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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: S-expression I/O in Ada
Date: Sat, 14 Aug 2010 13:11:25 +0200
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Natacha Kerensikova <lithiumcat@gmail.com> writes:
> On Aug 13, 11:08 am, Ludovic Brenta <ludo...@ludovic-brenta.org>
> wrote:
>> Natacha Kerensikova wrote on comp.lang.ada:
>> > Consider these two objects:
>>
>> > L--A--tcp-connect
>> >    |
>> >    L--A--host
>> >    |  |
>> >    |  A--foo.example
>>
>> This part cannot exist: the second L cannot have more than 2 children;
>> here you gave it three (A, A and the next L). The dot notation would
>> have prevented that mistake :)
>
> No, it has only two children: the A connected to "host" and the L you
> haven't quoted. All the nodes in both my graphs have exactly one
> "right child", which is their value, and zero or one (but not more)
> "lower child" being the next element in the list, i.e. the second
> argument of cons.

OK, so your A nodes weren't really atoms after all, they were cons
pairs, the car of which was the actual atom.  I misunderstood that.

> So if you really want a dot notation, here is the first object:
> (tcp-connect .
>    (
>        (host . (foo.example . nil))
>    .
>       ((port . (80          . nil)) . nil)
>    )
> )
>
> while the second one is:
>
> (tcp-connect .
>    (
>       (
>           (host . (foo.example . nil))
>       .
>          ((port . (80          . nil)) . nil)
>       )
>    .
>       nil
>    )
> )
> 
> I hope my vertical alignment will make these cons pairs clear enough.
>
>
> Hoping to finally understand,
> Natacha

OK, but these two are equivalent if you remember the axiom that (y
. nil) is really just y.  If you apply that transformation to both of
your examples you reach the same topology:

  (tcp-connect . ((host . (foo.example . nil)) . ((port . (80 . nil)) . nil)))
= (tcp-connect . ((host .     foo.example    ) . ((port .    80)      . nil)))
= (tcp-connect . ((host .     foo.example    ) .                (port . 80)))


  (tcp-connect . (((host . (foo.example . nil)) . ((port . (80 . nil)) . nil)) . nil))
= (tcp-connect . (((host .       foo.example  ) . ((port .    80)      . nil)) . nil))
= (tcp-connect . (((host .       foo.example  ) .                (port . 80))  . nil))
= (tcp-connect . ((host .        foo.example  ) .                (port . 80)))

-- 
Ludovic Brenta.