Lisp - Debugging

Debugging and tracing is an important aspect of active development in any programming language. LISP provides powerful and extensive debugging tools. In this chapter, we're discussing how to trace and debug applications in LISP.

Error System

Whenever their is any issue during program execution, LISP signal an error similar to exceptions raised in some languages like Java, C++ etc. In case of using an interactive environment like IDE, debugger is automatically enabled and LISP activates the debugger and drops us on the line of error. This way we can inspect and debug the state of the program.

Features of debugger

• Backtracing − During debugging phase, a debugger displays the complete sequence of function calls leading to the error. It helps in understanding the execution path.

• Variable tracing − We can inspect a variable's value in the backtrace frames. It helps in understanding the failure due to data flow.

• Expression Evaluation − In dubugger's frames, we can evaluate arbitrary LISP expressions. This helps in probing the environment and to understand the current state.

• Restartability − Debugger provides ways to ignore error and continuation of code execution as well. We can either provide a valid value or ignore error at all or jump to another line of execution.

• Stepping and Tracing − Using debugger, we can step through code line by line and trace function executions.

Example - Runtime Error

main.lisp

; define a function to divide numbers
(defun divide (a b)
   (/ a b))

; calculate function will throw error as y is 0
(defun calculate (x)
   (let ((y 0))
      (divide x y)))

; call calculate with 10
(calculate 10)

Output

When you execute the code, it returns the following result −

/: division by zero

When (calculate 10) is called, it causes an error as divide by zero. In case of debugger tools, it will show a backtrace similar to given below −

ERROR /: division by zero
Backtrace:
   (DIVIDE 10 0)
   (CALCULATE 10)

In divide frame, we can inspect values of a and b and in Calculate frame, we can inspect x and y. We can even restart the execution by changing the value of b.

Usage of print, format and Logging

• Strategic Output − We can use print or format statements at key statements in our code. This is a simple technique to check the flow of code with the data.

• Conditional Printing − We can print information on certain conditions using when or if statements.

• Logging − We can use Logging libraries for complex application to log information as per severity(info, debug, warn or error).

Tracing

• Track Function Calls − tracing using trace is a built-in facility in LISP and allows us to monitor function calls like when a function is entered and exited along with their arguments and return values.

• Untracing − Once tracing is done, it is better to untrace once monitoring is no longer needed.

main.lisp

; define a factorial function
(defun factorial (n)
   (if (zerop n)
      1
      (* n (factorial (1- n)))))
; start tracing of factorial function
(trace factorial)
(factorial 3)
; stop tracing of factorial function
(untrace factorial)

Output

When you execute the code, it returns the following result −

WARNING: TRACE: redefining function FACTORIAL in main.lisp, was defined in top-level
;; Tracing function FACTORIAL.
1. Trace: (FACTORIAL '3)
2. Trace: (FACTORIAL '2)
3. Trace: (FACTORIAL '1)
4. Trace: (FACTORIAL '0)
4. Trace: FACTORIAL ==> 1
3. Trace: FACTORIAL ==> 1
2. Trace: FACTORIAL ==> 2
1. Trace: FACTORIAL ==> 6
WARNING: UNTRACE: redefining function FACTORIAL in main.lisp, was defined in top-level

Here we can check, calls to factorial with arguments and returned value at each step.