The entities reflected by the CHR debugger are constraints and
rules. Constraints are treated like ordinary Prolog goals with the
usual ports: [call,exit,redo,fail]. In addition,
constraints may get inserted into or removed from the constraint store
(ports: insert,remove), and stored constraints containing
variables will be woken and reconsidered (port: wake)
when variables are touched.
The execution of a constraint consists of trying to apply the rules
mentioning the constraint in their heads. Two ports for
rules reflect this process: At a try port the active
constraint matches one of the heads of the rule, and matching
constraints for the remaining heads of the rule, if any, have been
found as well. The transition from a try port to an
apply port takes place when the guard has been successfully
evaluated, i.e. when the rule commits. At the apply port,
the body of the rule is just about to be executed. The body
is a Prolog goal transparent to the CHR debugger. If the rule
body contains CHR constraints, the CHR debugger will track them
again. If the rules were consulted, the Prolog debugger can be
used to study the evaluations of the other predicates in the
body.