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Newsgroups: comp.lang.ada
From: Anonymous Coward <spam@spam.com>
Subject: Re: Ada Quality and Style book discussion ("_Type" suffix)
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Date: Sat, 19 Nov 2005 21:52:09 GMT
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Date: 2005-11-19T21:52:09+00:00
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In article <437c5ec8$1_1@glkas0286.greenlnk.net>, 
Martin Dowie wrote:
> Anonymous Coward wrote:
>>    package Name_Collisions is
>>       [...]
>>       type Speed is new Float;
>>
>>       --No can do.  "speed" hijacked by type definition.
>>       --
>>       Speed : Speed;
>>
>>    end Name_Collisions;
> 
> But why not use scoping to solve this? I always separate type
> declarations from object declarations anyway, so I never see these
> sorts of problems.  

I would prefer not to make my architecture dependant on identifier
choices, since packages are much more than just name spaces.  If
package seperation prevents a name collision, it's only incidental in
my case.  

> Also, I don't think that "Speed" /is/ a good
> type name e.g.
> 
> package SI is
>    type Meters_Per_Second is ...;
> end SI;
> 
> package App is
>    Speed : SI.Meters_Per_Second;
>    ...
> end App;

That's getting into the specifics of how a speed might be used.  I
deliberately try to keep my examples as simple as possible to
illustrate my point, so you couldn't see from my example why
Speed(_Type) might be a good name - only why "Speed" alone would be a
bad name.  Here's a slightly more elaborate example:

   package Physics is

      type Distance_Type is private;
      type Speed_Type    is private;
      type Time_Type     is private;

      type Distance_Units_Type is (Meters, Feet, Miles);

      type Time_Units_Type is (Seconds, Minutes, Hours);

      function Distance (Distance       : in Float;
                         Distance_Units : in Distance_Units_Type)
        return Distance_Type;

      function Time (Time       : in Float;
                     Time_Units : in Time_Units_Type)
        return Time_Type;

      function "/" (Left : in Distance_Type; Right : in Time_Type)
        return Speed_Type;

      function "*" (Left : in Speed_Type; Right : in Time_Type)
        return Distance_Type;

      function "*" (Left : in Time_Type; Right : in Speed_Type)
        return Distance_Type;

   private

      type Distance_Type is new Float;
      type Speed_Type    is new Float;
      type Time_Type     is new Float;

   end Physics;

In this case, Speed_Type fully leverages encapsulation to keep units
of measure as abstract as possible.  Internally speed is represented
as meters per second, but that is none of the users business, so
"Speed" suffixed with "_Type" makes the most sense.