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Date: Tue, 08 Feb 2011 22:46:20 +0100
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Subject: Re: How do I write directly to a memory address?
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Date: 2011-02-08T22:46:20+01:00
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On 2/8/11 9:21 PM, Shark8 wrote:

>> void foo2(int jump)
>>     /* make sure jump % 2 == 0! */
>> {
>>      ...
>>
>> }
>>
>> Write that in Ada 2005 to demonstrate the glorious power of
>> your subtype constraints.  When was it that compilers will
>> support aspects reliably?
>
> Okay.
> Procedure Foo( Jump : In Integer ) is
--  make sure jump mod 2 == 0 *before* calling this function

You'd need a formal type in Foo's signature that lets the client
see what values can be supplied  without having to look at the body.
A first approximation is a precondition:

    procedure Foo2 (Jump : in Integer)
      with Pre => (Jump mod 2 = 0)
    is
      ...

A type in Ada 2012 can use an Invariant aspect referencing
an expression function,

    type Even is range -12 .. 345 with
      Invariant => Is_Even (Even);

    function Is_Even (E : Even) return Boolean is
       (E mod 2 = 0);

Then,

    procedure Foo2 (Jump : in Even);

seems a much better approximation of the C comment.