Interactive debugging of programs

The debug facility permits interactively controlled execution of a program.

Changing the TRACE action to one with a prefix ? (for example, TRACE ?A or the TRACE built-in function) turns on interactive debug and indicates to the user that interactive debug is active. Further TRACE instructions in the program are ignored, and the language processor pauses after nearly all instructions that are traced at the console (see the following for the exceptions). When the language processor pauses, indicated by a READ in the lower corner of the screen, three debug actions are available:

Entering a null line (with no characters, even blanks) makes the language processor continue execution until the next pause for debug input. Repeatedly entering a null line, therefore, steps from pause point to pause point. For TRACE ?A, for example, this is equivalent to single-stepping through the program.
Entering an equal sign (=) with no blanks makes the language processor rerun the clause last traced. For example: if an IF clause is about to take the wrong branch, you can change the value of the variable(s) on which it depends, and then rerun it.
Once the clause has been rerun, the language processor pauses again.
Anything else entered is treated as a line of one or more clauses, and processed immediately (that is, as though DO; line; END; had been inserted in the program). The same rules apply as in the INTERPRET instruction (for example, DO-END constructs must be complete). If an instruction has a syntax error in it, a standard message is displayed and you are prompted for input again. Similarly all the other SIGNAL conditions are disabled while the string is processed to prevent unintentional transfer of control.
During execution of the string, no tracing takes place, except that nonzero return codes from host commands are displayed. Host commands are always run (that is, are not affected by the prefix ! on TRACE instructions), but the variable RC is not set.

Once the string has been processed, the language processor pauses again for further debug input unless a TRACE instruction was entered. In this latter case, the language processor immediately alters the tracing action (if necessary) and then continues executing until the next pause point (if any). To alter the tracing action (from All to Results, for example) and then rerun the instruction, you must use the built-in function TRACE (see TRACE). For example, CALL TRACE I changes the trace action to I and allows re-execution of the statement after which the pause was made. Interactive debug is turned off, when it is in effect, if a TRACE instruction uses a prefix, or at any time when a TRACE O or TRACE with no options is entered.

You can use the numeric form of the TRACE instruction to allow sections of the program to be run without pause for debug input. TRACE n (that is, positive result) allows execution to continue, skipping the next n pauses (when interactive debug is or becomes active). TRACE -n (that is, negative result) allows execution to continue without pause and with tracing inhibited for n clauses that would otherwise be traced.

The trace action selected by a TRACE instruction is saved and restored across subroutine calls. This means that if you are stepping through a program (say after using

TRACE
?R

to trace Results) and then enter a subroutine in which you have no interest, you can enter TRACE O to turn tracing off. No further instructions in the subroutine are traced, but on return to the caller, tracing is restored.

Similarly, if you are interested only in a subroutine, you can put a TRACE ?R instruction at its start. Having traced the routine, the original status of tracing is restored and (if tracing was off on entry to the subroutine) tracing (and interactive debug) is turned off until the next entry to the subroutine.

Because any instructions can be run in interactive debug, you have considerable control over execution.

Note: While in interactive debug, pauses might occur because of PULL statements as well as because of interactive debug. For programs containing PULL statements, it is important to be aware of the reason for each pause. In programs, PULL statements are often paired with SAY statements. The user should enter the data for the PULL at the pause after the trace line for the PULL (the pause specifically for entering data for the PULL). The user should not enter the data at the pause after the corresponding SAY statement (this is an interactive debug pause).

Note: Some clauses cannot safely be re-run, and, therefore, the language processor does not pause after them, even if they are traced. These are:

Any repetitive DO clause, on the second or subsequent time around the loop.
All END clauses (not a useful place to pause in any case).
All THEN, ELSE, OTHERWISE, or null clauses.
All RETURN and EXIT clauses.
All SIGNAL and CALL clauses (the language processor pauses after the target label has been traced).
Any clause that raises a condition that CALL ON or SIGNAL ON traps (the pause takes place after the target label for the CALL or SIGNAL has been traced).
Any clause that causes a syntax error. (These may be trapped by SIGNAL ON SYNTAX, but cannot be re-run.)

Examples

Say expr     /* displays the result of evaluating the         */
             /* expression.                                   */

name=expr    /* alters the value of a variable.               */

Trace O      /* (or Trace with no options) turns off          */
             /* interactive debug and all tracing.            */

Trace ?A     /* turns off interactive debug but continues     */
             /* tracing all clauses.                          */

Trace L      /* makes the language processor pause at labels  */
             /* only.  This is similar to the traditional     */
             /* "breakpoint" function, except that you        */
             /* do not have to know the exact name and        */
             /* spelling of the labels in the program.        */

exit         /* stops execution of the program.               */

Do i=1 to 10; say stem.i; end
             /* displays ten elements of the array stem.      */