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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Problem with generic linked list package
Date: Sun, 10 Aug 2014 22:57:11 +0100
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Date: 2014-08-10T22:57:11+01:00
List-Id: <comp.lang.ada>

Laurent <daemon2@internet.lu> writes:

(actually I wrote this next section)
>    type Rec is record 
>       B : Boolean; 
>       I : Integer; 
>    end record; 
>    function "<" (L, R : Rec) return Boolean is 
>    begin 
>       return L.I < R.I; 
>    end "<"; 

> Would that work for a record like this or would I have to define a
> function for every imaginable record?
>
> ie:  type Rec is record
>              S: String;
>              I: Integer;
>              D: Date;
>      end record;
>
> Or could I do this (just a little piece of code hope you understand
> what I mean):
>
> els if Element.Rec.I < L.Head.all.Element.Rec.I then 
>          Add_To_Front (L, Element);

You have (after Shark8's suggestion, I think) a generic package which
starts something like

   generic
      type T is private;
      with function "<" (L, R : T) return Boolean is <>;
   package P is
      ...

If you want to instantiate it with a type Rec1

   type Rec1 is record
      B : Boolean;
      I : Integer;
   end record;

then you have somehow to provide a function to take the place of the "<"
in the generic formal part.

   package P_For_Rec1 is new P (T => Rec1, "<" => ?something?);

where ?something? is a function which takes two parameters of type Rec1
and returns Boolean. It will be a Good Thing if ?something? returns True
if the first parameter is "less than" the second, *whatever "less than"
means in your application*.

If you want the comparison to be on the I component of Rec1, then you
could supply Less where

   function Less (L, R : Rec1) return Boolean is (L.I < R.I);

(that's in Ada 2012), in which case you'd instantiate with

   package P_For_Rec1 is new P (T => Rec1, "<" => Less);

The "is <>" in the formal part of package P means that you could
alternatively write

   function "<" (L, R : Rec1) return Boolean is (L.I < R.I);

and

   package P_For_Rec1 is new P (T => Rec1);


If you now decide you want to instantiate P with

   type Rec2 is record
      S: String;
      I: Integer;
      D: Date;
   end record;

then you have to decide how

   function Less (L, R : Rec2) return Boolean is (?);

needs to work. Do you need to compare the S component? the I component?
the D component? or some combination? But however it has to work, it has
to be a new function.