.F.J
.LC.CS.SS
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1↑40.↑40

.UC.C
DEBUGGING FACILITIES
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.UL
↑INTRODUCTION
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 ↑DEBUGGING A COLLECTION OF ↑-LISP↑- FUNCTIONS INVOLVES ISOLATING
PROBLEMS WITHIN PARTICULAR FUNCTIONS AND/OR DETERMINING
WHEN AND WHERE INCORRECT DATA ARE BEING GENERATED AND TRANSMITTED.
IN THE ↑-UCI LISP↑- SYSTEM, THERE ARE FIVE FACILITIES
WHICH AID THE USER IN 
MONITORING HIS PROGRAM. ONE OF THESE IS THE ↑ERROR ↑PACKAGE
WHICH TAKES CONTROL WHENEVER AN ERROR OCCURS IN A PROGRAM AND
WHICH ALLOWS THE USER TO EXAMINE THE STATE OF THE
WORLD (SEE SECTION ON '↑-ERROR PACKAGE↑-').
ANOTHER FACILITY ALLOWS THE USER TO TEMPORARILY INTERRUPT
HIS COMPUTATION AND EXAMINE ITS PROGRESS.
THE OTHER THREE FACILITIES
(↑-BREAK↑-, ↑-TRACE↑- AND ↑-BREAKIN↑-)
ALLOW THE USER TO (TEMPORARILY) MODIFY SELECTED FUNCTION DEFINITIONS SO
THAT HE CAN FOLLOW THE FLOW OF CONTROL IN HIS PROGRAMS.
ALL OF THESE FACILITIES USE THE SAME SYSTEM FUNCTION,
↑-BREAK1↑-, AS THE USER INTERFACE.
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 ↑-BREAK↑-, ↑-BREAKIN↑-  AND ↑-TRACE↑-  TOGETHER
ARE CALLED THE ↑BREAK ↑PACKAGE.
↑-BREAK↑- AND ↑-TRACE↑-
CAN BE USED ON COMPILED  AND
SYSTEM FUNCTIONS AS WELL AS ↑-EXPR↑-'S, ↑-FEXPR↑-'S AND ↑-MACRO↑-'S.
↑-BREAKIN↑- CAN BE USED ONLY WITH INTERPRETED FUNCTIONS.
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 ↑-BREAK↑- MODIFIES THE DEFINITION OF A FUNCTION ↑-FN↑-,
SO THAT IF A BREAK CONDITION (DEFINED BY THE USER) IS SATISIFIED, THE
PROCESS IS HALTED TEMPORARILY ON A CALL TO ↑F↑N. THE USER
CAN THEN INTERROGATE THE STATE OF THE MACHINE, PERFORM ANY COMPUTATIONS,
AND CONTINUE OR RETURN FROM THE CALL.
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 ↑-TRACE↑- MODIFIES A DEFINITION OF A FUNCTION ↑F↑N SO THAT WHENEVER ↑F↑N
IS CALLED, ITS ARGUMENTS (OR SOME OTHER VALUES SPECIFIED
BY THE USER) ARE PRINTED. WHEN THE VALUE OF ↑F↑N IS COMPUTED IT IS PRINTED
ALSO.
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 ↑-BREAKIN↑- ALLOWS THE USER TO INSERT A BREAKPOINT ↑←INSIDE↑←
AN EXPRESSION DEFINING A FUNCTION. WHEN THE BREAKPOINT IS
REACHED AND IF A BREAK CONDITION (DEFINED BY THE USER)
IS SATISFIED, A TEMPORARY HALT OCCURS
AND THE USER CAN AGAIN INVESTIGATE THE STATE OF THE COMPUTATION.
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 THE TWO EXAMPLES  ON PAGES 1.3 AND 1.4 ILLUSTRATE THESE FACILITIES.
IN THE FIRST EXAMPLE, THE USER TRACES THE FUNCTION
↑-FACTORIAL↑-. ↑-TRACE↑- REDEFINES ↑-FACTORIAL↑- SO THAT
IT CALLS ↑-BREAK1↑- IN SUCH A WAY THAT IT PRINTS SOME
INFORMATION, IN THIS CASE THE ARGUMENTS AND VALUE
OF ↑-FACTORIAL↑-, AND THEN GOES ON WITH THE COMPUTATION.
WHEN AN ERROR OCCURS ON THE FIFTH RECURSION, ↑-BREAK1↑- REVERTS
TO INTERACTIVE MODE, AND A FULL BREAK OCCURS. THE SITUATION
IS THEN THE SAME AS THOUGH THE USER HAD ORIGINALLY PERFORMED 
↑-(BREAK FACTORIAL)↑- INSTEAD OF ↑-(TRACE FACTORIAL)↑-,
AND THE USER CAN EVALUATE VARIOUS ↑-LISP↑- FORMS AND
DIRECT THE COURSE OF THE COMPUTATION. IN THIS CASE, THE
USER EXAMINES THE VARIABLE ↑N, INSTRUCTS ↑-BREAK1↑- TO CHANGE
↑L TO 1 AND CONTINUE.
THE > COMMAND, FOLLOWING AN ↑-UNBOUND ATOM↑- OR ↑-UNDEFINED FUNCTION↑- ERROR,
TELLS ↑-BREAK1↑- TO USE THE NEXT EXPRESSION INSTEAD OF THE
ATOM WHICH CAUSED THE ERROR.
THE > COMMAND DOES A DESTRUCTIVE REPLACEMENT OF,
IN THIS CASE, 1 FOR ↑L, AND SAVES AN EDIT STEP BY CORRECTING THE
TYPO IN THE FUNCTION DEFINITION. THE REST OF THE TRACING PROCEEDS WITHOUT INCIDENT.
THE FUNCTION ↑-UNTRACE↑- RESTORES ↑-FACTORIAL↑- TO ITS
ORIGINAL DEFINITION.
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 IN THE SECOND EXAMPLE, THE USER HAS WRITTEN ↑ACKERMANN'S
FUNCTION. HE THEN USES ↑-BREAK↑- TO PLACE A CALL TO ↑-BREAK1↑-
AROUND THE BODY OF THE FUNCTION. HE INDICATES THAT ↑-ACK↑-
IS TO BE BROKEN WHEN ↑M EQUALS ↑N AND
THAT BEFORE THE BREAK OCCURS, THE ARGUMENTS TO ↑-ACK↑- ARE TO
BE PRINTED. WHILE CALCULATING ↑-(ACK 2 1)↑-, ↑-ACK↑-
IS CALLED TWICE WHEN ↑M = ↑N. DURING THE FIRST OF THESE
BREAKS, THE USER PRINTS OUT A BACKTRACE OF THE FUNCTION
NAMES AND VARIABLE BINDINGS. HE CONTINUES THE COMPUTATION
WITH A ↑G↑O WHI