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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Proposed change to BC iterator parameters
Date: Fri, 19 Dec 2003 11:53:02 +0100
Organization: At home
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Date: 2003-12-19T11:53:02+01:00
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Robert A Duff wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> Robert A Duff wrote:
>> 
>> > I'm still pushing for downward closures to be added to the language,
>> > which would add at least a minimal amount of support for iterators,
>> > and greatly simplify this and other data-structure proposals.
>> 
>> Wouldn't subroutine types do the trick? I wonder why Ada does not have
>> them.
> 
> By "downward closures", I mean the ability to pass more-nested
> procedures as parameters to less-nested procedures.  If that's what
> you mean by "subroutine types", then yes.

Yes. A subroutine object will be of course passed in "IN" mode, so there
will be no way to get an access object from it and store that non-locally,
which was the main concern caused the accessibility rules.

type Func is function (X : Float) return Float;
procedure Integrate (F : Func; X1, X2 : Float) return Float;

> Access-to-subprogram types don't do the trick because of the
> accessibility rules.

Right. However, there was an idea to have limited access types, which would
probably do it. But I think that to have subroutine types is more cleaner
and in Ada spirit.

> Hence the extra "context" parameters in
> Jeff Carter's iterators.
> 
> I also would like the ability to have anonymous procedures, like lambda
> in Lisp, or blocks in Smalltalk.  The idea is to pass the body of a loop
> as a parameter to the iterator, and I don't want to have to give a
> *name* to every loop body, and I want to write it *inside* the loop
> (i.e. inside the call to the iterator).

This should work with subroutine types. We only need to invent some syntax
sugar for "subroutine literals".

type Item is ...;
type Container is ...;
type Action is procedure (I : Item);
procedure Enumerate (X : Container; For_All : Action);
procedure Print (I : Item; Output : in out Device);
...
procedure Print (X : Container; Output : in out Device) is
begin
   Enumerate (X, Action'(Print (I, Device);));
-- or maybe
   Enumerate (X, procedure (I : Item) is begin Print (I, Device); end;);
end Print;

However mixing statements and expressions looks awful to my taste.

> I also want the ability to jump out of those anonymous procedures, to
> exit the loop, without passing extra parameters around.  The extra
> parameter is bad because it forces loops that *don't* want to exit
> prematurely to worry about whether they might want to exit prematurely.

If you have an ability to put a subroutine body in an expression then gotos
out of it are for free:

procedure Print
          (  X : Container;
             Output : in out Device;
             Count : Natural
          )  is
   To_Print : Natural := Count;
begin
   Enumerate
   (  X,
      procedure (I : Item) is
      begin
         if To_Print > 0 then
            Print (I, Device);
            To_Print := To_Print - 1;
         else
            goto Done; -- Done is visible here
         end if;
      end;
   );
<<Done>> null;
end Print;

But you will probably need a return-from-nested-subprogram statement (like
we have for exit):

function Search (X : Container; Key : Item_Key) return Item is
begin
   Enumerate
   (  X,
      procedure (I : Item) is
      begin
         if Get_Key (I) = Key then
            return I out Search; -- Return through all nested contexts
         end if;
      end;
   );
   raise Nothing_Found;
end Search;

> I don't expect to see any of this in Ada, ever, except perhaps for
> downward closures.  There is an AI on downward closures, but I don't
> remember it's status.  I hope it gets in.  It's the only case where Ada
> is inferior in power to Pascal.

-- 
Regards,
Dmitry A. Kazakov
www.dmitry-kazakov.de