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From: Stephen Leake <stephen_leake@acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Formal and informal type systems?
Date: 28 Sep 2004 19:47:40 -0400
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Date: 2004-09-28T19:47:40-04:00

Mark Lorenzen <mark.lorenzen@ofir.dk> writes:

> Consider the following data type and it's two value constructors:
> 
>    type Optional_Integer (Defined : Boolean) is
>       record
>         case Defined is
>            when True  => Value : Integer;
>            when False => null;
>         end case;
>       end record;
> 
> I think it would be quite nice to have the possibility to define the
> following subprograms:
> 
>    procedure Do_Something (I : Optional_Integer(Defined => False)) is
>    begin
>       null;
>    end Do_Something;
>    
>    procedure Do_Something (I : Optional_Integer(Defined => True, Value)) is
>    begin
>       Do_Something_More(Value);
>    end Do_Something;
> 
> The pattern matching could also be extended to case statements such as:
> 
>    procedure Do_Something (I : Optional_Integer) is
>    begin
>       case I is
> 	 when Optional_Integer'(Defined => False)       => null;
> 	 when Optional_Integer'(Defined => True, Value) =>
> 	    Do_Something_More(Value);
>       end case;
>    end Do_Something;

I'm sorry, but I have no idea what semantics you intend this new
syntax to represent. I don't know ML, so I can't guess. Please treat
this as a chance to entice me to learn more about ML.

How, exactly, is your Do_Something different from:

procedure Do_This (I : Integer) is
begin
   case I is
    when False       => null;
    when True =>
       Do_Something_More(Value);
   end case;
end Do_Something;

In particular, where is "Value" coming from? It appears to be a global
variable.

You mention "pattern matching", but I have no clue what dataspace you
are matching against.

-- 
-- Stephe