dutch(Xs,RedsWhitesBlues) :-
		distribute(Xs,Reds,Whites,Blues),
		append(Whites,Blues,WhitesBlues),
		append(Reds,WhitesBlues,RedsWhitesBlues).
 
/*	
    distribute(Xs,Reds,Whites,Blues) :-
	Reds, Whites, and Blues are the lists of red, white,
	and blue elements in Xs, respectively.
*/
 
	distribute([red(X)|Xs],[red(X)|Reds],Whites,Blues) :-
		distribute(Xs,Reds,Whites,Blues).
	distribute([white(X)|Xs],Reds,[white(X)|Whites],Blues) :-
		distribute(Xs,Reds,Whites,Blues).
	distribute([blue(X)|Xs],Reds,Whites,[blue(X)|Blues]) :-
		distribute(Xs,Reds,Whites,Blues).
	distribute([],[],[],[]).
 
%	Program 15.5: A solution to the Dutch flag problem