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Date: Thu, 10 Mar 2005 15:25:32 +0100
From: Georg Bauhaus <sb463ba@user1.uni-duisburg.de>
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Newsgroups: comp.lang.ada,comp.lang.c++
Subject: Re: Teaching new tricks to an old dog (C++ -->Ada)
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Date: 2005-03-10T15:24:42+01:00
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Falk Tannhäuser wrote:

> Well, I tried to google what Ada "downward closures" are but since I
> don't know the language, I'm not sure I properly understood how they
> work (shame on me!).

I believe downward closures are not at all Ada-specific.

In Ada, you pass a function f as a parameter to another
function g(..., f) *even* when f is declared at a place more
deeply nested than the declaration of g.
So f has access to objects up the nesting level, i.e. to
its environment, g has not.

For example, the Ada container library provides procedures
"iterate", consuming a container and a procedure pointer.
In abbreviated peudo-notation:

container library:

   procedure iterate(a container X, a procedure P);
     -- invoke P for every element in X

your program:

   procedure work (a container X) {
      ... local declarations

      procedure f (a cursor K) {
         -- local procedure with access to X's element a K,
         -- and to the local declarations
         ...
      }

      iterate(a container, f);
   }

When iterate executes, both the local declarations and f still
exist, they are accessible. But iterate is declared somewhere
else, f is more nested.

The accessibility rules imply that iterate can only invoke f, or
pass it on to other procedures, but f cannot for example be
assigned to a global variable because when work has executed,
f becomes inaccessible.

Here is a small and simple Ada 2005 example that I think comes
fairly close to the (here named) lambda expression you have given.

procedure find_gt_666 is

    vec: Vector;  -- of integers
    here: Cursor := No_Element;  -- end()

    procedure first_gt_666(x: Cursor) is
    begin
       if not Has_Element(here) and then Element(x) > 666 then
          here := x;
       end if;
    end first_gt_666;

begin
    ... populate vec

    iterate(vec, first_gt_666'access);

    ... use Element(here)
end find_gt_666;


> Of course, it would be nicer, easier and more flexible if you could write
> something like (syntax totally hypothetical!):
>   std::vector<int>::iterator it = std::find_if(vec.begin(), vec.end(),
>                                                lambda{x; x>666} );

Eiffel agents might come even closer to this as they can be defined
in place.

Georg