GNU bug report logs - #47677
[PATCH] condition-case success continuation

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From: Gregory Heytings <gregory <at> heytings.org>
To: Mattias Engdegård <mattiase <at> acm.org>
Cc: 47677 <at> debbugs.gnu.org
Subject: bug#47677: [PATCH] condition-case success continuation
Date: Mon, 26 Apr 2021 21:57:46 +0000

>
> This patch adds the condition-case handler syntax
>
>  (:success BODY)
>
> for code executed when the protected form terminates without error. BODY 
> is then executed with the variable bound to the result of the protected 
> form, and the result of BODY is then the value of the condition-case 
> form as usual.
>
> This plugs an annoying hole in elisp: there hasn't been any direct 
> access to the success continuation
>

Would a macro not be enough to cover that particular programming pattern? 
E.g. something like:

(defmacro success-case (value action continuation &rest handlers)
  `(catch 'success-case-fail
     ((lambda (,value) ,continuation)
      (catch 'success-case-success
        (throw 'success-case-fail
               (condition-case ,value
                   (throw 'success-case-success ,action) ,@handlers))))))

With this macro you can write for example:

(success-case result
    (read buffer)
  (use result)
  (end-of-file 'eof))

which is IMO even more concise and elegant.

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