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From: Lutz Donnerhacke <lutz@iks-jena.de>
Newsgroups: comp.lang.ada
Subject: Re: Implementing Memorize
Date: Wed, 22 Oct 2003 19:41:56 +0000 (UTC)
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* Dmitry A Kazakov wrote:
><lutz@iks-jena.de> wrote:
>>* Dmitry A Kazakov wrote:
>>> Solution depends on how exposed should be the context used to store
>>> results:
>>>
>>> 1. Completely hidden (allocated upon elaboration of the body)
>>> 2. Partially exposed as a generic parameter of the compilation unit
>>> 3. Fully exposed as a parameter of the subprogram
>>
>>It's more or less irrelevant.
> 
> You have to choose an abstraction first. For instance for (1), a
> generic variant could be:
> 
> generic
>    type Argument_Type is private;
>    type Parameter_Type is private;
>    with function Evaluate (X : Argument_Type; N : Parameter_Type)
>       return Argument_Type;
> function Evaluate_With_Side_Effects
>     (X : Argument_Type; N : Parameter_Type)
>       return Argument_Type;
> 
> function Evaluate_With_Side_Effects
>     (X : Argument_Type; N : Parameter_Type)
>       return Argument_Type is
> begin
>    if not <X & N already evaluated> then
>       <store> Evaluate (X, N);
>    end if;
>    return <cached>;
> end Evaluate_With_Side_Effects;

In this case I have reference Evaluate_With_Side_Effects from Evaluate. How
can I implement this?

> What you wrote is also (1), but on the basis of a tagged type.
> Theoretically one could also use a discriminated type, to complete the
> list of opportunities.

This does not solve the problem of recursion.

> precise if the original values were known. The solution here was (3) +
> keeping track of original value changes.

Understood, but I didn't grasp how to prepare the Evaluate function for this
usage.