From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2011-03-14T19:32:39+09:00
Subject: Re: Unespected result of define_method with "for" and "each"

On Mon, Mar 14, 2011 at 11:20 AM, lucapette <lucapette@gmail.com> wrote:
> Hi everyone,
>
> I was trying to define instance methods on a class based on an given array of strings. I run into different results using two different types of iteration. I can't understand the resulting behaviour just in a case, the following piece of code will be more meaningful than words:
>
> class Toy
>
> end
>
> %w{e1 e2}.each { |m|
>    Toy.class_eval do
>        define_method m do
>            puts m
>        end
>    end
> }
>
> for m in %w{u1 u2}
>    Toy.class_eval do
>        define_method m do
>            puts m
>        end
>    end
> end
>
> p=Toy.new
>
> puts p.e1 # ok
> puts p.e2 # ok
> puts p.u1 # WTF
> puts p.u2 # ok
>
> I tested it with ruby 1.8.7 and with 1.9.2 and I get the same results . Obviously, the third result is the only I don't understand :). It seems that define_method defers the evaluation of "m" object when used with a for.
>
> I'm relatively new to Ruby and  maybe this is the expected behaviour. I would like to understand the difference between the to version,
> thank you.

It is expected, and the reason is the way closures work and the fact
that the block passed to "each" is a new scope, while the body of the
"for" is not a new scope. In the first case, the closures created by
class_eval and define_method close over a local variable m, *which is
different for each iteration*, because it is local to the block passed
to each, and the block is a new scope, so every iteration creates a
new variable m. In the second case, though, the m is a local variable
but to the whole script, it's not local to the body of the for, since
the body of the for does not create a new scope. And so, in this case
the closures created by class_eval and define_method close over a
single variable m, which after all is said and done refers to the
string "u2", so when you are calling the method u1, it returns this
string.

Jesus.