Re: assignments using different types

[cgreen@yosemite.atc.com (Christopher Green)]

In article <32EE4431.6D24@ti.com>, David Dessert  <d-dessert@ti.com> wrote:
>I have a question about readability and efficiency of assignments
>when using strict types in Ada83.  I'm using the TI-Tartan C40
>compiler, which has few Ada95 features.
>
>Example:
>   type Distance_Type is new float range 0.0 .. 1000.0;
>   type Time_Type is new float range 0.0 .. 10.0;
>   type Velocity_Type is new float;
>
>   Distance : Distance_Type := 100.0;
>   Time     : Time_Type     := 10.0;
>   Velocity : Velocity_Type;
>
>...
>
>   -- A difficult to read, but legal Ada assignment.
>   Velocity := Velocity_Type(float(Distance) / float(Time));
>
>
>Can anyone provide me with suggestions to improve the readability
>of the above assignment?  Of course, I'd like efficiency to be
>similar to that of using the same type for all the variables.

If your compiler does something useful with "pragma Inline", the following
is no less efficient and quite a bit more expressive.  I've extended the
example to show the expressiveness of using the type system and operators
together...

    type Distance_Type is new float;
    type Time_Type is new float;
    type Velocity_Type is new float;
    type Acceleration_Type is new float;

    -- note that the inlined functions' names are chosen to be unique:
    -- this avoids problems with attempting to inline functions that
    -- have overloaded names.

    function V_Is_Dx_Dt (Left : in Distance_Type; Right : in Time_Type)
      return Velocity_Type;
    pragma Inline (V_Is_Dx_Dt);

    function A_Is_Dv_Dt (Left : in Velocity_Type; Right : in Time_Type)
      return Acceleration_Type;
    pragma Inline (A_Is_Dv_Dt);

    -- these are so we can invoke our definitions as operators.

    function "/" (Left : in Distance_Type; Right : in Time_Type)
      return Velocity_Type renames V_Is_Dx_Dt;

    function "/" (Left : in Velocity_Type; Right : in Time_Type)
      return Acceleration_Type renames A_Is_Dv_Dt;

    -- the type conversions are buried in the function bodies.

    function V_Is_Dx_Dt (Left : in Distance_Type; Right : in Time_Type)
      return Velocity_Type is
    begin
      return Velocity_Type (float (Left) / float (Right));
    end V_Is_Dx_Dt;

    function A_Is_Dv_Dt (Left : in Velocity_Type; Right : in Time_Type)
      return Acceleration_Type is
    begin
      return Acceleration_Type (float (Left) / float (Right));
    end A_Is_Dv_Dt;

    -- and later on, simply write...

    Distance     : Distance_Type := 100.0;
    Time         : Time_Type     := 10.0;
    Velocity     : Velocity_Type;
    Acceleration : Acceleration_Type;

    Velocity     := Distance / Time;
    Acceleration := Velocity / Time;

Chris Green                                  Email cgreen@atc.com
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