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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Subject: Re: Anybody in US using ADA ? One silly idea..
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Date: Tue, 28 Jan 2003 18:44:55 GMT
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Hyman Rosen <hyrosen@mail.com> writes:

> Robert A Duff wrote:
> > No, because you can't pass a nested procedure to a less-nested
> > procedure.
> 
> So you can instantiate a generic procedure and pass its address
> to a less-nested procedure, even though the generic uses procedures
> defined at a more-nested level?

No.

You can pass a nested procedure to a less-nested generic during
instantiation.  You cannot pass the 'Access of a nested procedure to a
less-nested procedure.

generic
    with procedure Action(Item: Element);
procedure Iterate(X: Data_Structure);
-- Calls Action once for each Item in the data structure.

function Count(X: Data_Structure) return Natural is
    Result: Natural := 0;

    procedure Incr_Result(Item: Element) is
    begin
        Result := Result + 1;
    end;
    procedure Iter is new Iterate(Incr_Result);
begin
    Iter(X);
    return Result;
end Count;

The above works fine (barring typos -- I didn't compile it).
But if you try to do the same thing using access-to-procedure,
it won't work:

type Action_Proc is access procedure(Item: Element);
procedure Iterate(X: Data_Structure; Action: Action_Proc);

function Count(X: Data_Structure) return Natural is
    Result: Natural := 0;

    procedure Incr_Result(Item: Element) is
    begin
        Result := Result + 1;
    end;
begin
    Iterate(X, Incr_Result'Access); -- Illegal!
    return Result;
end Count;

- Bob