From: HarryO <harryo@...>
Date: 2002-01-20T08:00:47+09:00
Subject: Re: $_ as default parameter for a function

On Sun, 20 Jan 2002 09:42:41 +1100, HarryO wrote:

> On Sun, 20 Jan 2002 07:27:41 +1100, Thomas wrote:
> 
> 
>> How do I write a function that uses the value of $_ in the calling
>> scope, so I can get the same behaviour as the many Kernel methods.
> 
> While I don't have an answer to the specific question, it seems to me
> that talking about scope for $_ doesn't make sense, since it's a global.
> 
> My first guess is that what happens when the parser sees the "def" for
> this method, it takes the CURRENT value for $_ and uses that as the
> default value for that parameter.
> 
> Maybe you can do something like ...
> 
>   def fred(x = nil)
>     if x.nil?
>       x = $_
>     end
>   end

Well, just because I was interested, I tried this. Here's what happened

  def f(x = nil)
      puts "f() -- $_ = #{$_}"  # => "" (so I guess $_ is nil)

      if x.nil?
          x = $_
      end

      puts x
  end

  $stderr.print "-> "

  gets

  puts "$_ = #{$_}"  # => whatever I've typed in

  f()  # => ""

So, it looks like $_ gets scribbled on as we enter the method.
As I say, global variables, whose values are changed by code I
didn't write explicitly are not my idea of useful programming
features :-( !!