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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Variant record limitation - what's a better solution?
Date: Tue, 09 Jul 2013 08:49:33 +0100
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Date: 2013-07-09T08:49:33+01:00
List-Id: <comp.lang.ada>

peter.h.m.brooks@gmail.com (Peter Brooks) writes:

> I'm still having trouble with this. I've tried using functions, as
> suggested, but it doesn't allow overloaded functions in the varient
> part. Here's what I tried:

I think that people meant you should hide the awkwardness of the
*unavoidably* different names by using functions to encapsulate the
differences.

Something like this (using Ada2012 expression functions, hope that
doesn't confuse):

   package Lorries is

      type Lorry_Load is new Integer range 1 .. 36;

      type Lorry_Size is (Small, Medium, Large);

      type Lorry_Type (Lorry : Lorry_Size) is private;

      type Lorry_Number_Plate is new String (1 .. 8);

      function Plate (L : Lorry_Type) return Lorry_Number_Plate;

      function Load (L : Lorry_Type) return Lorry_Load;

   private

      subtype Small_Lorry_Load is Lorry_Load range 1 .. 2;
      subtype Medium_Lorry_Load is Lorry_Load range 1 .. 18;
      subtype Large_Lorry_Load is Lorry_Load range 1 .. 36;

      type Lorry_Type (Lorry : Lorry_Size) is
         record
            Number_Plate : Lorry_Number_Plate;
            --  Location : Depot_Name_Type;
            --  Destination : Depot_Name_Type;
            case Lorry is
               when Small  => Small_Load : Small_Lorry_Load;
               when Medium => Medium_Load : Medium_Lorry_Load;
               when Large  => Large_Load : Large_Lorry_Load;
            end case;
         end record;

      function Plate (L : Lorry_Type) return Lorry_Number_Plate is
         (L.Number_Plate);

      function Load (L : Lorry_Type) return Lorry_Load is
         (case L.Lorry is
            when Small => L.Small_Load,
            when Medium => L.Medium_Load,
            when Large => L.Large_Load);

   end Lorries;