Is it true that Arc has no return statement? If so, how does one, for example, return a result from within an iteration before the end of the iteration?
This issue came up on the 8 Queens Problem thread ( http://arclanguage.com/item?id=4370 ). I asked the question in a comment, but it's buried kind of deep.
In Ruby, one could do the following (yes, there are better ways to do this in Ruby, but it illustrates return ):
(mac point (name . body)
" Creates a form which may be exited by calling `name' from within `body'. "
(w/uniq g
`(ccc (fn (,g)
(let ,name [,g _]
,@body)))))
breakable: works on any expression, not just loops and other control structures. Basically, if you need something to suddenly return a value without continuing, just add breakable: to the point you want to do so and use (break ...) to break out.
Maybe should have called it 'brittle or 'fire-extinguisher ^^
I wonder how ccc in Arc/Scheme compares to return in Ruby with respect to efficiency. Hopefully ccc is very lightweight. It really does make me question why return wasn't defined to be part of the language.
Basically the most important realization is that the return address on the stack may be modeled as the address for a different procedure, which by a staggering coincidence just so happens to point to the code after the function call. So basically a return is itself equivalent to a function call.
ccc is one of the strangest things in Scheme (and Arc now). Hard to get everything behind it, but it can be used to implement return statements, try-catch à la C++/Java, coroutines à la Lua, generators à la Python and of course continuation-based web apps à la Arc...
A very strange beast, and as far as I know the thing Scheme has that CL hasn't (and cannot trivially implement).
Actually, the form of closure used by Arc is highly similar to a continuation passing style, so although it doesn't use 'ccc, it does use continuations.
cl-cont is a library that implements closures and runs on many CL implementations. So it's not a matter of CL can't do it, CL just has to use a library to do it.
I think there are conceptual problems mixing unwind-protect-like constructs and call/cc.
The cl-cont lib appears to only support a subset of CL (in particular unwind-protect is not supported).
So cl-cont doesn't demonstrate that CL can support it, only that a restricted subset of CL can.
I don't know if it is possible to sensibly accommodate unwind-protect and call/cc in the same language, it seems smarter people than I have avoided doing so.
Scheme's dynamic-wind serves the same purpose as unwind-protect (except with entry conditions as well, since a continuation could suddenly return into your protected code), and plays nicely with continuations. I'm not sure about the details, though.
Thanks for that, pretty interesting. I guess that makes writing entry and exit conditions a little more interesting, since they could end up being invoked multiple times.
i am learning how to write scheme compilers now it it looks like you just jump around your CCC's like GOTO labels - very fascinating and lower over head then functions calls (if you compile to machine level)
Thanks. And from your comment ( http://arclanguage.com/item?id=4432 ) on the 8 Queens thread, the Ruby snippet in the OP above could be implemented as:
That said catch-throw is in ArcN, although they do have different intended usages. breakable: is supposed to be used as a modifier for existing control structures, while (catch ...) is supposed to be a control structure in its own right.
(def a (return)
(each i xs
(if ( ... )
(return i)))
nil)
arc> (ccc a)
1
This is a very low-level solution, if you want to put your hand into axioms. almkglor's solutions, based on point & breakable macros (from Anarki) are much cleaner, even if they just expand in the above code (or something very close).
The major difference between return statements in other languages and ccc is that you can use any continuation that you can access, which means you can conditionally return to two different places in the code. As an example, here's a loop that supports "break" and "continue" (and can even break with a return value):
(mac my-loop body
`(ccc (fn (break)
(while t (ccc (fn (continue) ,@body))))))
Also, I need help. I can't use my regular account because I have forgotten my password and the only password recovery thing is only accessible when you are already logged in. I am pretty sure I put my email address on my regular account, so I don't see why I can't recover it. Please post any suggestions at http://www.arclanguage.org/item?id=4449
