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Re: A new module for performing tail-call elimination

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Date Thu, 16 Jul 2015 12:58:44 -0700
From Ethan Furman <ethan@stoneleaf.us>
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Subject Re: A new module for performing tail-call elimination
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On 07/16/2015 12:45 PM, Terry Reedy wrote:
> On 7/16/2015 2:02 PM, Ethan Furman wrote:
>> On 07/16/2015 06:35 AM, Antoon Pardon wrote:
>
>>> Here is one way to do the odd, even example.
>>>
>>> def even(n):
>>>      return odd_even('even', n)
>>>
>>> def odd(n):
>>>      return odd_even('odd', n)
>>>
>>> def odd_even(fn, n):
>>>      while fn is not None:
>>>          if fn == 'even':
>>>              fn, n = (None, True) if n == 0 else ('odd', n - 1)
>>>          elif fn == 'odd':
>>>              fn, n = (None, False) if n == 0 else ('even', n - 1)
>>>          else:
>>>              raise ValueError("Unknown state: %s" % fn)
>>>      return n
>>
>> Wow -- definitely uglier and less easy to understand than the original
>> (even if the original had had the error-checking).
>
> What you call ugly is an example of the core of the proof that alternation and indefinite
>  while looping are enough for turing completeness, and that no recursive syntax is needed.
> Another way to put it is that while loops perform recursion in the operational sense, as
>  opposed to the syntactical sense.

And we add new constructs and new language features to make certain ways of thinking/programming easier -- async and await come to mind. ;)  After all, we don't really even need while loops as we 
could get by with if and goto.

--
~Ethan~

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Thread

A new module for performing tail-call elimination "Th. Baruchel" <baruchel@no.spam.gmx.dot.com> - 2015-07-13 15:44 +0000
  Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-15 11:29 +0200
    Re: A new module for performing tail-call elimination Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-07-16 18:07 +1000
      Re: A new module for performing tail-call elimination Robin Becker <robin@reportlab.com> - 2015-07-16 10:13 +0100
      Re: A new module for performing tail-call elimination Robin Becker <robin@reportlab.com> - 2015-07-16 10:28 +0100
        Re: A new module for performing tail-call elimination Marko Rauhamaa <marko@pacujo.net> - 2015-07-16 12:56 +0300
      Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-16 12:41 +0200
        Re: A new module for performing tail-call elimination Steven D'Aprano <steve@pearwood.info> - 2015-07-17 04:58 +1000
          Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-17 11:00 +0200
      Re: A new module for performing tail-call elimination Chris Angelico <rosuav@gmail.com> - 2015-07-16 21:11 +1000
      Re: A new module for performing tail-call elimination Jeremy Sanders <jeremy@jeremysanders.net> - 2015-07-16 13:29 +0200
      Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-16 15:35 +0200
      Re: A new module for performing tail-call elimination Chris Angelico <rosuav@gmail.com> - 2015-07-16 23:47 +1000
        Re: A new module for performing tail-call elimination Paul Rubin <no.email@nospam.invalid> - 2015-07-17 20:06 -0700
      Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-16 16:21 +0200
      Re: A new module for performing tail-call elimination Chris Angelico <rosuav@gmail.com> - 2015-07-17 00:27 +1000
      Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-16 17:14 +0200
      Re: A new module for performing tail-call elimination Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-16 10:17 -0600
      Re: A new module for performing tail-call elimination Ethan Furman <ethan@stoneleaf.us> - 2015-07-16 10:54 -0700
      Re: A new module for performing tail-call elimination Ethan Furman <ethan@stoneleaf.us> - 2015-07-16 11:02 -0700
      Re: A new module for performing tail-call elimination Terry Reedy <tjreedy@udel.edu> - 2015-07-16 15:45 -0400
      Re: A new module for performing tail-call elimination Ethan Furman <ethan@stoneleaf.us> - 2015-07-16 12:58 -0700
      Re: A new module for performing tail-call elimination Robin Becker <robin@reportlab.com> - 2015-07-17 09:57 +0100
    Re: A new module for performing tail-call elimination Alain Ketterlin <alain@universite-de-strasbourg.fr.invalid> - 2015-07-16 13:23 +0200
  Re: A new module for performing tail-call elimination Terry Reedy <tjreedy@udel.edu> - 2015-07-15 17:19 -0400
  Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-16 09:45 +0200
  Re: A new module for performing tail-call elimination Terry Reedy <tjreedy@udel.edu> - 2015-07-16 15:34 -0400
    Re: A new module for performing tail-call elimination Marko Rauhamaa <marko@pacujo.net> - 2015-07-16 22:45 +0300
      Re: A new module for performing tail-call elimination Terry Reedy <tjreedy@udel.edu> - 2015-07-17 15:47 -0400
        Re: A new module for performing tail-call elimination Marko Rauhamaa <marko@pacujo.net> - 2015-07-17 23:55 +0300
          Re: A new module for performing tail-call elimination Terry Reedy <tjreedy@udel.edu> - 2015-07-17 20:40 -0400
            Re: A new module for performing tail-call elimination Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-07-19 10:39 +1200
          Re: A new module for performing tail-call elimination Chris Angelico <rosuav@gmail.com> - 2015-07-18 10:47 +1000
          Re: A new module for performing tail-call elimination Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-07-19 10:39 +1200
      Re: A new module for performing tail-call elimination Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-07-19 10:39 +1200
        Re: A new module for performing tail-call elimination Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-07-19 01:09 +0100
        Re: A new module for performing tail-call elimination MRAB <python@mrabarnett.plus.com> - 2015-07-19 01:19 +0100
        Re: A new module for performing tail-call elimination Marko Rauhamaa <marko@pacujo.net> - 2015-07-19 09:29 +0300
  Re: A new module for performing tail-call elimination Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2015-07-17 10:06 +0200

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