"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.
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.
> 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.
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
