[Python-Dev] Proper tail recursion

[Christian Tismer]

Christopher T King wrote:

> On Thu, 15 Jul 2004, Bob Ippolito wrote:
> 
>>On Jul 15, 2004, at 11:10 AM, Christopher T King wrote:
>>
>>But in this case what is tail-call optimization going to do for you?  
>>You still require a stack at least the size of the height of your tree 
>>because of traverse(t.left) since that can not be tail-call optimized 
>>away, with the proposed algorithm.
> 
> In Andrew's example, he noted that it would only help for list-like 
> structures (i.e. those with mostly right nodes).
> 
>>I think Guido is in the right here, if you want to work around the 
>>recursion limit, use Stackless..  It should already be able to go just 
>>about as deep as you want.

Right.

> You're right -- even if Stackless doesn't do tail call optimization,
> implementation should be trivial.  But there's no guarantee when or even
> if Stackless will be merged with CPython, and until that happens,
> Stackless isn't an option for many (most?) people.

Wrong.

Stackless will always be there, one or two weeks later but will
be there.

There is a guarantee: It will never be part of CPython.
Proof: Just follow the postings in python-dev over the last five
years, and how Guido handled these.
If the proposals seemed to have a tiny chance, it was finally
shut down by some "And however you argue, I don't want it" like
statement. "If you need it, use it, that's where Stackless is for".
In a sense, I like this Groucho Marx approach of
"whatever it is, I'm against it". It is just not coherent with
all the feature-creaping-ins of the past years, this is
not consistent.

Some way I can accept this. But the whole story is partially like
cheating.
First, I was dissed because of Guido disliking continuations. OK!
Next, I was dissed because of rewriting lots of the core is
not feasible. OK!
Btw., I have almost solved this by some automation which could
apply sensible patches to almost every extension module.
But who cares.
Afterwards, I was dissed because compatibility to Jython
was made into a problem. OK!
Nice that I could turn a not-so-beloved kid into a really loved one.

After all (as said many times, already), I don't care, and I continue
with Stackless. Also by supporting more supportive languages like
Prothon (although I still don't understand why prototypes are better
than classes).

Anyway, removing recursive-descend evaluation from Python would
mean to remove Guido from Python, which is certainly not an
option. At least I won't try this for his life-time, and it
is also by no means my wish. I prefer to stick within my role as
the dark side of Python. :-)

If I only could *understand* him, that would really help.

ciao -- chris

p.s.: since yesterday, Psyco belongs to the "well supported special
modules for Stackless". Psyco is the only module that I know of that
heavily depends on frame layout. You have to recompile it, but then
it works really fine. I hope not to impose resentments on Psyco
by that :)
-- 
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