[pjmlp]

That is partially what I was referring to.

The other part, which might not be visible on that video is the integration of debugger into the REPL, and the ability to redo a piece of code after breaking into the debugger and fixing it.

So you can do something like, REPL => error (ask to start debugger) => track down and fix error => restart error expression => finish the execution of the original REPL expression with the respective result.

[abhirag]

The only debugging experience I find enjoyable in Emacs is debugging Clojure using cider[1] and I think it comes close to what you are describing but I think you might already be aware of that :)

[1] -- (https://github.com/clojure-emacs/cider/blob/master/doc/debug...)

[bitwize]

SLIME doesn't give you this?

[pjmlp]

I don't know, back when I cared about Lisp development on Emacs, SLIME wasn't a thing, as you might understand from my XEmacs reference.

[juki]

SLIME does let you redefine functions and restart a stack frame (assuming the Lisp implementation supports that). You can even do it without using Emacs/SLIME, although that's not very convenient of course. A silly example with SBCL,

    ~ $ rlwrap sbcl --noinform
    CL-USER> (defun foobar (x y)
               (if (evenp x)
                   (/ x y)
                   (* x y)))
    
    FOOBAR
    CL-USER> (foobar 10 0)
    
    debugger invoked on a DIVISION-BY-ZERO in thread
    #<THREAD "main thread" RUNNING {1001BB64C3}>:
      arithmetic error DIVISION-BY-ZERO signalled
    Operation was /, operands (10 0).
    
    Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.
    
    restarts (invokable by number or by possibly-abbreviated name):
      0: [ABORT] Exit debugger, returning to top level.
    
    (SB-KERNEL::INTEGER-/-INTEGER 10 0)
    0] backtrace 3
    
    Backtrace for: #<SB-THREAD:THREAD "main thread" RUNNING {1001BB64C3}>
    0: (SB-KERNEL::INTEGER-/-INTEGER 10 0)
    1: (FOOBAR 10 0)
    2: (SB-INT:SIMPLE-EVAL-IN-LEXENV (FOOBAR 10 0) #<NULL-LEXENV>)
    
    0] down
    (FOOBAR 10 0)
    
    1] source 1
    
    (IF (EVENP X)
        (#:***HERE*** (/ X Y))
        (* X Y)) 
    1] (defun foobar (x y)
         (if (and (evenp x) (not (zerop y)))
             (/ x y)
             (* x y)))
    
    WARNING: redefining COMMON-LISP-USER::FOOBAR in DEFUN
    FOOBAR
    1] restart-frame
    
    0

[pjmlp]

Thanks for the example.