[ ruby-Bugs-7698 ] unexpected behaviour: block arguments read / write outside of their block

Bugs item #7698, was opened at 2007-01-09 14:57
You can respond by visiting: 
http://rubyforge.org/tracker/?func=detail&atid=1698&aid=7698&group_id=426

Category: Core
Group: 1.8.x
Status: Open
Resolution: None
Priority: 3
Submitted By: Boris Schmid (bor_)
Assigned to: Nobody (None)
Summary: unexpected behaviour: block arguments read / write outside of their block

Initial Comment:
unexpected behaviour: block arguments are readable outside of their block. 

This is something that caught me by surprise when I was using a block within a block where both used the same block argument. After some experimentation, I figured out that block arguments (the bit in between | |) for which variables with the same name were defined prior to the block influence those variables: see example

irb(main):014:0> item = nil
=> nil
irb(main):015:0> count = 0
=> nil
irb(main):016:0> array.each {|item| count += item}
=> [1, 2, 3, 4, 5]
irb(main):017:0> count
=> 15
irb(main):018:0> item
=> 5
irb(main):019:0>

Normally, if I hadn't defined "item = nil", I wouldn't be able to query item in line 018, but now I am. This was unexpected for me, nor am I quite sure when you'd like it to happen (you could use array.last, or hash.keys.last).

It messes up things when you use the same block argument in nested blocks (I picked up the habit of doing so. I'd have expected that block arguments only exist within their block.

   #irb(main):014:0> hosts = [1,2,3,4,5]
   #=> [1, 2, 3, 4, 5]
   #irb(main):015:0> hosts.each {|host|
   #irb(main):016:1* puts host
   #irb(main):017:1> [10,10,10].map {|host| host }
   #irb(main):018:1> puts host
   #irb(main):019:1> }
   #1
   #10
   #2
   #10
   #3
   #10
   #4   
   #10
   #5
   #10
   #=> [1, 2, 3, 4, 5]



Unrelated, but I ran into it while trying to figure out what was happening in the above example: apparently all @local variables are predefined to exist. I'll remember that.

boris:falcon:~:irb
irb(main):001:0> @bit
=> nil
irb(main):002:0> bit
NameError: undefined local variable or method `bit' for main:Object
        from (irb):2
irb(main):003:0>



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You can respond by visiting: 
http://rubyforge.org/tracker/?func=detail&atid=1698&aid=7698&group_id=426