Re: Understanding generic package functions

[Stephen Leake <stephen_leake@stephe-leake.org>]

Nick Gordon <annihilan@gmail.com> writes:

> Okay, basically I'm writing a demo of Ada for a class, and I'm trying
> to showcase Ada's math functions. 

Is this a class on Ada, or something else?

> This is the code:
>
>    function Recoded_Exp(Power: in Float) return Float is -- Base e
>       use Ada.Numerics.Generic_Elementary_Functions;
>       use Ada.Numerics;
>       ExpResult : Float;
>    begin
>       ExpResult := e ** Power;
>       return ExpResult;
>    end Recoded_Exp;

Style notes: it's best not to use "use" unless it really cleans up the
code (my rule is "unless the name is used at least three times"), and
not to define a local variable unless it is really needed.

> I've seen something that suggests I have to instantiate the generic
> package, but I don't understand how.

This is how:

with Ada.Numerics.Generic_Elementary_Functions;
function Recoded_Exp(Power: in Float) return Float is -- Base e
   package Float_Elementary is new Ada.Numerics.Generic_Elementary_Functions
     (Float_Type => Float);
   use Float_Elementary;
begin
   return Ada.Numerics.e ** Power;
end Recoded_Exp;

"Instantiating a generic package" means declaring a package using a
generic template instead of providing all the code directly. So this
line:

   package Float_Elementary is new Ada.Numerics.Generic_Elementary_Functions
     (Float_Type => Float);

declares the package "Float_Elementary", using the code from
Generic_Elementary_Functions, with "Float_Type" replaced by "Float".

-- 
-- Stephe