• Robert Fraser <fraserofthenight gmail.com> Aug 01 2007

Robert Fraser <fraserofthenight gmail.com> writes:

downs Wrote:

 I realize this has been requested before, by me and a few others, gained the
votes of a significant 
 amount of people here, been completely ignored by Walter (no offense meant, I
know he can't read 
 every proposal), you know how it goes. The reason I'm bringing it up again is
that it could 
 potentially, and implicitly, help (or even completely) fix a quite serious bug
I discovered this 
 morning.
 
 To recap: the proposal is to make the parameter ("()") part of some common
constructs such as 
 while, do/while and with (I'll get to that in a second) part of the scope of
the actual loop body - 
 this would allow, for instance, checking the value of an inner variable in the
condition of a 
 do/while loop.
 Disadvantages: it breaks compatibility with C++ in cases which are forbidden
anyway (shadowing 
 declarations). .. That's all I can think of. As far as I know, it doesn't
change the behavior of 
 existing code.
 
 Now, for the bug.
 Consider the following example program.
 
 scope class test { this() { writefln("this"); } ~this() { writefln("~this"); }
}
 void main() {
          writefln("Pre");
          with (new test) { writefln("Inside With"); }
          writefln("Post");
 }
 
 What would be expected: Pre  this  Inside With  ~this  Post.
 What happens: Pre  This  Inside With  Post  ~this.
 Okay, I thought, so the new test is in the outside scope. Sucks, but workable.
I changed it to
 
 	{ with (new test) { writefln("Inside With"); } }
 
 ...
 I got the same results.
 Turns out constructing an instance of a scoped class as the parameter of a
with construct makes the 
 compiler COMPLETELY ignore the scope.
 And I can't help thinking, if the with loop's parameters were part of the
inner scope, that bug 
 _probably_ wouldn't have happened. Which is why I'm holding off on a bug
report until there's some 
 feedback on what the compiler should do in this situation.
 
 With greetings and stuffies,
   --downs


This is something I've been secretly lusting for for a while (in Java). I
especially hate having to declare _and initialize_ do/while conditions outside
the loop.

do
{
    String line = reader.nextLine();
    [...]
} while(null != line);

... is much prettier than...

String line = null;
do
{
    line = reader.nextLine();
    [...]
} while(null != line);

Of course, you don't need the initializer in D, but it still requires variables
be declared outside the scope.So this gets my vote.

You should still file a bug about the with statement thing anyway.