From: Amos <amos+lists.ruby-talk@...>
Date: 2001-03-18T06:16:43+09:00
Subject: [ruby-talk:12789] scope stuff

This morning while I was still lying in bed, I was trying to see if
my dense mind could recall the various issues regarding this local
scope stuff, a thread I've only been partially able to follow due to
limited time.  This is how things currently work, right?

b = false
i = 0
5.times {|i|
  print i, " "
}
print "\n", i, "\n"
print b, "\n"

-> 0 1 2 3 4
-> 4
-> false

Going over things out load, the symbols between the bars (|) are not
local to that block, which can frustrate folks because of the side
effects that can have.

Must admit, that aspect of blocks wasn't immediately apparent to me
when I was first going through the pickaxe book.  In my simplistic
mind, I just saw the items between the bars as being the same thing
as function arguments.  Silly me!

Okay, so what if the symbols between the bars were pretty much the
same as function arguments?  That is, like with functions arguments,
the symbols between the bars were, at least by default, local to
that scope.  Then perhaps the following might be possible?

b = false 
i = 0 
5.times {|i| 
  print i, " " 
} 
print "\n", i, "\n" 
print b, "\n" 
 
-> 0 1 2 3 4 
-> 0
-> false 
 
b = false  
i = 0  
5.times {|::i|  
  print i, " "  
}  
print "\n", i, "\n"  
print b, "\n"  
  
-> 0 1 2 3 4  
-> 4
-> false  

b = false   
i = 0   
5.times {|::i, b=true|   
  print i, " " if b
}   
print "\n", i, "\n"   
print b, "\n"   
   
-> 0 1 2 3 4   
-> 4 
-> false   

In the second chunk of code the scope operator (::) is used to
designate that the following symbol is to be bound to the calling
scope, if that makes any sense.

In the third block, the second argument between the bars is
assigned a default value.  The idea behind that is so that the
invocation of yield won't blow up because of the wrong number of
arguments, yet it allows one to define a symbol to have local
scope. 

Well, such as it is, that's what my mind spit out this morning.

-- 
Amos