New Arc Out(arclanguage.org)

36 pointsby ramsvor 17 Jahren19 comments

http://arclanguage.org/install

19 comments

jrockwayvor 17 Jahren

Perhaps more interesting is that mzscheme changed a core language feature (mutable cons cells) without providing any backcompat.

pgvor 17 Jahren

Yeah, that was a pretty dramatic move. It means MzScheme is no longer an implementation of the Scheme standard.

The latest standard, R6RS, goes no further than grumbling about mutable pairs and raising the possibility of deprecating them in the future:

http://www.r6rs.org/final/html/r6rs-rationale/r6rs-rationale...

magvvor 17 Jahren

To be fair, the base language of PLT never claimed to support any relevant standard. Instead implementations of standards are built as libraries on top of the base language. So PLT does provide both R5RS and R6RS, both with mutable pairs.

It is when you want to use PLT-specific non-RnRS libraries from RnRS code, you run into pair incompatibilities.

pgvor 17 Jahren

But before this change, I believe every PLT Scheme implementation was a superset of standard Scheme. (Which every Scheme implementation is.) Now it's actually incompatible.

Tichyvor 17 Jahren

So is MzScheme effectively like Erlang now? I thought the immutable memory in Erlang does make some things difficult to code, which would have been a reason for me to prefer Scheme.

khafrevor 17 Jahren

Actually, mutable cons cells are in PLT, they're just created using mcons instead of cons, and accessed with mcar/mcdr. While this does make porting code a bit difficult, it's a very old change - Matthew's original blog post on the subject is dated from end-2007:

http://blog.plt-scheme.org/2007/11/getting-rid-of-set-car-an...

magvvor 17 Jahren

Not really. Only pairs are immutable now; vectors, strings, structures, hash tables and all the rest are still mutable. There's also separate mutable pairs. And mutable bindings, as always.

So no, far from Erlang in mutability.

byvor 17 Jahren

I doubt you would find the immutable aspect in Erlang a problem in practice. I don't know MzScheme, but Erlang has a its own very different philosophy. I suggest Joe's thesis as an interesting read.

http://www.sics.se/~joe/thesis/armstrong_thesis_2003.pdf

Tichyvor 17 Jahren

Well I found it a problem when I tried to program a Sieve Of Eratosthenes for project Euler. Also when I considered a grid based alife simulation. I guess the only way would be to use a process per cell?

byvor 17 Jahren

Sometimes the mutable structures get passed around in the function arguments, so you might do a sequence of tail recursive calls and on each call you pass in the next board state to the same function. I would hope the Erlang Questions mailing list would help you if you posted a small example and explained your troubles to them. There are lots of very clever Erlang people there.

http://www.erlang.org/faq.html

Tichyvor 17 Jahren

Might try it again - not that I didn't enjoy it, it was fun.

torrvor 17 Jahren

Just curious: Why was mzscheme chosen to implement Arc rather than some other Scheme implementation?

pgvor 17 Jahren

Originally we used Scheme48. I forget exactly why we switched. I vaguely recall some problem with threads. I think we chose PLT Scheme because it seemed like the default Scheme implementation.

khandekarsvor 17 Jahren

In case you decide to compile MzScheme 372 from sources on a Fedora 10 box that has SELinux in enforcing mode, you need to take a few (simple) additional steps:

http://aadnyavali.wordpress.com/2009/06/07/making-arc-3-play...

I'm still downloading Fedora 11, but suspect that the same exercise will be needed for that as well.

s-phi-nlvor 17 Jahren

I wonder whether Arc will end up in effect maintaining mutable-cons mzscheme. If this happens, I hope that Arc's focus won't shift to the implementation details.

astinevor 17 Jahren

I wonder if PG can tell us what the new changes are. I'm sure that there is a changelist, but I don't see it.

kylecvor 17 Jahren

Actually, pg has said he hasn't kept track of the changes:

http://arclanguage.org/item?id=9399

Fortunately, someone else has taken the time to analyze the differences between arc2 and arc3:

http://arcfn.com/2009/06/whats-new-in-arc3.html

pgvor 17 Jahren

There were also a couple changes since then. IIRC the most significant are that ontable is gone (each expanded to take its place) and that you can call readc etc with no arg (and read from stdin).

astinevor 17 Jahren

yep, just found that. Thanks.