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3 points by rocketnia 303 days ago | link | parent

"It could just be that the tables are not serialised as something Racket reads as mutable hashes when deserialising it back again?"

I'm pretty sure the Racket reader never reads a mutable hash, but that it's possible for a custom reader extension to do it.

Some of Racket's approach to module encapsulation depends on syntax objects being deeply immutable. In particular, a module can export a macro that expands to (set! my-private-module-binding 20) but which uses `syntax-protect` so that the client of that macro can't use the `my-private-module-binding` identifier for any other purpose. If the lists constituting a program's syntax were usually mutable, then it would be hard to stop the client from just mutating that expansion to make something like (list my-private-module-binding 20), giving it access to bindings that were meant to be private.

I think this is why Racket's `read-syntax` creates immutable data. As for why `read` does it too, I think it's just a case of `read` being a relatively unused feature in Racket. They don't have many use cases for `read` that they wouldn't rather use `read-syntax` for, so they don't usually have reasons for the behavior of `read` to diverge from the behavior of `read-syntax`.

All this being said, they could pretty easily add built-in syntaxes for mutable hashes, but I think it just hasn't come up. Who ever really wants to read a mutable value? In Racket, where immutable values are well-supported by the core libraries, you aren't gonna need it. In the rare case you want it, it's easy enough to do a deep traversal to build the mutable structure you're looking for (like my example `correcting-arc-read` does).

It only comes up as a particular problem in Arc. Arc's language design doesn't account for the existence of immutable values at all, so working around them when they appear can be a bit quirky.



3 points by hjek 302 days ago | link

> I'm pretty sure the Racket reader never reads a mutable hash, but that it's possible for a custom reader extension to do it.

No..?

    > (require racket/serialize)
    > (define basket (make-hash))
    > (hash-set! basket 'fruit 'apple)
    > (write-to-file (serialize basket) "basket.txt")
    > (define bucket (deserialize (file->value "basket.txt")))
    > (hash-set! bucket 'fruit 'banana)
    > bucket
    '#hash((fruit . banana))

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4 points by rocketnia 302 days ago | link

The Racket reader reads a seven-element list there, not a mutable hash.

Looks like you're proposing to use a two-stage serialization format. One stage is `read` and `write`, and the other is `serialize` and `deserialize`. At the level of language design, what's the point of designing it this way? (Why does Racket design it this way, anyway?)

I can see not wanting to have cyclic syntax or syntax-with-sharing in the core language just because it's a pain in the neck to parse and another pain in the neck to interpret. Maybe that's reason enough to have a separate `racket/serialize` library.

But isn't the main issue here that Arc's `read` creates immutable tables when the rest of the language only deals with mutable ones? The mutable tables go through `write` just fine, but `read` doesn't read the same kind of value that was written out. If and when this situation is improved, I don't see where `racket/serialize` would come into play.

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2 points by hjek 301 days ago | link

> Looks like you're proposing to use a two-stage serialization format.

I wasn't really proposing anything, only pointing out that it's not the case that Racket never can read mutable hashes, and then illustrating that with a code example.

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3 points by rocketnia 300 days ago | link

Hmm, all right. I didn't want to believe that was the point you were trying to make. In that case, I think I must not have conveyed anything very clearly in the `correcting-arc-read` comment.

I've been trying to respond to this, which was your response to that one:

"Racket also has mutable hashes created using `make-hash` rather than `hash`. It could just be that the tables are not serialised as something Racket reads as mutable hashes when deserialising it back again?"

In the `correcting-arc-read` comment, I used `make-hash` in the implementation of `correcting-arc-read`, so I assumed your first sentence was for the edification of others. I found something to respond to in the second, which kinda pattern-matched to a question I had on my mind already ("Can't the Racket reader just construct a mutable table since what was written was a mutable table?").

As for my response to your "No..?"...

One of the purposes of `correcting-arc-read` is that (when it's used as a drop-in replacement for Arc's `read`) it makes `readfile1` return mutable tables. So if anyone had to be convinced that a reader that returned mutable hashes could be implemented at all in Racket, I thought I had shown that already. When it looked like you might be trying to convince me of something I had already shown, I dismissed that idea and thought you were instead trying to clarify what your proposed alternative to `correcting-arc-read` was.

Seems like I've been making bad assumptions and that as a result I've been mostly talking to myself. Sorry about that. :)

Maybe I oughta clarify some more of the content of that `correcting-arc-read` comment, but I'm not sure what parts. And do you figure there are any points you were making that I could still respond to? I'd better not try to assume what those are again. :-p

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3 points by kinnard 301 days ago | link

Sounds like I opened a can of . . . lists . . .

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