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From: hreba <f_hreba@yahoo.com.br>
Newsgroups: comp.lang.ada
Subject: Re: Incompatible types for array conversion
Date: Wed, 22 Jul 2015 10:22:19 -0300
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Xref: number.nntp.giganews.com comp.lang.ada:194290
Date: 2015-07-22T10:22:19-03:00
List-Id: <comp.lang.ada>

On 07/22/2015 10:00 AM, hreba wrote:
> On 07/22/2015 02:20 AM, Niklas Holsti wrote:
>> On 15-07-22 04:19 , hreba wrote:
>>> I have a generic vector package:
>>>
>>> generic
>>>     dim:        positive;
>>>     type Real is digits <>;
>>> package Vectors is
>>>     type Vector is array (1..dim) of Real;
>>>     function "*" (a: Vector; b: Real) return Vector;
>>> end Vectors;
>>>
>>> In another package I want to model the definition of a (physical)
>>> position vector by its direction and module:
>>>
>>> package Basic is
>>>     type Real is new Interfaces.IEEE_Float_64;
>>>     type Length is new Real;
>>>
>>>     package Positions is new Gen.Vectors (3, Length);
>>>     package Directions is new Gen.Vectors (3, Real);
>>>
>>>     subtype Position is Positions.Vector;
>>>     subtype Direction is Directions.Vector;
>>>
>>>     function "*" (l: Direction; r: Length) return Position;
>>> end Basic;
>>>
>>> Now my attempt to define this function:
>>>
>>>     function "*" (l: Direction; r: Length) return Position is
>>>     begin return Positions."*"(Position(l), r); end "*";
>>>
>>> gets me the error message in the subject line.
>>>
>>> Do I have to convert the vector element by element in a loop or is there
>>> a more elegant way?
>>
>> The conversion is rejected because of the conversion condition in RM 4.6
>> (24.5/2): "The component subtypes shall statically match". The component
>> (sub)types are Real and Length, which are different types, although
>> Length is derived from Real. If you change the declaration of Length
>> to be
>>
>>     subtype Length is Real;
>>
>> the conversion becomes legal.
>>
>
> Thanks a lot for the hint!
>
Well, I was a little too fast with my response. It works, but it is not 
what I need. I use type Real for physical dimensionless quantities and 
Length for lengths. They must be incompatible, you cannot add them. So I 
came up with a new version of my function:

    function "*" (l: Direction; r: Length) return Position is
    begin
       return Positions."*"
         (Position'(Length(l(1)), Length(l(2)), Length(l(3))), r);
    end "*";

(Thanks God I am working with classical physics and not string theory 
with its 26 dimensions.)

-- 
Frank Hrebabetzky		+55 / 48 / 3235 1106
Florianopolis, Brazil