Názor k článku Programování v jazyce BASIC je životním stylem – již šedesát let od Pavel Křivánek - The function FOO has a BREAK. CL-USER 76 >...

The function FOO has a BREAK.

CL-USER 76 > (defun foo (a)
               (+ (break) (sin a)))
FOO

CL-USER 77 > (foo 10)

Break.
  1 (continue) Return from break.
  2 (abort) Return to top loop level 0.

Type :b for backtrace or :c <option number> to proceed.
Type :bug-form "<subject>" for a bug report template or :? for other options.

We are in the break lets get a quick backtrace and move to the right stack frame:

CL-USER 78 : 1 > :bq

INVOKE-DEBUGGER <- BREAK <- FOO <- EVAL <- CAPI::CAPI-TOP-LEVEL-FUNCTION <- CAPI::INTERACTIVE-PANE-TOP-LOOP <- MP::PROCESS-SG-FUNCTION

CL-USER 79 : 1 > :n
Call to INVOKE-DEBUGGER

CL-USER 80 : 1 > :n
Call to BREAK

CL-USER 81 : 1 > :n
Interpreted call to FOO

What is the current source?

CL-USER 82 : 1 > :lambda
(LAMBDA (A) (DECLARE (SYSTEM::SOURCE-LEVEL #<EQ Hash Table{0} 8220104A8B>)) (DECLARE (LAMBDA-NAME FOO)) (+ (BREAK) (SIN A)))

Let's change the code:

CL-USER 83 : 1 > (setf (third (fifth *)) '(* a 4))
(* A 4)

Let's see the new code.

CL-USER 84 : 1 > :lambda
(LAMBDA (A) (DECLARE (SYSTEM::SOURCE-LEVEL #<EQ Hash Table{0} 8220104A8B>)) (DECLARE (LAMBDA-NAME FOO)) (+ (BREAK) (* A 4)))

Move to the BREAK stack frame and return 2 from there:

CL-USER 85 : 1 > :p
Call to BREAK

CL-USER 86 : 1 > :ret 2
42

As you can see, we have changed the running source code, without the need to restart anything.