"Easy: I have a macro called "defs" that handles mutual recursion"
While I appreciate 'defs, it's a non-answer. The even/odd example I posted and the evenp/oddp example dido posted are idiomatic Arc code. While you and I don't care much about Arc compatibility, it's something dido wants for Arcueid, so these examples should work without modification.
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I'm about to disagree with myself, but first I want to reiterate and clarify what I was saying at "caught me on a technicality":
For this discussion I don't see a much of a reason to distinguish between macros which insert mutable boxes and macros which insert functions. Either system can pretty much support the other as a special case: We can translate spliced boxes into spliced getter/setter functions, and we can translate spliced functions into spliced functions-in-the-box. Because of that equivalence, these systems share the disadvantage of being challenging to serialize.
If dido considers compilation to be important (do you, dido?), then this hygiene approach might be unsuitable, and thus the use of first-class namespaces might be unsuitable. (As I explained at "match the variables the macro author expected," first-class namespaces make hygiene more important.)
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"What I'm talking about happens entirely at compile-time using boxes."
Ah. I think you have a point!
For compiling Nulan to JavaScript, I guess the boxes you're using aren't arbitrary getter/setter functions, and they aren't merely some mutable container either; they're globally associated with a JavaScript variable name. When you compile the macroexpansion result and it contains a (get-variable-box ...) form, you decide on its JavaScript variable name at that time. If the macroexpansion result contains a box, you use the attached variable name to compile it to JavaScript. Am I getting this right? This sounds very workable. :) And whaddayaknow, Nulan works. ^_-
I seem to remember understanding this before, when you and I talked about Nulan compilation in depth, but I guess I had to retrace the steps just now.
Anyhow, get-variable-box is fantastic IMO, but first-class namespaces still might not be ideal for Arcueid due to Arc's unhygienic macros.
dido, are you comfortable with breaking existing Arc macro idioms in favor of hygiene?
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I have a convoluted but surprisingly comprehensive idea of how to integrate get-variable-box into a system that's compatible with unhygienic Arc macros, but I've put it in a separate simultaneous post: http://arclanguage.org/item?id=17464
Actually, it's two separate posts, because it's otherwise too long for the forum. If this becomes a tl;dr scenario, I won't be surprised. ^_^
"While I appreciate 'defs, it's a non-answer. The even/odd example I posted and the evenp/oddp example dido posted are idiomatic Arc code. While you and I don't care much about Arc compatibility, it's something dido wants for Arcueid, so these examples should work without modification."
For this example, let's suppose there was a file "foo.arc" that contained idiomatic Arc code that implements evenp/oddp. This code works in Arc 3.1. It will work in my system as well, because undefined symbols automatically create new boxes. Basically, it'll work, but name collisions are possible, just like in Arc 3.1.
If you then write a new file "bar.arc" that uses hyper-static idioms (var, defs, etc.), it can import "foo.arc" and everything will work fine. "foo.arc" will clobber any existing evenp/oddp definitions, but "bar.arc" will not clobber "foo.arc". And of course "bar.arc" can use "w/include" and "w/exclude" to prevent "foo.arc" from clobbering things.
If you wanted to make it so that "foo.arc" behaves correctly without needing to use "w/include" and "w/exclude", you would indeed need to rewrite it to use "defs". But it's still usable even without a rewrite. So it's a perfectly graceful degradation.
My system is designed so that it can correctly use all existing Arc 3.1 code, while new code is written with the hyper-static idioms. Then, slowly, old code can be migrated to use hyper-static scope, until eventually you could make Arc purely hyper-static.
There's three issues I see with my proposal:
1) If you're writing Arc code in a hyper-static fashion, you really want "arc.arc" to be changed to be hyper-static. But old Arc code will need the non-hyper-static "arc.arc". I think the simplest solution to this is to have two versions of "arc.arc", one that uses hyper-static scope, and one that doesn't. Then you would need to make sure to load the non-hyper-static version before loading Arc 3.1 code. This could be automated a tiny bit by using a macro, something like "w/arc3".
2) "load" occurs at run-time, which is why my definition of "w/include" needed to use "eval". Nulan doesn't have this problem because file importing occurs at compile-time. Perhaps the best way to solve this is to keep "load" as-is, and add in a new "import" macro that does all its work at compile-time.
3) If you think (eventually) making Arc purely hyper-static is a bad thing, you won't like my proposal.
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"Am I getting this right? This sounds very workable. :) And whaddayaknow, Nulan works. ^_-"
Yes, that's more or less correct. The one detail that's different is... Nulan doesn't have a "get-variable-box" function. The reason is because "quote" internally uses (the equivalent of) "get-variable-box". So in Nulan, rather than using "get-variable-box", you'd just use "quote". And if you want to break hygiene, you'd explicitly use the "sym" function.
I mostly followed along, but I don't understand "It will work in my system as well, because undefined symbols automatically create new boxes." You were talking about having them create new boxes at assignment time, and I was recommending compiling-a-reference-time instead so that we don't get an unbound variable error in the first definition.
How it works is, anytime the compiler sees an undefined symbol, it creates a new box for it like as if it had been created with "var".
Another way to think about it is... the compiler would replace this:
(= foo (fn () ... bar ...))
(= bar (fn () ... foo ...))
With this:
(var bar)
(var foo)
(= foo (fn () ... bar ...))
(= bar (fn () ... foo ...))
What happened is, when it encountered the undefined variable "bar", it created a new box for it. Then it encountered the undefined variable "foo", so it created a new box for it. Then it did the assignments like normal.
Given how you said "compiling-a-reference-time", I think we're talking about the same thing. Why did you mention assignment time?
We've just had a long exchange about you creating boxes at assignment time and me using compiling-a-reference time instead. Here's a recap:
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You: Here's how you do it. The definition of "=" is the same: if the variable exists, mutate it, otherwise create a new variable. But now you add in a new primitive called "var"[...]
Me: The behavior I'd use is that any compile-time variable access (even under a lambda) creates a new, uninitialized variable binding if a binding doesn't already exist.
You: Yeah I'd do that too, if I wanted to graft dynamic variables onto a hyper-static system. But since Arc uses dynamic variables, I proposed to graft hyper-static onto it instead.
Me: How do you make the even/odd code work? Under the approach you described, the first line refers to an undefined variable (odd), and I interpret that as an error. I was recommending a fix.
You: Easy: I have a macro called "defs" that handles mutual recursion
Me: While I appreciate 'defs, it's a non-answer. The even/odd example [...] should work without modification.
You: It will work in my system as well, because undefined symbols automatically create new boxes.
Ah, sorry, huge miscommunication and misunderstanding on my part. I've actually been agreeing with you all along.
A large part of the problem is that I've been thinking about my proposal as two separate parts: one part deals with backwards compat with Arc, and the other part describes a hyper-static system for Arc.
When I was talking about "defs", I was talking about the hyper-static part. But you were talking about the backwards compat part. Hilarity (?) ensues.
