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Date: Thu, 26 Aug 2010 01:55:51 +0200
From: Georg Bauhaus <rm-host.bauhaus@maps.futureapps.de>
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Subject: Re: S-expression I/O in Ada
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Date: 2010-08-26T01:55:51+02:00
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On 8/26/10 12:38 AM, Simon Wright wrote:

>> (A B (C D E) F)
>>
>> '(' ~>  push key_1     -- First key will be returned
>>
>> key_1 : A B key_2     -- '(' ~>  push key_2
>> key_2 : C D E         -- ')' ~>  pop (key_1 on top)
>>   |
>>   v
>> key_1 : A B key_2 F   -- ')' ~>  pop (stack empty)
>> key_2 : C D E
>>
>> First_Key := key_1;
>> return;
>
> Oh. I had quite misunderstood your meaning.
>
> There are issues - like copying a sexp and then deleting or changing
> part of one of them, how to know when to delete or duplicate.
>
> Seems to me that the key is a disguised pointer to a shared structure?

key_N is a pointer in disguise, and modifications of key_N entries
anywhere need care, recursively.  When all Map operations are
hidden behind the container operations of package S_Expressions,
can copying (in order to prevent structural sharing) still be
avoided by implementing copy-on-write?

In any case, an optimized version might use a variation of skip lists
for Element_Type instead of List, I think. Its pointers point to
any key_N is the list, simplifying depth first tree operations
(like removing a subtree).

Georg