Q about tail recursion

[Tim Peters]

[Felix Thibault]
> That implicit return is what I was missing...does the 'no tail-call
> optimizations ever' mean there's no point in doing functions like this:
>
>     def fun(seq, sofar=0):
>         ...
>         this = seq.pop()
>         return fun(seq, sofar+this)
>
> instead of like this:
>
>     def fun(seq):
>        ...
>        this = seq.pop()
>        return this + fun(seq)

If you're programming in Python, use a loop <0.1 wink>.  The 2nd version is
likely to be a tiny bit faster today, simply because it passes one fewer
argument.

> ...
> Does this mean that besides sending a value back to the caller, return
> also says "We're done with this namespace/local scope/frame(?), go back to
> where we got called from," ?

Yes.

>> after-all-this-i'm-still-not-sure-what-the-question-was<wink>-ly y'rs
>>    - tim

> Well, then here's the real question on my mind that I've been too
> embarassed to ask till now:
>
> Is there a Python version of this scheme function

Yes.

> that has informative variable names

Yes.

> so I can figure out what it does ?

No <wink>.

> Or just - what does this do:
>
> (define Y
> 	(lambda (le)
> 		((lambda (f) (f f))
> 		 (lambda (f)
> 		   (le (lambda (x) ((f f) x))))))) ?

Nothing to be embarrassed about here!  This is difficult stuff at first --
Curry didn't call Y the "paradoxical combinator" for nothing <0.9 wink>.
You can find an excellent explanation, including a Scheme version using
sensible names, here:

   http://www.ececs.uc.edu/~franco/C511/html/Scheme/ycomb.html

See any book or paper on the implementation of functional languages via
combinators for a more pragmatic view.  Raymond Smullyan's "To Mock A
Mockingbird" is a lovely "general interest" intro.

it's-called-"y"-because-that's-what-everyone-says-the-first-
    time-they-see-it<wink>-ly y'rs  - tim