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bug#29684: exception printers - request for improvement

From: David Pirotte
Subject: bug#29684: exception printers - request for improvement
Date: Sun, 1 Jul 2018 19:23:21 -0300

Hello Ludovic,

> ...
> It seems to me that you can achieve similar effect, at least in some
> cases, by parameterizing ‘current-error-port’.  It also makes more sense
> to me to parameterize ‘current-error-port’ rather than set
> ‘exception-format’, because the ‘exception-format’ is very constrained:
> it has to implement at least everything ‘simple-format’ implements.
> Besides, note that there’s also ‘set-exception-printer!’.  It’s a
> different kind of customization, but it can serve a similar purpose.

To be honest, exactly how does not really matter, I leave that to you 
I have used the code, actually written by Daniel, who also claimed it was not
complete, iirc, to raise the issue, so we can discuss it between us:

        I think we should be very (very) friendly to new Guile users, and 
        those who do not know scheme (yet);

        I think I should be able to use Guile-CV to teach basic image 
        to a class of young teenagers, who would not even know what scheme is, 
        means, imo, no guile customization should be required:

        (use-modules (cv))
        (im-load "my-preferred-penguin.png")

        - at this point, even if the image is very small, guile is 'lost' [*]
        - same if I teach them to compose a few image ops, they commit
        a mistake while typing .. bang, the exception printer is 'lost'

It is perfectly fine, on the opposite side, to ask advanced scheme users (who 
want to, but I really don't see the point) configure guile s the repl and the
exception printer so it tries to print the full content of huge lsts, vectors, 


[*]     one could argue i could have chosen another data structure, but well, I
        did not :), and imo, we can't tell users who have to manipulate large
        structures (arrays, vector, think about non s/w engineers scientists,
        biologists ...),        "... don't manipulate large structures in a 
repl, wrap them
        in a record first ..."

        it happened to me at university, trying to make a demo of Guile-CV to a 
        of postdoc candidates: I updated guile the day before and forgot to 
        both the repl and the exception printers, opened an image, and ... bang!

        I was in emacs, so ... after a few seconds they all laughed and ... by
        the time I could recover, repatched guile ... they were all ...
        gone! (and convinced they should stick to python and open-cv or java and
        imagej ...

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