bug#37888: 27.0.50; Streams and errors in element generation

[Michael Heerdegen]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> (defun test-stream (n)
>   (stream-cons n (if (< n 0) (error "test") (test-stream (1- n)))))
>
> (setq my-stream (test-stream 10))
>
> (condition-case nil (seq-length my-stream)
>   (error (message "Hmm, didn't work so well")))
>
> Now, what happened to `my-stream' after evaluating this?  If you try to
> use it, you get a quite confusing error:
>
> (seq-length my-stream)
>
> |-- stream--force: Wrong type argument: streamp, (((n . -1) t) nil ...)

Would something like this make sense (Noam)?

From 4c778f26ff8d56d0e7018305aa3d46caa2f9fb38 Mon Sep 17 00:00:00 2001
From: Michael Heerdegen <michael_heerdegen@web.de>
Date: Wed, 23 Oct 2019 16:55:01 +0200
Subject: [PATCH] WIP [stream] Fix Bug#37888

---
 packages/stream/stream.el | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/packages/stream/stream.el b/packages/stream/stream.el
index 9f73e8b861..d401fb7e3c 100644
--- a/packages/stream/stream.el
+++ b/packages/stream/stream.el
@@ -86,8 +86,8 @@ That value is the one passed to `stream-make'."
    ((eq (car-safe stream) stream--evald-identifier)
     (cdr stream))
    ((eq (car-safe stream) stream--fresh-identifier)
-    (setf (car stream) stream--evald-identifier)
-    (setf (cdr stream) (funcall (cdr stream))))
+    (prog1 (setf (cdr stream) (funcall (cdr stream)))
+      (setf (car stream) stream--evald-identifier)))
    (t (signal 'wrong-type-argument (list 'streamp stream)))))

 (defmacro stream-cons (first rest)
--
2.23.0


Michael.