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From: ka@sorry.no.email (Kenneth Almquist)
Newsgroups: comp.lang.ada
Subject: What evil would happen?
Date: 15 Jul 2003 01:16:50 -0400
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Date: 2003-07-15T01:16:50-04:00
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wojtek@power.com.pl (Wojtek Narczynski) wrote:
> I feel like with SML like tagged union datatypes I'd be done long
> ago... Is there an AI for this maybe?

I'm not sure what the problem is here.  In SML you can write
things such as:

    datatype tree = Leaf of int
                  | Tree of {
                        value : int,
                        left : tree,
                        right : tree
                    }
                  | Empty

In Ada, you achieve the same effect by writing:

    type Tree_Tag is (Leaf, General_Tree, Empty);
    type Tree(Tag : Tree_Tag);
    type Tree_Ptr is access Tree;
    type Tree(Tag : Tree_Tag) is record
        case Tag is
            when Leaf =>
                Leaf : Integer;
            when General_Tree =>
                Value : Integer;
                Left : Tree_Ptr;
                Right : Tree_Ptr;
            when Empty =>
                null;
        end case;
    end record;

The Ada code is slightly more complex than the SML code because in
Ada we have to include a type declaration for the tag, and have to
declare an access type.  (The reason we have to declare an access
type in Ada but not in SML is that in SML the fields of a tree cannot
be modified after a tree is created, so the semantics of SML don't
need to distinguish between a tree and a pointer to a tree.)
				Kenneth Almquist