Subject: bug#31641: 26.1; iter-do variable not left unused warning

From: Lars Ingebrigtsen <[email-masked]>
Date: Sat, 06 Feb 2021 11:31:49 +0100

Stefan Monnier <[email-masked]> writes:

> You can clean up this semantics and the warning at the same time by
> using an expansion like:
>
>     (let (#3=#:iter-do-result11
>           (#1=#:iter-do-iterator-done8 nil)
>           (#2=#:iter-do-iterator10 i))
>       (while (not #1#)
>         (let ((_ (condition-case #4=#:iter-do-condition9
>                      (iter-next #2#)
>                    (iter-end-of-sequence
>                     (setf #3# (cdr #4#))
>                     (setf #1# t)))
>         (unless #1# [BODY])
>       #3#)

So I tried this...

diff --git a/lisp/emacs-lisp/generator.el b/lisp/emacs-lisp/generator.el
index 9eb6d95964..dfd2513350 100644
--- a/lisp/emacs-lisp/generator.el
+++ b/lisp/emacs-lisp/generator.el
@@ -725,17 +725,18 @@ iter-do
         (condition-symbol (cps--gensym "iter-do-condition"))
         (it-symbol (cps--gensym "iter-do-iterator"))
         (result-symbol (cps--gensym "iter-do-result")))
-    `(let (,var
-           ,result-symbol
+    `(let (,result-symbol
            (,done-symbol nil)
            (,it-symbol ,iterator))
        (while (not ,done-symbol)
-         (condition-case ,condition-symbol
-             (setf ,var (iter-next ,it-symbol))
-           (iter-end-of-sequence
-            (setf ,result-symbol (cdr ,condition-symbol))
-            (setf ,done-symbol t)))
-         (unless ,done-symbol ,@body))
+         (let ((,var
+                (condition-case ,condition-symbol
+                    (iter-next ,it-symbol)
+                  (iter-end-of-sequence
+                   (setf ,result-symbol (cdr ,condition-symbol))
+                   (setf ,done-symbol t)))))
+           (unless ,done-symbol
+             ,@body)))
        ,result-symbol)))
 
 (defvar cl--loop-args)

But then this fails:

1 unexpected results:
   FAILED  iter-lambda-variable-shadowing

Uhm...  oh, it fails even without that change?  ...  If I just say "make
check", then it doesn't fail, but if I say "make generator-tests", then
it fails?  Very odd.

> BTW, I think we can remove the duplicate #1 test by moving the body of
> the `while` into its test, e.g.:
>
>     (let (#3=#:iter-do-result11
>           (#1=#:iter-do-iterator-done8 nil)
>           (#2=#:iter-do-iterator10 i))
>       (while
>         (let ((_ (condition-case #4=#:iter-do-condition9
>                      (iter-next #2#)
>                    (iter-end-of-sequence
>                     (setf #3# (cdr #4#))
>                     (setf #1# t)))))
>           (unless #1#
>             [BODY]
>             t)))
>       #3#)

Indeed.  I've now pushed something like this to Emacs 28.

> It's too bad that [BODY] can throw `iter-end-of-sequence`, since
> otherwise we could move the `condition-case` outside of the loop and get
> something more efficient.

Yup.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no