From: "Matthias Wächter" <matthias@...>
Date: 2007-09-13T22:30:41+09:00
Subject: Scope Issues (avoid name clashes for block parameters)

On 30.08.2007 16:22, Todd Benson wrote:
> On 8/30/07, kazaam <kazaam@oleco.net> wrote:
>> Why is a local variable my_new_array not working but just @ instance or $ global variables and why do I have
>> initialize this varriable? I think you don't need this in ruby? But without initialisation it just shows nil?
> 
> It's about scope.  I'm sure the veterans here can give you a thorough
> explanation, (and please pipe in here, because I don't want to spread
> disinformation), but basically the block tries to retain its own scope
> as best it can (from what I understand).  So, new variables that are
> not "swallowed" from the scope outside of the block need to be
> initialized, and those initialized within the block don't get outside
> of the block (what happens in Vegas stays in Vegas, blah blah).
> 
> Simple example...
> 
> irb(main):001:0> a = 1,2,3,4
> => [1, 2, 3, 4]
> irb(main):002:0> a.each { |i| b = i } #nothing happens here outside of
> the block for b
> => [1, 2, 3, 4]
> irb(main):003:0> b
> NameError: undefined local variable or method `b' for main:object
> 
> 
> Here's one that will make you think a little that is somewhat OT, but
> demonstrates namespace danger...
> 
> irb(main):001:0> a = 1,2,3,4
> => [1, 2, 3, 4]
> irb(main):002:0> a.inject{ |s, a| s + a }
> => 10
> irb(main):003:0> a
> => 4
> 
> The a is not the accumulator, but it has changed even though the
> method doesn't have a ! following it.  These are the small
> idiosyncrasies that we have to be aware of with namespace and scope.

Is there something I can do to avoid name clashes for block parameters?

f.e. standard:

x="external"
a= 1,2,3,4

a.each {|x| puts x*x}
puts x                           # => 4

vs. clash avoision (note the exclamation mark):

x="external"
a= 1,2,3,4

a.each {|!x| puts x*x}
puts x                           # => "external"



- Matthias