Re: Block-local variables - obsolete?

On Dec 28, 2007 8:39 AM, David A. Black <[e-mail]> wrote:

> On Sun, 23 Dec 2007, Rick DeNatale wrote:

> > So the 'warning' about shadowing is telling you that.
> >
> >   1) in the case of a parameter, the binding is different than in 1.8
>
> Hopefully anything that warns of a deprecation or change like that
> will disappear once we're at a stabler version (1.9.1?).
>
> >   2) In the case of the local, that there's a  variable with the same
> > name, in an outer scope which would otherwise be visible within the
> > scope of the block.
>
> There's no reason for a warning, then. If you're using the "; x"
> construct, you're saying that you want a block-local x, whether or not
> there's an x outside the block. So you certainly don't need to be
> warned that there's an x outside the block. The whole point is that
> you don't want to have to worry about that, one way or the other.

I  absolutely agree with this.

> Also, the whole reason that we're getting this new syntax in the first
> place, as I understand it, is because people want a way to not worry
> about outer variables. I've even heard it talked about in terms of
> "What if you cut-and-paste a block?" The ";" thing handles that
> problem -- so certainly if you use it, you don't care about what's in
> the containing scope, and shouldn't be warned about it.
>
> I guess the ";" is the "anti-closure" operator, or something :-) In
> any case, it should just let us do our thing and not give the warning.
> Shadowing the outer variable fact, is a sign that the programmer's
> intentions have succeeded, so a warning is out of place.

FWIW, this new feature of Ruby 1.9 is, to a first approximation,
something which looks very much like Smalltalk block temporaries.

There are some subtle differences of course. In Smalltalk, the
existance of variables, although not their type which is always any
object, must be declared.  Method local variables (Smalltalk calls
them temporaries) are declared at the beginning of a method in
vertical bars:

Smalltalk
foo: arg
  | var |
  var := arg
  ...

Ruby
def foo(arg)
    # No declaration of var needed
    var := arg
    ...
end
Smalltalk block arguments are declared at the beginning of the block
somewhat like in Ruby

Smalltalk: [ :blkArg1 :blkArg2 | ... ]
Ruby:       { |blk_arg1, blk_arg2 | ... )

Although :blkArg1 looks like a synbol to a rubyist, in Smalltalk it
represents a block argument named blkArg1.  Unlike Ruby, in Smalltalk
block arguments can only appear on the lhs of an assignment.

Smalltalk block local variables are declared similarly to method temporaries.

Smalltalk: [:blkArg1 :blkArg2 | | blkTemp1 blkTemp2| ... ]
Ruby:       {| blk_arg1, blk_arg2; blk_temp1, blk_temp2 | ... }

Semantically they work the same was as in Ruby 1.9 they take
precedence over a variable with the same name in an outer scope.

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/