From: David Alan Black <dblack@...>
Date: 2001-05-03T08:43:33+09:00
Subject: [ruby-talk:14559] Re: Arggg Bitten by the block var scope feature!!!

On Thu, 3 May 2001, Wayne Scott wrote:

>
> I just spend a day debugging a elegant script that had this
> function.
>
>     #simplified for this discussion
>     def fetch(arg2)
>         # cheesy block to flush cache if it has gotten too big
>      	if @cache.length > 200
>   	    # one pass at flushing cache
> 	    @cache.dup.each do |arg2, file|       ## (1)
>     		file.flush
>     		@cache.delete arg2
>     	    end
>     	end
> 	unless @cache[arg2]			  ## (2)
> 	    @cache[arg2] = @cl.new(@arg1, arg2)
> 	end
> 	@cache[arg2]
>    end
>
> The bug is that at (1) the variable 'arg2' is not local to the block
> like I was expecting, but was just a copy of the function parameter
> 'arg2'.  So when I hit the normal part of the function at (2) the
> parameter is trashed if I happened to have executed (1).

Wouldn't it be easier and clearer, and less dangerous (in any
language), not to reuse the parameter variable name for a temporary
variable inside the method?  I'm not sure why you'd want to call that
block variable arg2, except specifically to take advantage of the
powerful feature of Ruby that interprets that to mean you want to use
the same variable :-)

|arg2, file| at the beginning of your block basically means: each time
through the block, do this:

  arg2 = <first arg>
  file = <second arg>

So you really don't want Ruby to assume that you mean some new arg2,
any more than you would if you did:

  def thing(arg2)
    [1,2,3].each do |n| arg2 += n end
  end


David

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