4.3 The Stepper

4.3.3 Reading Stepper output

The following example shows a sample session with the Stepper:

> (step (cdr '(1 2 3)))
  (CDR (QUOTE (1 2 3))) -> :n
    (FUNCTION CDR) -> :n
    #<Compiled-Function CDR 101302F>
    (QUOTE (1 2 3)) = (1 2 3)
  (2 3)
(2 3)

The Stepper output demonstrates Lisp's evaluation process:

Lisp retrieves the function binding of the first element in the list, the functioncdr, which is a compiled function.
Lisp evaluates the remaining elements of the list. The next element is a quoted list, which evaluates to the list(1 2 3).
Lisp applies the function binding of the first element of the list, the symbolcdr, to the result of evaluating the remaining elements of the list.
Lisp returns the list(2 3).

The following variables, which are extensions to Common Lisp, allow you to adjust the indentation and number of columns in the Stepper's output:

*max-step-indentation*
*step-columns-per-level*

In addition, the variables*debug-print-length* and*debug-print-level* control the length and level of printing within the Stepper. Whenstep is called on a form, the variable*print-level* is bound to the value of*debug-print-level*, and the variable*print-length* is bound to the value of*debug-print-length*. You can change the printing level and length by binding the variables*debug-print-level* and*debug-print-length* to a fixnum integer greater than or equal to 1. See Chapter 3, "Debugging Lisp Programs" for more information.

The User's Guide - 9 SEP 1996

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