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Subject: ML-like alternatives to out parameters for functions
From: Mark Lorenzen <mark.lorenzen@ofir.dk>
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Date: 17 Mar 2003 23:17:12 +0100
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Date: 2003-03-17T23:17:12+01:00
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There has lately been some discussions about Ada's missing
capabilities for specifying out parameters for functions. Apparently
this is a needed feature and an AI (AI-323) has been issued.

An out parameter can be used when the validity of the result of
evaluating a function is not always well-defined. For example:

function Calculate (Argument : in AT; Result_Valid : out Boolean) return RT;


Use of the function could be as:

Result := Calculate (Arg, Valid);
if Valid then
  .. do stuff with Result ..
else
  .. handle error ..
end if;


It seems to me that using a discriminated record type as return type
in order to mimic ML's Option datatype could solve such a (frequently)
occurring problem. But of course I am not the first to come up with
this suggestion, so what is wrong with it? Is it too slow?

The package spec is shown below and should be self-explanatory.

Regards,
- Mark Lorenzen

generic
   type Value_Type is private;
package Optional_Value is

   type Optional_Value_Type is private;

   Undefined_Value : exception;

   function Some
     (Value : in Value_Type)
     return Optional_Value_Type;

   function None
     return Optional_Value_Type;

   -- May raise Undefined_Value;
   function Value_Of
     (Element : in Optional_Value_Type)
     return Value_Type;

   function Is_Defined
     (Element : in Optional_Value_Type)
     return Boolean;

private

   type Optional_Value_Type (Defined : Boolean := False) is
      record
        case Defined is
           when True  =>
              Value : Value_Type;
           when False =>
              null;
        end case;
      end record;

end Optional_Value;