[racket users] describe variant issue?

Kevin Forchione

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Sep 15, 2020, 5:47:17 PM9/15/20

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Hi guys,

I’m not sure why the describe library’s variant is always returning ‘simple regardless of numeric value. For instance, the docmentation says:

(variant 1) -> fixnum-integer

But when I run it under Dr Racket 7.8 [cs] it returns ‘simple for this, as well as for 10.3 as well as pi, 2/3, and 0+1i.

Any ideas why this is happening?

Thanks!

Kevin

Sam Tobin-Hochstadt

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Sep 15, 2020, 6:11:41 PM9/15/20

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This is a difference in behavior between Racket BC and Racket CS, and
not something in the describe library:

[samth@homer:~/work/teaching/c211 (master) racket-7.8] racket
Welcome to Racket v7.8.
> (struct->vector 5)
'#(struct:fixnum-integer ...)
> ^D
[samth@homer:~/work/teaching/c211 (master) plt] racket
Welcome to Racket v7.8.0.9 [cs].
> (struct->vector 5)
'#(struct:simple ...)

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Kevin Forchione

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Sep 17, 2020, 1:17:19 PM9/17/20

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> On Sep 15, 2020, at 3:11 PM, Sam Tobin-Hochstadt <sa...@cs.indiana.edu> wrote:
>
> This is a difference in behavior between Racket BC and Racket CS, and
> not something in the describe library:
>
> [samth@homer:~/work/teaching/c211 (master) racket-7.8] racket
> Welcome to Racket v7.8.
>> (struct->vector 5)
> '#(struct:fixnum-integer ...)
>> ^D
> [samth@homer:~/work/teaching/c211 (master) plt] racket
> Welcome to Racket v7.8.0.9 [cs].
>> (struct->vector 5)

> '#(struct:simple …)

Is this going to be the go-forward stance for Racket? Or is this still a work in progress? Will it be left up to the code to determine types through predicate checking?

Thanks!

Kevin

Sam Tobin-Hochstadt

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Sep 17, 2020, 1:30:22 PM9/17/20

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We may change Racket CS so that it produces the same results, but in
general the results of `struct->vector` on values that are opaque is
not something that should be relied on.

The describe library is, I believe, unmaintained, and hasn't been
updated in many years.

Sam

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Matthew Flatt

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Sep 17, 2020, 1:33:57 PM9/17/20

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A PR to improve `struct->vector` for CS would be welcome. There
relevant code is here:

https://github.com/racket/racket/blob/master/racket/src/cs/rumble/struct.ss#L1221

As you'll see, the synthesized name is currently whatever comes out of
Chez Scheme's `inspect/object` interface. It could instead involve a
case dispatch on fixnums, bignums, etc., to improve on the `simple`
label that `inspect/object` produces. There may also be Chez Scheme
versus Racket terminology to bridge, such as `byte-string` instead of
`bytevector`.

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