When developing source code, mistakes may prevent your programs from working properly, or even at all. Sometimes you can see what is causing a bug in a program immediately, and correcting it is trivial. For example, you might make a spelling mistake while typing, which you may instantly notice and correct.
More often, however, you need to spend time studying the program and the errors it caused before you can debug it. This is especially likely when you are developing large or complex programs.
A debugger tool is provided to make this process easier. This tool is a graphical front-end to the command line debugger which is supplied with your Lisp image. In order to get the best use from the debugger tool, it is helpful if you are familiar with the command line debugger supplied.
The debugger tool can be used to inspect the behavior of programs which behave in unexpected ways, or which contain Common Lisp forms which are syntactically incorrect.
The command line debugger is invoked automatically whenever errors occur. There are two ways that you can invoke the debugger tool:
The command line debugger can be entered by signaling an error in interpretation or execution of a Common Lisp form. For each error signaled, a further level of the debugger is entered. Thus, if, while in the debugger, you execute code which signals an error, a lower level of the debugger is entered. The number in the debugger prompt is incremented to reflect this.
(defun thing (number)
(/ number 0))
This function which attempts to divide a number given as an argument by zero.
(thing 12) The call to thing invokes the command line debugger.
Note that you can also invoke the command line debugger by tracing a function and forcing a break on entry to or exit from that function. See the tutorial chapter (Section 2.3) for the example code used in Figures 2.3 and 9.1.