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World: r4wp

[Rebol School] REBOL School

Steeve
3-Oct-2012
[1050x2]
because you could use recurse from anywhere in the function not only 
at the tail
[B] it's not by definition, it's just the way you do it ;-)
MarcS
3-Oct-2012
[1052x4]
hmm, okay
i don't follow your point re: recursing from anywhere (i.e., from 
non-tail position)
via the use of throw/catch
if you're not in tail position you'd still have to push stack frames
Ladislav
3-Oct-2012
[1056]
Marc's code is an implementation of tail-recursive function. Such 
implementations have been posted by some people in the past, but 
there are some minor differences between them. For example, the way 
how the arguments were passed differed from Marc's idea.
Steeve
3-Oct-2012
[1057x2]
I thought you wanted to simulate tail/recursion only (it's what youre 
current code do anyway)
in that case you don't need to store the stack frame
To answer to the question: why tail recursion from anywhere ?
What if you want conditionned recursion ? 
Recurse could appear inside a IF block not at at the tail.
MarcS
3-Oct-2012
[1059x4]
aha, misunderstood your point
yes, there's a shortcoming if you're recursing from within a conditional 
-- thanks for spotting that; will rework
fix: http://0branch.com/highlight/snippets/rfunc3.r
though there's still the while loop wrapping the body
Ladislav
3-Oct-2012
[1063]
The WHILE loop is necessary for what you are doing, do not worry 
about it.
MarcS
3-Oct-2012
[1064x2]
damned silly mistakes -- was focused on getting the bindings etc. 
right, overlooked this prob
thanks for the help guys
Gregg
3-Oct-2012
[1066]
The usage comment in the third version is very nice. 


On doc strings, try to keep them short, leaving detailed notes to 
the usage or other comments. e.g.

rfunc: func [

    {Defines a tail-call optimized user function, using RECUR for self-calls.}
    spec [block!] "Function spec (see: func)"
    body [block!] "Function body (modified) (see: func)"
    /local wrapped
] [
    ...
] 

Short doc strings are nicer with HELP.
MarcS
3-Oct-2012
[1067]
thanks for the tip
Gregg
3-Oct-2012
[1068]
Also, I try to use special names for augmentations and such. In this 
case, 'with would conflict with my WITH func.
MarcS
3-Oct-2012
[1069x3]
hmm, yeah -- was wondering about that
that was originally _args_ but i switched it out so that you could 
get a more readable recur call (i.e., cheekily make use of 'with' 
so that it read better)
(for the BIND)
Gregg
3-Oct-2012
[1072x3]
Easy enough to add sigils to make it special. I don't general go 
as far as a gensym approach for things like this.
Yes. Too early for my brain here, but 'is known-word honored in the 
bind? e.g. "set with reduce (args)"
That is, 'with is special no matter what known-word you use, correct?
Ladislav
3-Oct-2012
[1075]
Marc, your "private variables" like 'with, _recur_ can be made more 
private not needing to use the PROtECT function, in fact.
Gregg
3-Oct-2012
[1076]
Ah, no, I think I get it now.
Steeve
3-Oct-2012
[1077]
I don't think you need 'with  at all, you can throw back the new 
parameters as an argument of the throw function, like:
>> throw/name reduce args 'recurse
Ladislav
3-Oct-2012
[1078x2]
also, there is a problem with the result I think
(I mean the function result)
MarcS
3-Oct-2012
[1080x2]
steeve: ooh, good point
ladislav: ah yes, there is
Steeve
3-Oct-2012
[1082]
Here is a version with no locals, no temporary context, no shit and 
not tested ;-)

rfunc: func [spec body][
	func spec compose/deep [
		forever [
		   set [(spec)] catch/name [
			  return (func spec body) (spec)
		   ] 'recur
		]
	]
]
recur: func [args][throw/name reduce args 'recur]
MarcS
3-Oct-2012
[1083x2]
wow
works well - full thing here: http://0branch.com/highlight/snippets/rfunc4.r
Steeve
3-Oct-2012
[1085x3]
to return a value and stop the function you need to use 'return instead 
of 'break
I think
recur (Must be in tail position)
 -> not needed anymore
MarcS
3-Oct-2012
[1088]
well, it has to be a tail call
Steeve
3-Oct-2012
[1089]
it's a tail call but it can be anywhere in the function body
MarcS
3-Oct-2012
[1090x2]
so you couldn't do   x + recur [ y ]
sounds like we mean different things by 'tail position'
Steeve
3-Oct-2012
[1092x3]
it seems
Yep, it seems
Yep, it seems
MarcS
3-Oct-2012
[1095]
i mean this: http://en.wikipedia.org/wiki/Tail_call
Steeve
3-Oct-2012
[1096x2]
(lag)
It's tail call recursion yes, but tail position usualy means "physically 
placed" at the tail (in our context, at the end of the body of the 
function)
Anyway...
MarcS
3-Oct-2012
[1098]
not be pedantic, but i linked to that for the opening two sentences:


In computer science, a tail call is a subroutine call that happens 
inside another procedure as its final action; it may produce a return 
value which is then immediately returned by the calling procedure. 
The call site is then said to be in tail position, i.e. at the end 
of the calling procedure.
Steeve
3-Oct-2012
[1099]
Okay T_T