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Re: Little fun with vararg

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On 8/4/2014 8:58 AM, Jay Carlson wrote:

On Aug 4, 2014, at 9:40 AM, Andrew Starks <andrew.starks@trms.com> wrote:

On Sun, Aug 3, 2014 at 2:03 PM, Tom N Harris <telliamed@whoopdedo.org> wrote:

I'll have to remember this the next time I feel the urge to write `return
value, recursive_call(...)` which isn't a tail call. Although I've
occasionally thought the VM could pull some stack manipulation tricks to make
it a tail call.

Hmm... I had to look this up. I thought that `return value, recursive_call(...)` would be a tail call. I guess it isn't, given that `return fund(args)` is the required form.

This gets into what "tail call" means, and perhaps specifically what "properly tail recursive" means. As a matter of language specification, the call to f in
 function f()
 return 1, f()
 end
is difficult to require to be a tail call, since the return values stack will exhaust all memory regardless of whether activation records do.
You can watch Scheme fight it out here:
https://groups.csail.mit.edu/mac/ftpdir/scheme-mail/HTML/rrrs-1997/msg00048.html
Jay

I always understood the documentation that "tail call" to mean
 function f(args)
 --optional other code
 return g(otherargs)
end
and "recursive tail call" to mean
function f(args)
 --other code including code to end recursion
 return f(modified_args)
end
and in both cases the return statement must return exactly the unmodified return values of the function it calls; otherwise it is not a tail call. IMHO, the Lua documentation is clear on this point.

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