I do feel stupid. However, this creates new bindings. From the code that shader posted, I think he doesn't want to create a new lexical binding. Is there some way of using destructuring for assignment?
Both Adlai's and rntz's version do what I was trying to do.
However, I was looking for a way to assign a value to the variable referenced by the one I was passed. i.e.
(= v 'b)
(= (val v) 6) ; unknown function/macro val which returns contents of v so that '= can assign 6 to 'b.
This would allow the expression above to be:
(each v '(a b c) (= (val v) (readb))) ;assigns three bytes from standard in to 'a, 'b, and 'c respectively.
Whether this is done by overloading 'unquote to work like that outside of a quasiquote, or making something called 'val, I don't know. Maybe it could be done by making val or unquote a setform? That would solve this problem, but I think it might be more useful if it worked in more places than just assignment.
If b is a lexical variable, eval won't help, since it isn't run in the lexical scope in which it's called. An Arc lexical variable will get compiled to an MzScheme lexical variable, and I don't know of a way to refer to an MzScheme lexical variable by name determined at run time. If there isn't, I suspect your only hope might be to modify the Arc compiler to generate code like
((a) (set! a n))
((b) (set! b n))
((c) (set! c n))
((d) (set! d n))
for all the lexical variables available at that point.
The behaviour of the  syntax makes perfect sense, if you think about it. First, [...] is just an abbreviation for (fn (_) ...) to save typing. Second, `(...) is just an abbreviation for (quasiquote '(...)). Thus [`(...)] is [(quasiquote ...)], which is (fn (_) ((quasiquote ...)). If  stripped off the outer layer of parentheses, then [+ _ 1] would become (fn (_) + _ 1), which is clearly wrong. Thus, we see that it makes more sense to use fn, since the whole point of  and ` is to save typing and make code clearer, neither of which they can do here.
Another, possibly nicer, way to think about it is that  just represents a delayed function call; you're trying to delay an object, and since  is a function call, you're calling it instead. Either way, the point is that this is the consistent and desirable way for  to work.
: On Anarki, you also get _1, _2, etc., and __ for varargs, but that's not important for this discussion.
in is just a input-port that I would already have bound, or passed in. If you'll notice, that was just a standalone expression, with no context, to illustrate what I was trying to do.
Thanks for the code, that was exactly what I was thinking of. Too bad there isn't a macro or function that just returns the value of a variable in such a way that it can be assigned to. Can anyone think of how to implement such a thing?
(= v 'a)
(= (val v) 6) ;the variable a is now assigned the value 6.
Basically an (unquote) without the wrapping quote ;)
Well, there's always 'eval, but it has issues and stigmas, and I've read that other Lispers look at you funny when you use it your code...
'in is also a built-in macro in arc.arc. Since Arc is a Lisp-1 (grr...), you can't bind it to an input stream without potentially breaking some other code...
(mac in (x . choices)
`(let ,g ,x
(or ,@(map1 (fn (c) `(is ,g ,c)) choices)))))
Back to the symbol dereferencing question -- I think one reason that this kind of thing is shied away from is that it starts to smell an awful lot like pointers. However, the control over evaluating that you get with macros is more than enough to take care of symbol dereferencing problems like this.
Right. I forgot to check the symbol before I used it like I sometimes do, via 'help or 'src (Anarki).
Is there anything wrong with "pointers"? As in this case, they can often be quite useful, and make the code (at least to me) simpler. Maybe I just think in pointers, and need to learn to use destructuring binding more. That won't work unless you know all of the variable names in advance, but I guess that's what a hash table is for :)