From: 7stud -- <bbxx789_05ss@...>
Date: 2008-01-29T12:48:30+09:00
Subject: Re: what's the diff between puts y and puts "#{y}" in class_

Ryan Ingram wrote:
> On 1/28/08, Raj Singh <neeraj.jsr@gmail.com> wrote:
>> Could you please further elaborate on that. To me it  seems like both of
>> them have the same scope because they are both being used in the same
>> method.
> 
> Compare to this:
> 
> class T
>     def first
>         "hello"
>     end
>     def second
>         "chunky bacon"
>     end
> end
> 
> y = "first"
> 
> T.class_eval <<-END
>     def hello
>         puts #{y}
>     end
> END
> 
> T.new.hello # prints "hello"
> 
> The key is that the "<<-END" is creating an interpolated string in the
> original context; the interpolation of #{y} isn't happening during the
> call to "hello", but rather as part of the string passed to
> class_eval.
> 
> You are probably confused by the fact that in the sample code, the
> interpolation is happening inside of another string constant, which
> makes it look like it's happening as part of "hello".  It's not.
> 
>   -- ryan

Thanks for the explanation,   I've been watching this thread and waiting 
for an explanation that I could comprehend.  If I understand your 
explanation correctly, the steps are:

1) The substitution in the string:

str =<<STR
def hello
   puts #{y}
end
STR

is performed.  The scope is the global scope, so the string becomes:

str =<<STR
def hello
   puts first
end
STR

2)  The string is class_eval'ed in the scope of class T, producing:

class T
   def first
      "hello"
   end

   def second
      "chunky bacon"
   end

  def hello
     puts first
  end
end

3)  When the method hello is called, the line:

puts first

outputs the return value of the first method.


Applying those steps to the op's example:

>y = " another hi"
>
>class Wes
>end
>
>Wes.class_eval <<-END
>def hello
>    puts "#{y}"
>    puts y # I get wrong number of arguments error
> end
>END

1)  The string:

><<-END
>def hello
>    puts "#{y}"
>    puts y # I get wrong number of arguments error
>end
>END

becomes:

<<-END
def hello
    puts "another hi"
    puts y # I get wrong number of arguments error
end
END

2) The class Wes is transformed into:

class Wes
  def hello
    puts "another hi"
    puts y
  end
end

3) Then a statement such as:

w = Wes.new
w.hello

produces an error because there is no variable or method named 'y' 
inside class Wes:


rb:4:in `hello': undefined local variable or method `y' for 
#<Wes:0x25328> (NameError)




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