Sv: Is this a bug in while-let or do I missunderstand it?

[arthur miller <arthur.miller@live.com>]

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>Comprarison with a for loop is somewhat strained here. The while-let

I didn't meant to say that while-let was equivalent to that for-loop;
but tried to illustrate the expectaions. I hope it was clear from the rest.

>loop in Elisp is directly analogous to this C++ while loop:
>
>    #include <iostream>
>
>    int main() {
>        while (bool run = true) {
>            std::cout << "running\n";
>            run = false;
>        }
>        std::cout << "out of loop\n";
>    }
>
>and yes, it’s an infloop, too.

Actually didn't know we can introduce new variable in while declaration in
C++; in C it is verbotten:

~/repos/test $ gcc -o test test.c
test.c: In function 'main':
test.c:3:10: error: expected expression before 'int'
    3 |   while (int i = 0) {
      |          ^~~
test.c:4:5: error: 'i' undeclared (first use in this function)
    4 |     i++;
      |     ^
test.c:4:5: note: each undeclared identifier is reported only once for each function it appears in

But that is just a regression (thought it as in C++ too :-)).

>What you’re looking for, though, seems to be a while loop with a
>break, which is expressed as a catch/throw in Elisp.

Yes, that is what I came to as well, if you check the rest of the
response to Eli as I suggested to mention catch/throw or cl-block/cl-return-from
in the docs.

Even better is named-let, which seems to be a general
version of while-let:

(defmacro while-test (spec &rest body)
  (declare (indent defun))
  (let* ((name (gensym "while-let-"))
         (bindings (if (and (consp spec) (symbolp (car spec)))
                       (list spec)
                     spec)))
    `(named-let ,name ,spec
       ,@body
       (if (not (and ,@(mapcar #'car bindings)))
           nil
         (,name ,@(mapcar #'cadr bindings))))))

(pp (macroexpand-1
     '(while-test ((run t))
        (setf run nil))) (current-buffer))

(named-let while-let-141 ((run t))
  (setf run nil) (if (not (and run)) nil (while-let-141 t)))

(pp (macroexpand-1
     '(while-let ((run t))
        (setf run nil))) (current-buffer))

(catch 'done140
  (while t (if-let* ((run t)) (progn (setf run nil)) (throw 'done140 nil))))

As seen, they both expand to equivalent infinite loop.

For the illustration, named-let expands to a nice while loop itself:

(pp (macroexpand-all
     '(while-test ((run t))
        (setf run nil))) (current-buffer))

(let ((run t))
  (let (retval)
    (while
        (let ((run run))
          (progn
            (setq run nil)
            (if (not (and run)) nil (progn (setq run t) :recurse)))))
    retval))

In my personal opinion while-let, while meant to be a "shortcut" to
certain style of expressions is a bit unfortunate name, since the "-let"
part of the name suggest establishing an environment around the body,
however that environment is read only which is not normal semantic of
let-bindings.  In other words, the devil is in the details which perhaps was
 not intentional?

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