💾 Archived View for thebird.nl › gn-gemtext-threads › topics › lisp › define-condition.gmi captured on 2023-06-14 at 14:31:14. Gemini links have been rewritten to link to archived content

-=-=-=-=-=-=-

Common Lisp Condition Handling

One of Lisp's great features is its condition system.

Every Common Lisp implementation has a condition system.

... conditions are more general than exceptions in that a condition can

represent any occurrence during a program's execution that may be of

interest to code at different levels on the call stack.

The following three quotes are abridged from a conversation with

Ed Langley over #common-lisp:matrix.org (Thu Dec 23 07:46:30 PM EST 2021)

Ed Langley's GitHub

Why is condition handling special in common lisp compared to other languages?

Because code can define recovery strategies without handling the condition.

Any bit of code can be wrapped with restart-case to define a

restart. This restart defines a way you could continue from any condition

that’is signaled within the dynamic extent of the restart-case.

Your condition handler can do something like (find-restart ‘continue

c) to find the CONTINUE restart associated with condition C. If the

restart exists, it can use invoke-restart to jump to the restart which

runs and then execution continues like normal after the restart-case form.

-- end of coversation

Restarts, restarts, restarts: interactive choices in the debugger

Restarts are the choices we get in the debugger, which always has

the RETRY and ABORT ones. By handling restarts we can start over the

operation as if the error didn't occur (as seen in the stack).

(divide 3 0)
;; Y can not be zero. Please change it
;;    [Condition of type SIMPLE-ERROR]
;;
;; Restarts:
;;  0: [CONTINUE] Retry assertion with new value for Y.  <--- new restart
;;  1: [RETRY] Retry SLIME REPL evaluation request.
;;  …