10.33.5 Examples

Here are two example constraint solvers written in CHR.

1. The program below defines a solver with one constraint, leq/2, which is a less-than-or-equal constraint, also known as a partial order constraint.

             :- module(leq,[leq/2]).
             :- use_module(library(chr)).
             
             :- chr_constraint leq/2.
             reflexivity   leq(X,X) <=> true.
             antisymmetry  leq(X,Y), leq(Y,X) <=> X = Y.
             idempotence   leq(X,Y) \ leq(X,Y) <=> true.
             transitivity  leq(X,Y), leq(Y,Z) ==> leq(X,Z).
        

   When the above program is loaded, you can call the leq/2 constraint in a query, e.g.:

             | ?- leq(X,Y), leq(Y,Z).
             leq(X,Y),
             leq(X,Z),
             leq(Y,Z) ?
        

2. The program below implements a simple finite domain constraint solver.

             :- module(dom,[dom/2]).
             :- use_module(library(chr)).
             :- use_module(library(sets), [intersection/3]).
             
             :- chr_constraint dom(?int,+list(int)).
             :- chr_type list(T) ---> [] ; [T|list(T)].
             
             dom(X,[]) <=> fail.
             dom(X,[Y]) <=> X = Y.
             dom(X,L) <=> nonvar(X) | memberchk(X,L).
             dom(X,L1), dom(X,L2) <=> intersection(L1,L2,L3), dom(X,L3).
        

   When the above program is loaded, you can call the dom/2 constraint in a query, e.g.:

             | ?- dom(A,[1,2,3]), dom(A,[3,4,5]).
             A = 3