[Python-ideas] return from (was Re: Tail recursion elimination)

[David Mertz]

I was mostly disliking the idea of TCO during this discussion.  However,
the idiom of 'return from' seems sufficiently elegant and explicit--and has
exactly the semantics you'd expect from 'yield from'--that I am actually +1
on that idea.

Being an explicit construct, it definitely becomes a case of "consenting
adults" not of implicit magic.  I.e. you are declaring right in the code
that you don't expect to see a frame in a stack trace, which is fair
enough.  I mean, if you *really* wanted to you could muck around with
'sys._getframe(N).f_whatever' already which would give inaccurate
tracebacks too.  Probably there would be a way to removed frames from the
stack even, using some such trick in current python.


On Sun, Jan 19, 2014 at 3:13 PM, Terry Reedy <tjreedy at udel.edu> wrote:

> Proposal (mostly not mine): add 'return from f(args)', in analogy with
> 'yield from iterator', to return a value to the caller from an execution
> frame running f(args) (and either reuse or delete the frame that ran
> 'return from'). The function name 'f' would not have to match the name of
> the function being compiled, this would actually be TCO, even if it were
> nearly always used for recursive tail calls. That does mean that is would
> work for mutually tail recursive functions.
>
> On 1/19/2014 6:57 AM, Joao S. O. Bueno wrote:
>
>> OTOH, since we are at it, we'd better check
>> 2009 BDLF's opinion on the subject:
>>
>> http://neopythonic.blogspot.com.br/2009/04/tail-recursion-
>> elimination.html
>>
>
> I read throught the comments and near the very end, in July 2013, Dan
> LaMotte said... '''
> Definitely seems to be complicated/impossible to determine a function is
> tail recursion 'compliant' statically in python, however, what if it were
> an 'opt in' feature that uses a different 'return' keyword?
>
>     def f(n):
>     if n > 0:
>     tailcall f(n - 1)
>     return 0
> '''
> In additional paragraphs, he noted, among other things, that this makes
> the feature 'opt-in' on a function by function basis.
>
> Guido replied "Dan: your proposal has the redeeming quality of clearly
> being a language feature rather than a possible optimization. I don't
> really expect there to be enough demand to actually add this to the
> language though. Maybe you can use macropy to play around with the idea
> though?"
>
> 水逝云飞 then suggested 'return from'. My only contribution is to point out
> the analogy with the new, and initially strange, 'yield from'.
>
> Guido seems to have said that if a) someone tries out the idea with
> macropy, and b) someone demonstrates enough demand, he might consider
> adding such a feature. So this seems to me the best option to pursue to get
> something into CPython. I also think it is the best proposal so far.
>
> As for a), I have not looked as macropy, but:
> On 1/19/2014 4:33 PM, Haoyi Li wrote:> MacroPy's @tco decorator is about
> as easy as you could ask for. 'pip
> > install macropy', 'from macropy.experimental.tco import macros, tco' >
> is about as easy as you could ask for. Works for arbitrary tail-calls >
> too, not just tail recursion.
>
> That leaves b) for those of you who want the feature.
>
> Any PEP should admit that the feature might be abused. Someone might write
>   return from len(composite)
> Unless return from refuses to delete the frame making a call to a C
> function, the effect would be to save a trivial O(1) space as the cost of
> deleting the most important line of a stack trace should len() raise. But I
> think this falls under the 'consenting adults' principle. A proposed doc
> should make it clear that the intended use is to make deeply recursive or
> mutually recursive functions run and not to replace all tail calls.
>
> --
> Terry Jan Reedy
>
>
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