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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: not null
Newsgroups: comp.lang.ada
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Date: Fri, 6 Mar 2009 18:48:58 +0100
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Date: 2009-03-06T18:48:59+01:00
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On Fri, 6 Mar 2009 08:59:41 -0800 (PST), Harald Korneliussen wrote:

> On Mar 6, 4:21�pm, "Dmitry A. Kazakov" <mail...@dmitry-kazakov.de>
> wrote:
>> On Fri, 6 Mar 2009 04:01:45 -0800 (PST), Harald Korneliussen wrote:
>>> On Mar 4, 5:09�pm, Adam Beneschan <a...@irvine.com> wrote:
>>
>>>> I'm having difficulty understanding the point. �You have to have the
>>>> idea of the lack of a reference. �How else would you implement a
>>>> linked list abstraction?
>>
>>> You can use algebraic data types to do that in a type safe manner.
>>
>> No, you cannot, if safe = static. The nodes of the list have contracts to
>> support certain operations. The only problem with null is that the virtual
>> object corresponding to null does not fulfill the contract:
> 
> I don't understand your code example. But kind of like some Java
> functions won't compile unless they're surrounded by an appropriate
> try/catch block, you can make in Haskell a linked list get_next
> function that causes the compiler to protest - at compile time,
> naturally - if used in a context where the possibility of returning
> Nothing (null) isn't considered.

But this has nothing to do with null as a value. It does to exception
handling. The problem was moved to the client, which is the point.

(It is a pity that Ada does not have contracted exceptions.) 

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de