> On Apr 22, 2016, at 1:47 PM, 'John Clements' via Racket Users
> <racket-users@googlegroups.com> wrote:
>
> Currently, the handin-server runs student expressions in an ‘eval’ which
> intercepts errors and re-raises them with a message that includes the failing
> expression. All good.
>
> However, it doesn’t catch all of them. Specifically, if the exception
> contains any values that are opaque to struct->vector, it gives up and
> re-raises the exception as-is.
>
> This turns out to cause a problem with “match” failures, which include such
> values. This causes a problem for my students, because they’re unable to see
> the text of the test cases that they failed.
>
> It’s easy enough to hack around this in my code by re-wording ‘match’
> failures in the same way that wrap-evaluator does. In general, though, it
> seems like there’s no good reason that ‘match’ failures shouldn’t go into the
> same bin as division by zero, applying a non-function, and all of the other
> things that can go wrong during evaluation.
>
> In order to fix this, then, I’m trying to determine why this check exists:
> what exceptions do you *not* want to re-word here?
Okay, answering my own question and asking another:
The fundamental reason for the existence of this logic is that the
handin-engine is trying to be careful, and modify exceptions that can safely be
reconstructed. If the “skipped” value is #t, or if one or more of the values in
the structure are opaque, this is impossible. At this point, the handin engine
just throws up its hands and decides to follow the hippocratic oath, “first do
no harm,” and let the exception continue as is.
So, this leads to a different question: *why* is the match exception different
from all the other exceptions? Based on my reading of the struct-info
documentation, it appears that the ‘match’ form constructs exceptions whose
inspector is not the current one, or perhaps that it fails to declare itself as
transparent.
My guess is that this is just an oversight, and that a ‘match’ exception should
be just as transparent as, say, a division by zero exception. Here’s code that
illustrates the difference:
#lang racket
(with-handlers ([exn:fail?
(λ (exn)
(struct-info exn))])
(/ 1 0))
(with-handlers ([exn:fail?
(λ (exn)
(struct-info exn))])
(match 13
['a 4]))
So, here’s my question:
Is there a good reason for the difference between the `match` exception
(skipped is #t) and the division-by-zero exception (skipped is #f) ?
John
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