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Re: Functional Programming and python

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On 9/30/2013 5:02 PM, Tim Chase wrote:
> On 2013-09-30 19:04, Franck Ditter wrote:
>> two points make me crazy :
>> 1. Tail recursion is not optimized. We are in 2013, why ? This is
>> known technology (since 1960). And don't answer with "good
>> programmers don't use recursion",
>
> I seem to recall hearing that the primary reason it hadn't been
> implemented is because of Python's super-dynamism (to make up a
> word).

Right. A tail call is a call immediately followed by a return (in the 
byte code or equivalent). A recursive tail call is a tail call to the 
function containing the tail call. In Python, the function to be called 
in a call is not determined until the call is made. This is because the 
function expression is evaluated to a function object at runtime, not 
when the function is compiled.

It would be trivial to detect and somewhat optimize all tail calls in 
CPython. But the result would be to delete traceback info with *all* 
tail calls, not just for recursive tail calls.

Some functions have multiple recursive calls, as with divide and conquer 
algorithms and graph traversal. The last recursive call might or might 
not be a tail call. When it is, having calls omitted from the traceback 
might be undesireable. It might be disconcerting, for instance, if the 
number of calls in the traceback for a balanced binary tree algorithm 
did *not* match the level where the error took place.

> That a function could be a tail recursion in one call, but
> the calling the same name could then become rebound.  I'm making up
> the example, but I think it was something like this:
>
>    def kablooie(*args):
>      if not args:
>        def kablooie(*args):
>          woah()
>      do_something(args)
>      kablooie(args[1:])
>
> where tail recursion optimization would do weird things.

-- 
Terry Jan Reedy

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Where to suggest improvements in the official Python documentation? Aseem Bansal <asmbansal2@gmail.com> - 2013-08-03 10:53 -0700
  Re: Where to suggest improvements in the official Python documentation? Joel Goldstick <joel.goldstick@gmail.com> - 2013-08-03 14:23 -0400
  Re: Where to suggest improvements in the official Python documentation? Terry Reedy <tjreedy@udel.edu> - 2013-08-03 16:43 -0400
    Re: Where to suggest improvements in the official Python documentation? Aseem Bansal <asmbansal2@gmail.com> - 2013-08-04 09:30 -0700
      Re: Where to suggest improvements in the official Python documentation? Joel Goldstick <joel.goldstick@gmail.com> - 2013-08-04 14:34 -0400
        Re: Where to suggest improvements in the official Python documentation? Aseem Bansal <asmbansal2@gmail.com> - 2013-08-20 07:22 -0700
          Re: Where to suggest improvements in the official Python documentation? Joel Goldstick <joel.goldstick@gmail.com> - 2013-08-20 10:57 -0400
      Functional Programming and python rusi <rustompmody@gmail.com> - 2013-09-22 19:21 -0700
        Re: Functional Programming and python Vito De Tullio <vito.detullio@gmail.com> - 2013-09-23 20:24 +0200
          Re: Functional Programming and python Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-09-24 10:06 +0300
          Re: Functional Programming and python rusi <rustompmody@gmail.com> - 2013-09-24 00:08 -0700
            Re: Functional Programming and python Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-09-24 10:42 +0300
              Re: Functional Programming and python rusi <rustompmody@gmail.com> - 2013-09-24 05:14 -0700
                Re: Functional Programming and python Jussi Piitulainen <jpiitula@ling.helsinki.fi> - 2013-09-24 17:51 +0300
                Re: Functional Programming and python rusi <rustompmody@gmail.com> - 2013-09-24 08:07 -0700
                Re: Functional Programming and python Chris Angelico <rosuav@gmail.com> - 2013-09-25 01:26 +1000
                Re: Functional Programming and python rusi <rustompmody@gmail.com> - 2013-09-24 10:25 -0700
        Re: Functional Programming and python Franck Ditter <nobody@nowhere.org> - 2013-09-30 19:04 +0200
          Re: Functional Programming and python Chris Angelico <rosuav@gmail.com> - 2013-10-01 03:17 +1000
          Re: Functional Programming and python Neil Cerutti <neilc@norwich.edu> - 2013-09-30 18:36 +0000
            Re: Functional Programming and python Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-10-01 00:14 +0000
              Re: Functional Programming and python Neil Cerutti <neilc@norwich.edu> - 2013-10-01 13:59 +0000
          Re: Functional Programming and python Piet van Oostrum <piet@vanoostrum.org> - 2013-09-30 14:55 -0400
            Re: Functional Programming and python Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-09-30 22:37 +0200
              Re: Functional Programming and python Piet van Oostrum <piet@vanoostrum.org> - 2013-10-01 13:31 -0400
          Re: Functional Programming and python Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2013-09-30 20:29 +0200
          Re: Functional Programming and python Tim Chase <python.list@tim.thechases.com> - 2013-09-30 16:02 -0500
          Re: Functional Programming and python Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-10-01 00:41 +0000
          Re: Functional Programming and python alex23 <wuwei23@gmail.com> - 2013-10-01 11:03 +1000
          Re: Functional Programming and python Terry Reedy <tjreedy@udel.edu> - 2013-09-30 21:03 -0400
            Re: Functional Programming and python rusi <rustompmody@gmail.com> - 2013-09-30 18:36 -0700
              Re: Functional Programming and python Chris Angelico <rosuav@gmail.com> - 2013-10-01 17:28 +1000

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