It enters the debugger due to the error. In the debugger menu you see additional entries YES and NO.
yes or no?
[Condition of type SIMPLE-ERROR]
Restarts:
0: [YES] YES
1: [NO] NO
2: [RETRY] Retry SLIME REPL evaluation request.
3: [*ABORT] Return to SLIME's top level.
4: [ABORT] abort thread (#<THREAD "repl-thread" RUNNING {1003167FA3}>)
See? All debugger menus are actually implemented as the restarts, set up at various call stacks. In this implementation of the debugger the way to select YES is to enter 0 or click it on the emacs buffer. In another debugger, it could be entering ":r 0" to the stdin. You can also implement your debugger function, which `read-line` the input stream and only recognizes a specific string, y or n, and set your function to `debugger-hook` to use it (I forgot to mention this in the above example).
Ok right. That's why I feel in Clojure it doesn't provide as much, because it doesn't hook into a debugger.
Having said that, practically, you wouldn't release a tool which has users interact at the debugger, at least for any level of more serious commercial work no?
CL's debugger is just a function, and the key is not to be distracted by the name "debugger".
As said, you can have a custom "debugger" which does not look like a debugger, but a general purpose user interface in the text terminal. For example, being "custom", it can also hide the restarts which are only meaningful for true debugging (like restart 2,3,4). (see detail [1])
It is possible for a debugger to even invoke a GUI (imagine a debugger hook which pops up a window with an OK button) or a web UI (sends a Websocket message to the client, wait for the user to click something on the browser, receive a reply and invoke the restart). In fact, SLIME is written this way: it overloads the debugger hook, and it sends and receives messages to the Emacs TCP client (thus you can click on the menu on an Emacs buffer to restart -- this action is notified back to the underlying lisp process via TCP, which then calls invoke-restart).
[1] You can obtain the list of restarts with COMPUTE-RESTARTS and filter unassociated restarts (similar to a method dispatch but is temporary / has dynamic extent -- see http://clhs.lisp.se/Body/09_adbd.htm).
