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4 points by rocketnia 56 days ago | link | parent

This came up during ar development, although I don't remember who brought it up. I think aw was about to implement it but was concerned that it would be annoying at the REPL to have all the program's bindings be looked up eagerly (aka for them to not be "hackable"). It could be annoying for mutually recursive macros, too, although those are already tricky enough that it probably wasn't a big concern at the time.

I remember recommending an upgrade: Instead of generating:

  `(,my-func ,a ,b)
Generate something that preserves the late binding of mutable global variables, like this:

  `((',(fn () my-func)) ,a ,b)
I seem to remember aw wasn't convinced this cruft was going to be worth whatever hygiene it gained, even after I recommended a syntactic upgrade:

  `(,late.my-func ,a ,b)
Since aw values concision a lot, perhaps the issue was that most programmers would surely just write ,my-func in the hope that it would be rare (or even incorrect) to ever want to rebind it, and then other programmers would suffer from that decision.

But Pauan followed a design path like this in Nulan and/or Arc/Nu. In Pauan's languages... actually, I think I remember a couple of approaches, and I don't remember which ones were real.

One approach I think I remember is that `(my-func a b (another-func c) d) inserted mutable boxes for my-func and another-func into the result, but it would just implicitly unquote a, b, c, and d, because variables at the beginning of a list are likely to be mutable globals and variables in other positions are likely to refer to gensyms.

There might have even been an auto-gensym system in that quasiquote operator at some point.

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I liked this approach a lot at the time. That was the same time I was working on Penknife. I was trying to avoid name collision with first-class namespaces in Penknife, but I still wanted macros to work, and I was trying to take a very similar approach. (I couldn't take exactly the same approach because my macroexpansion results were strings; instead, I think I allowed regions of the macroexpansion result to be annotated with a first-class namespace to use for name lookup.)

When Penknife's compile times were abysmally long, I started to realize that even if I found an optimization, this was going to be a problem with macros in general. Anyone can write an inefficient macro, and anyone who can put up with it can build a lot of stuff on top of it that other users won't be able to appreciate. So I started to require separate compilation in my language designs.

With separate compilation in mind as a factor for the language design, it no longer made sense to put unserializable values into the compiled code. Instead, in Penknife's case, I devised a system of namespace paths to replace them. The namespaces were still first-class values, but one thing you could do with a Penknife macro was get hold of its first-class definition-site namespace, so the macroexpanded code could refer to variables in distant namespaces by specifying a chain of names of macros to look them up from. And this kept things hackable, too, since you could mutate a macro's definition-time namespace explicitly (not that many programs or REPL sessions would bother to do that).

Not long after, I set a particular goal to make a language (Era) where the built-in functionality was indistinguishable from libraries. Builtins aren't hackable from within the language, so the module system needs to make it possible (and ideally easy) for people to write non-hackable libraries.

(Technically they only need to be non-hackable to people who don't know the source code, because once you know the source code, you know you're not dealing with builtins. I intend to take advantage of this to make the language hackable after all, but it's going to take essentially a theorem prover in the module system before it's useful to tell the module system you have the source code of a module, as opposed to just using that source code to compile a new module of your own behind the module system's back.)

Anyhow, this means I haven't put hackability at the forefront for a long time.

I think the embedding-first-class-values approach will work, and I think late binding is workable (by using late.my-func) and there's a workable variation of that late binding approach to enable separate compilation too (by using namespace paths made out of chains of macro names). So I like it, but I just have this stuff to recommend for it to make it really tick. :)

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By the way, it's good to see you! I wondered how you were doing.





3 points by waterhouse 42 days ago | link

With interpreter semantics, in which a macro gets expanded anew every time a function is called, the late binding comes for free. ;-) Then, if you want the runtime performance that comes from compilation, you optimize for the case where people are not redefining functions, and invalidate old compilation results when they do. I think that rough plan should be doable, though I haven't gotten around to implementing enough of a system to say how well it works. But I think that's the only way to get anything close to good performance in Javascript VMs (not that they expand macros, but I expect they inline function calls and such, which requires similar assumptions about global definitions), and it seems to have been done.

For separate compilation, it does seem clear that what gets serialized will be references like "the object [probably a function] named 'foo in module bar", and structures (s-expressions or otherwise) containing such references. Given that compilation implies macroexpansion, you do have to assume (or verify) that the macros from other modules are what they used to be—and that non-macros (used in functional position at least) are still non-macros. If you have a full-blown Makefile kind of build system, then by default I suppose every file's output depends on the contents of every other file that it uses; or, as an optimization, depends merely on the exact set and definitions of macros exposed from those files. (In the C++ system I encounter at work, code is separated into .cpp and .h files, and editing a .h file causes the recompilation of every .cpp file that recursively depends on it, but editing a .cpp file only causes its own recompilation. If you wanted to imitate that, I guess you'd put macros into a distinctively named set of files, and forbid exportable macros anywhere else.)

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Thanks! I've sold out and have been working for a medium-sized company doing mostly C++ and bash (the latter is unbelievably useful) for the past 3.5 years. I make intermittent progress on the side doing other things.

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