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

[Michael Heerdegen]

Hi,

consider this case (it appeared to me in real life):

--8<---------------cut here---------------start------------->8---
(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")))
--8<---------------cut here---------------end--------------->8---

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 ...)

i.e. the stream is broken.  I wonder if we could improve this case.  I'm
not sure how, however.  Should referencing the element whose generation
caused the error raise the same error again, or a standardized kind of
error?  Should the whole stream be invalidated in some way?  FWIW, in my
use case I expected the stream to raise the same error again.  Would it
be reasonable to make the stream just retry to calculate the problematic
element?


TIA,

Michael.


In GNU Emacs 27.0.50 (build 27, x86_64-pc-linux-gnu, GTK+ Version 3.24.12)
 of 2019-10-23 built on drachen
Repository revision: 39ba44f18445c7759de5ac91ce25e53123c2abeb
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux bullseye/sid