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Date: Mon, 27 Feb 2006 22:40:18 +0100
From: Georg Bauhaus <bauhaus@futureapps.de>
Organization: future apps GmbH
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Subject: Re: limited types (Was: Records that could be arrays)
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Date: 2006-02-27T22:39:33+01:00
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Dmitry A. Kazakov wrote:

> The question Georg asked was: where typed containers of elements of related
> types might themselves appear related. The number of cases is huge.
> Generics fundamentally cannot help here, they aren't dynamically
> polymorphic.

But then wouldn't you again be mixing apples and herrings?

  type APPLE_CONTAINER is new CONTAINER with null record;
    -- specializes in apples, only

  type HERRING_CONTAINER is new CONTAINER with null record;
    -- specializes in herrings, only

  x: access CONTAINER'class := ...;

  x.insert(an_apple); -- right or wrong? compile time?


> It is dynamic typing. The constraint check would happen at run-time. Too
> late. You just have tossed a stinking herring in a jug apple cider...
> 
> 
>>>3. Merging
>>
>>You'll need to define a relational operator over your element type, but
>>then you can say:
>>
>>Merge (C1.First, C2.First);
> 
> 
> Same as above. I don't mix herrings and apples,

In fact, herrings with little pieces of apple aren't
that unusual, add onions and mayonnaise and you are
almost set - not everyone's taste, maybe. :-) Seriously,
wouldn't type-forcing containers be in the way of
composition?

How do you make a vector of doughnuts with different
toppings? Something like

 zip(doughnuts.first, toppings.first);

seems reasonable to me.