Here are two example constraint solvers
written in CHR.
- 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 saved in a file and loaded in SWI-Prolog,
you can call the leq/2 constraints in a query, e.g.:
?- leq(X,Y), leq(Y,Z).
leq(_G23837, _G23841)
leq(_G23838, _G23841)
leq(_G23837, _G23838)
true .
When the query succeeds, the SWI-Prolog top level prints the content
of the CHR constraint store and displays the bindings generated during
the query. Some of the query variables may have been bound to attributed
variables, as you see in the above example.
- The program below implements a simple finite domain constraint
solver.
:- module(dom,[dom/2]).
:- use_module(library(chr)).
:- 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 saved in a file and loaded in SWI-Prolog,
you can call the dom/2 constraints in a query, e.g.:
?- dom(A,[1,2,3]), dom(A,[3,4,5]).
A = 3.
