The Cut and the Suspended Goals

It is very important to mention here the influence of non-logical predicates, especially the cut, on the execution of delayed goals. The cut relies on a fixed order of goal execution in that it discards some choice points if all goals preceding it in the clause body have succeeded. If some of these goals are delayed, or if the head unification of the clause with the cut wakes some nondeterministic delayed goals, the completeness of the resulting program is lost and there is no clean way to save it as long as the cut is used.

The user is strongly discouraged to use non-local cuts together with coroutining, or to be precisely aware of their scope. The danger of a cut is twofold:

• a cut can be executed if some calls preceding it in the clause (or children of these calls) delay and they fail when they are woken

• the head unification of a clause with cuts can wake some delayed goals. If they are nondeterministic, the cut in the body of the waking clause will commit even the woken goals

In order to detect these situations, the ECLiPSe debugger has an option to print a warning whenever a cut in one of the above two conditions is executed. These warnings can be toggled using the P command.