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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: {Pre,Post}conditions and side effects
Date: Wed, 13 May 2015 14:47:55 +0200
Organization: cbb software GmbH
Message-ID: <1d9ioha0kn05$.1durah75rm1yn.dlg@40tude.net>
References: 
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Date: 2015-05-13T14:47:55+02:00
List-Id: <comp.lang.ada>

On Wed, 13 May 2015 13:53:59 +0200, G.B. wrote:

> On 13.05.15 11:35, Dmitry A. Kazakov wrote:
>> The
>> proper design is a composite type Sorted_Array with an*invariant*  that
>> ensures the array sorted.
> 
> This shifts responsibility.

Yes, to where it belong to.

> Also, why does the array need to be sorted during
> its entire life time? How do you know that?

>From its type, of course.

> A contract for Binary_Search could require that
> 
> (a) input data is any plain old array of numbers
> (b) in sorted order, by "<="

That is not a contract of binary search. That is of a type of which
[binary] search is an operation. Individual contracts of operations is a
mess, tolerated only when it would be too complicated to deal with this at
the type level. Sorted constraint is not such a thing.

> What is the difference between
> 
> -  the use of a sorted array (by precondition)
>     in one place and

Untyped mess

> -  the use of a sorted array (by invariant)
>     in the very same place,

Properly designed software with types reflecting problems space entities.
Note that this would not only ensure no run-time errors. It also will allow
safe searching unsorted arrays. You will convert unsorted array type to
sorted one, which conversion will sort it.

> Does every single, isolated algorithm on data warrant
> a corresponding type?

Certainly so. Operations are defined on types.
 
-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de