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From: Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups: comp.lang.ada
Subject: Re: How to get Ada to ?cross the chasm??
Date: Sat, 05 May 2018 10:50:23 +0100
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Date: 2018-05-05T10:50:23+01:00
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Dennis Lee Bieber <wlfraed@ix.netcom.com> writes:

> On Fri, 4 May 2018 18:29:17 -0500, "Randy Brukardt" <randy@rrsoftware.com>
> declaimed the following:
>
>>"Paul Rubin" <no.email@nospam.invalid> wrote in message 
>>news:87zi1gz3kl.fsf@nightsong.com...
>
>>> This list contains a reference to itself:
>>>
>>>    x = 1 : 2 : 3 : x
>>
>>And is illegal in any reasonable programming language... ;-)
>>
>>> If you try to print it, you'll see "1,2,3,1,2,3,1,2,3,1,2,3..." ad
>>> infinitum until you interrupt the program.  It is infinitely long, but
>>> occupies a finite amount of memory.
>>
>>If you say so. I can't see any sensible way to implement such a thing; 
>>indeed I would have said it was impossible to implement but apparently you 
>>have an example to the contrary.
>
> 	Not as a one-liner... but...
>
> PythonWin 2.7.13 (default, Jun 26 2017, 14:28:43) [MSC v.1500 64 bit
> (AMD64)] on win32.
> Portions Copyright 1994-2008 Mark Hammond - see 'Help/About PythonWin' for
> further copyright information.
>>>> x = [1, 2, 3]
>>>> x.append(x)
>>>> print x
> [1, 2, 3, [...]]
>>>> 
>
> 	Slight difference is that the entire list becomes an element, it is not
> a one level sequence of repeating entities

I'd say that's more than a slight difference!  If you needed this in
Python you could get close with

>>> import itertools
>>> x=[1,2,3]
>>> x=itertools.cycle(x)

but the result is not a list thought it is, of course, an "iterable".

In Ada:

   type Node;
   type List is access Node;
   type Node is record N: Integer; Next: List; end record;
   ...
   H : List := new Node'(1, new Node'(2, new Node'(3, null)));
   ...
   H.Next.Next.Next := H;

In C:

  typedef struct node { int n; struct node *next; } node;
  ...
  node *x = &(node){ 1, &(node){ 2, &(node){ 3 } } };
  x->next->next->next = x;

My Ada is very rusty, so there may well be much better ways to do this.

-- 
Ben.