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Date: Tue, 17 Jun 2008 00:02:51 +0200
From: Georg Bauhaus <rm.tsoh.plus-bug.bauhaus@maps.futureapps.de>
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Subject: Re: About String
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Date: 2008-06-17T00:02:52+02:00
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Robert A Duff wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> 
>> On Sun, 15 Jun 2008 18:06:00 -0400, Robert A Duff wrote:
>>
>>> I don't see why a loop needs a declarative part.
>>> My idea is that every statement list is a scope.
>> What is so special in statement lists? Pushing this idea further, we should
>> also conclude that any sequence of expressions is a scope. For example, an
>> aggregate:
>>
>> String'(X : constant Character := F(A(I)), 1 => X, 2 => 'a', 3 => X);
>>
>> a function call etc.
> 
> Well, I suppose something like that could work, but it seems like it
> would be confusing -- how many components are there, which pieces of
> text belong to which components (or which parameters, in the function
> call case)?
> 
>> [ That might be useful to overcome some silly limitations on discriminants:
>>
>> type T (S : Storage_Count) is record
>> -- Buffer : Storage_Elements_Array (0..S-1);  -- This would be illegal, so
>>    Buffer : Storage_Elements_Array
>>       (Last : constant Storage_Offset := S-1, 0..Last);
>> end record; ]
> 
> I don't see the point.  Why not just make it legal (the line marked
> "This would be illegal").  Rules would be needed about when to evaluate
> that expression -- presumably on elaboration of a discriminant
> constraint, or an aggregate, or a discriminant default.  I don't think
> having a name for Last makes this any easier.
> 
>>>> However I find nested declare/begin/end more readable because it clearly
>>>> disambiguates between the scope of the loop and the scope of one iteration
>>>> of.
>>> I don't get it.  You declare something inside a loop body, or the 'then'
>>> part of an 'if' -- it's local to that.
>> Local in which sense? There are statical nesting and dynamic execution of
>> the loop body upon iterations.
> 
> The same sense as declare blocks.  That is, in my "circus",
> this:
> 
>     ... loop
>         <decls>
>         <statements>
>     end loop;


   ... loop
      Get(Buffer(K));
      <decls>   -- depending on Buffer
      <statements>
   end loop;

I guess that in order to maintain the order(?)
SCOPE [DECLS] STATEMENTS END-SCOPE
we would be needing

   ... loop
      Get(Initial(K));
      begin
         <decls>
         <statements>
      end;
   end loop;

> has identical semantics to:
> 
>     ... loop
>         declare
>             <decls>
>         begin
>             <statements>
>         end;
>     end loop;

   ... loop
      Get(Buffer(K));
      declare
         <decls>
      begin
         <statements>
      end;
   end loop;

Compared to the solution above, that would be an exchange
of BEGIN <-> DECLARE and the addition of a BEGIN where there
is nothing between <decls> and <statements> above.

A quick shot at syntax, what impact does this have, if any?
I'm adding a few rather artificial errors, but nevertheless:

   ... scope
      function Foo(N: Integer := K);
      return Value;
   end scope;

   ... scope
      declare
         function Foo(N: Integer := K);
         return Value;
      begin
         null;
      end;
   end scope;

By no more than a gut feeling I'd say that from a syntax
point of view, the omission of explicit declare blocks
adds a few more opportunities for ambiguity: RETURN is
at least not likely introducing a statement in the second
example.