From: Mark Hubbart <discordantus@...>
Date: 2004-12-03T03:57:56+09:00
Subject: Re: any way to manipulate variables and bindings?

On Fri, 3 Dec 2004 03:27:45 +0900, itsme213 <itsme213@hotmail.com> wrote:
> def foo
>     bar [:a, :b, :c]
>     a
>     b
>     c
> end
> 
> I would like a, b, and c to be local variables with values 1, 2, 3
> (positions of :a, :b, :c). Is there some implementation of
>     def bar (symbol_list)
>         symbol_list.each_with_index { |s, i|
>             ???
>         }
>     end
> that will do this?

Not really. Technically, you could do something like this:

def bar(&block)
  ary = block.call
  ary.each_with_index{|s,i| eval("#{s}=#{i}", block.binding)}
end
    ==>nil
bar{[:a,:b,:c]}
    ==>[:a, :b, :c]
[a,b,c]
    ==>[0, 1, 2]


This would create local variables with those values. However, they
wouldn't be accessible in anything other than irb:

mark@eMac% ruby
def bar(&block)
  ary = block.call
  ary.each_with_index{|s,i| eval("#{s}=#{i}", block.binding)}
end
bar{[:a,:b,:c]}
p [a,b,c]
-:6: undefined local variable or method `a' for main:Object (NameError)
mark@eMac% 

This is because ruby decides at _compile time_ whether an identifier
represents a variable or a method call. Since Ruby doesn't see
anything being assigned to a b and c at compile time, it assumes they
are methods, and raises an error.

You could hack around this by pre-defining your variables, but that's ugly.

mark@eMac% ruby
def bar(&block)
  ary = block.call
  ary.each_with_index{|s,i| eval("#{s}=#{i}", block.binding)}
end
a,b,c = *[]
bar{[:a,:b,:c]}
p [a,b,c]
[0, 1, 2]
mark@eMac% 


HTH,
Mark