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4 points by aw 77 days ago | link | parent

Especially for modules, I do like using function and macro values in macro expansions.

Consider a module which contains a macro such as `for`. I can import the macro into my module simply by copying the macro value into my module, e.g.: `(= for other-module!for)`. By using function and macro values in the macro expansion (as you do in `for2`), the functions and macros that the imported macro uses don't also need to be copied into my module.

I haven't worried myself about how most terms end up getting a comma prefix, but I agree there could be an alternative syntax where injecting the value of a symbol (instead of the symbol itself) could be the default.

To handle special forms, a solution is to come up with a globally unique random prefix long enough that no one could generate it accidentally (e.g. "xVrP8JItk2Ot"), and then rename the special forms `assign`, `fn`, etc. to `xVrP8JItk2Ot-assign`, `xVrP8JItk2Ot-fn` etc; and then add `assign` and `fn` macros etc. that expand into the prefixed special form. Now `assign`, `fn`, etc. are macros that can be used as values in a macro expansion.

(Making `fn` a macro also has the advantage that features such as default arguments can be implemented in Arc).

I was at first concerned that using such a random prefix was ugly, however eventually I realized that no one ever needs to see the prefixed versions :-)

> (aka for them to not be "hackable")

I was indeed curious about maximizing hackability as a general principle... but with modules even if a macro such as `for` is imported into my default namespace and thus expansions such as `with` wouldn't use my version, in practice it's easy enough to create a new module with my version `with` etc. and simply load the `for` source into the new module. (Or simply reload the `for` source into my default module).

    > (eval (list + 1 2))
    3
This is unusual in Lisp implementations, and I was quite pleased when I discovered that Racket supported this.

    (#M:with (i nil gs3342 1 gs3343 (#F:+ 10 1))
This is a nifty idea... hmm, is there a reason to use different prefixes for macros and functions? We could have a read syntax which evaluated to the value of the symbol (whatever it is), and that would work for both functions and macros.


2 points by waterhouse 71 days ago | link

> To handle special forms, a solution is to come up with a globally unique random prefix long enough that no one could generate it accidentally (e.g. "xVrP8JItk2Ot"), and then rename the special forms `assign`, `fn`, etc. to `xVrP8JItk2Ot-assign`, `xVrP8JItk2Ot-fn` etc;

Would this prefix be present in the source files of the language implementation? Or generated at runtime, or something else? If the latter, it seems like gensyms are the right approach. If the former, what's the use case—someone writing programs that assume someone might have redefined "assign" and want to work with the builtin? (If it's just hacking arc3.1 to do what we want, I think you can create more or less arbitrary unique objects (vectors, cons cells, a new kind of struct), give them bindings in the base Arc environment, and replace e.g. '(eq? (xcar s) 'quote) with '(eq? (xcar s) the-quote-object).)

Speaking of gensyms, I had a plan recently. I like PG's approach of just sequentially named variables, but (a) my ideal language would probably expose its interned-symbol table, and it'd therefore be more orthogonal and simpler for it to directly expose the "create a symbol without interning it" function, so uninterned symbols should be available for free; (b) it is possible that people will name a variable "gs1", so that's another reason to use true gensyms; (c) many macros use gensyms, but I might like to bootstrap a system that likes macros the expansion of which incurs zero side effects, and incrementing a gensym counter is a side effect. For (c) there might be another approach, but I came up with this one: Have the printer assign sequential numbers to each uninterned symbol it encounters (with a weak hash table). This has the nice effect that, when you do print macroexpansions, the numbers you see will begin at 1, rather than in the hundreds or thousands and varying depending on how many libraries have been loaded.

> (Making `fn` a macro also has the advantage that features such as default arguments can be implemented in Arc).

Yes, that is nice. I actually did this in my Lisp in Summer Projects submission[1] many years ago.

> is there a reason to use different prefixes for macros and functions? We could have a read syntax which evaluated to the value of the symbol (whatever it is), and that would work for both functions and macros.

Eh, no strong reason. I'd thought about doing a reverse variable lookup for every value and printing the variable name if it existed, but it makes less sense if e.g. the value 10 gets printed as #V:<some global variable that happened to be bound to 10>. Also, I figured an IDE-type setup (such as DrRacket) might use different colors or fonts to distinguish macros/functions/special forms/other. Last, you do need a way to print lambdas that don't have a global name (and might not even have a local name), and I'm guessing you'd want that to appear as #f:<some attempt at indicating line number / REPL input>, looking analogous to but different from the global case.

[1] https://github.com/waterhouse/emiya : "[O]ptional arguments are implemented by redefining "fn" (Arc's equivalent of "lambda") as a macro that expands to a call to "underlying-fn", which is bound to the fn special object--again, by the user. Then everything is recompiled, so that this redefinition does not cost anything at runtime; some care is needed here, to avoid an infinite loop, if a function used to recompile "fn" itself uses "fn"."

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