From: dblack@...
Date: 2003-02-06T09:23:21+09:00
Subject: Re: Local variables & blocks

Hi --

On Thu, 6 Feb 2003, Brian Candler wrote:

> On Wed, Feb 05, 2003 at 10:23:52PM +0900, dblack@candle.superlink.net wrote:
> > > So that x in
> > >
> > >   a.each do |item|
> > >     x = ...
> > >   end
> > >   p x
> > >
> > > would not be a block local variable.  The code above will work without
> > > preceding "nil" assignment.
> >
> > I know I'm joining this thread rather late, but I have a minority
> > opinion anyway, so it may not matter :-)
> >
> > I'm one of the few people who have never been bothered by the way
> > things are now.  But I realize it's going to change, so that's OK.
> >
> > However, the above example, where x comes into being in the block and
> > then persists, looks and feels very strange to me.  I suspect that if
> > this is adopted, it won't be long before it starts showing up in lists
> > of "Ruby Gotchas" and such.
>
> Well, it's a side-effect of the new rules. The alternatives would be:

Yes, I was expressing concern about precisely that effect :-)

> > For me, part of the problem is just the visual backwardness of it --
> > having something propagate from a right-hand field back into a
> > left-hand field, which has a "shoe on wrong foot" feel to me.  But
> > more fundamentally, I've lost sight of what problem people are
> > perceiving and trying to solve.
> >
> > Originally, the problem was shadowing.
>
> ... more importantly, lack of it.
>
> For me, the biggest problem with the current rules is the non-locality of
> code, or 'action at a distance'. Take the following snippet:
>
>    ...
>    obj.each { |i|
>       j = fn(i)
>    }
>    ...
>
> Is 'i' a block local variable? Is 'j' a block local variable?
>
> The answer now is: "it depends". It depends on whether you happen to have
> used a local variable called 'i' or 'j' earlier in the method. You have to
> scan your whole method to find out. If you rely on one behaviour or the
> other - especially in threads, where you likely rely on i or j being local,
> then things will break badly if you (or someone else maintaining your code)
> adds a line or two using 'i' or 'j' as a local variable higher up.

This is the part I've never understood.  Even if you write a method
with no blocks, you still have to know whether or not you've used a
variable name.  I'm not sure I really believe that people often add
lines to methods (their own or, certainly, other people's) without
familiarizing themselves with what's going in the method.  I certainly
don't believe that they should do so :-)  And the presence or absence
of blocks doesn't seem to me to have any special status in this
connection.


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav