Node:CHR Control Flow Model, Next:CHR Debugging Predicates, Previous:CHR Debugging, Up:CHR Debugging
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.