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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: User defined implicit conversion
Date: Wed, 30 Oct 2019 16:26:12 -0500
Organization: JSA Research & Innovation
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Xref: reader01.eternal-september.org comp.lang.ada:57393
Date: 2019-10-30T16:26:12-05:00
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"Alain De Vos" <devosalain71@gmail.com> wrote in message 
news:d9269cae-93e5-428c-9c7c-a7a1f5679806@googlegroups.com...
> Casting from one type to another is a real pain.
> Does the functionality "User defined implicit conversion" exists?
> If not are there strategies, general rules, to limit conversion to a 
> minimum ?
>
> We don't want as concatination :
> result := to_type_a (from_type_a (X) & from_type_a (Y)) ;
> when you could have :
> result := X & Y ;

The only reason you would have that is if you forgot to define "&" for 
Type_A. Depending on the definition of Type_A, that's unlikely to happen 
automatically.

Ergo, adding
      function "&" (Left, Right : Type_A) return Type_A is
              (to_type_a (from_type_a (X) & from_type_a (Y)));

Solves the problem in typical use.

Implicit conversion itself is the same is eliminating strong typing. If you 
really want that, you probably need to use subtypes rather than types. That 
is, "Type_A" should be a subtype rather than a type.

For example, instead of:

     type Type_A is new String;

use:

     Subtype Type_A is String;

And now you never need to use a type conversion between Type_A and String.

                                           Randy.