How Arguments are Printed - SBCL 1.0.31 User Manual
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#### 5.3.2 How Arguments are Printed

A frame is printed to look like a function call, but with the actual argument values in the argument positions. So the frame for this call in the source:

         (myfun (+ 3 4) 'a)

would look like this:

         (MYFUN 7 A)

All keyword and optional arguments are displayed with their actual values; if the corresponding argument was not supplied, the value will be the default. So this call:

         (subseq "foo" 1)

would look like this:

         (SUBSEQ "foo" 1 3)

And this call:

         (string-upcase "test case")

would look like this:

         (STRING-UPCASE "test case" :START 0 :END NIL)

The arguments to a function call are displayed by accessing the argument variables. Although those variables are initialized to the actual argument values, they can be set inside the function; in this case the new value will be displayed.

`&rest` arguments are handled somewhat differently. The value of the rest argument variable is displayed as the spread-out arguments to the call, so:

         (format t "~A is a ~A." "This" 'test)

would look like this:

         (FORMAT T "~A is a ~A." "This" 'TEST)

Rest arguments cause an exception to the normal display of keyword arguments in functions that have both `&rest` and `&key` arguments. In this case, the keyword argument variables are not displayed at all; the rest arg is displayed instead. So for these functions, only the keywords actually supplied will be shown, and the values displayed will be the argument values, not values of the (possibly modified) variables.

If the variable for an argument is never referenced by the function, it will be deleted. The variable value is then unavailable, so the debugger prints ``#<unused-arg>`' instead of the value. Similarly, if for any of a number of reasons the value of the variable is unavailable or not known to be available (see Variable Access), then ``#<unavailable-arg>`' will be printed instead of the argument value.

Note that inline expansion and open-coding affect what frames are present in the debugger, see Debugger Policy Control.