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From: Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>
Newsgroups: comp.lang.ada
Subject: Re: Ada 0y wish list: parameters of package parameters
Date: 20 Feb 2001 13:28:31 -0500
Organization: NASA Goddard Space Flight Center
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Date: 2001-02-20T18:42:08+00:00
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Dr Adrian Wrigley <amtw@linuxchip.demon.co.uk> writes:

> <snip>
> 
> My solution was to add generic functions, which converted
> between the types:
> 
>          with package Structure_1 is new Algrbraic_Structure ( <> ) ;
>          with package Structure_2 is new Order_Structure     ( <> ) ;
>          with function Convert (X : Structure_1 . Item_Type)
>               return Structure_2.Item_Type;
>          with function Convert (X : Structure_2 . Item_Type)
>               return Structure_1.Item_Type;
> 
> This allows the package to use the two types interchangably,
> provided that the Convert functions are called, if necessary.
> You could use Unchecked_Conversion between the types, if you
> believe them to be the same.

Better to use an inlined Copy function; the compiler will warn you if
something changes during maintenance.

> The alternative I could see in my case was to replace the package
> generic parameters with all the contents of the specification that
> were needed. Every time a type occurs which need to be the same in
> Structure_1 and Structure_2, you only have it once. This is general
> (I think), but can be very verbose, and obscures the abstraction.

Perhaps Item_Type should be a generic parameter of both Structure_1
and Structure_2?

-- 
-- Stephe