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Chapter 7: Debugging
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move back to the code which called this predicate and keep moving backwards
up the code looking for choice points.
Exception This arrow represents an exception which was raised in the initial goal, either
by a call to raise_exception/1 or by an error in a built-in predicate. See
Section 8.5 [Exception], page 136. Control now passes out of the Exception
port of the descendant box and the system continues to pass the exception to
outer levels. Textually we move back to the code which called this predicate
and keep moving backwards up the code looking for a call to on_exception/3.
In terms of this model, the information we get about the procedure box is only the control
flow through these five ports. This means that at this level we are not concerned with which
clause matches, and how any subgoals are satisfied, but rather we only wish to know the
initial goal and the final outcome. However, it can be seen that whenever we are trying to
satisfy subgoals, what we are actually doing is passing through the ports of their respective
boxes. If we were to follow this, then we would have complete information about the control
flow inside the procedure box.
Note that the box we have drawn round the predicate should really be seen as an invocation
box. That is, there will be a different box for each different invocation of the predicate.
Obviously, with something like a recursive predicate, there will be many different Calls and
Exits in the control flow, but these will be for different invocations. Since this might get
confusing each invocation box is given a unique integer identifier.
In addition to the five basic ports discussed above, there are two more ports for invocations
involving a blocked goal:
Block
This port is passed through when a goal is blocked.
Unblock
This port is passed through when a previously blocked goal is unblocked.
7.2 Basic Debugging Predicates
Development systems provide a range of built-in predicates for control of the debugging
facilities. The most basic predicates are as follows:
debug
Switches the debugger on, and ensures that the next time control reaches a
spypoint, it will be activated. In basic usage this means that a message will be
produced and you will be prompted for a command. In order for the full range
of control flow information to be available it is necessary to have the debugger
on from the start. When it is off the system does not remember invocations
that are being executed. (This is because it is expensive and not required for
normal running of programs.) You can switch Debug Mode on in the middle of
execution, either from within your program or after a ^C (see trace/0 below),
but information prior to this will just be unavailable.
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