From: Greg Weeks <greg.weeks@...>
Date: 2007-11-17T06:06:37+09:00
Subject: "instance_eval" (eg, sent to a class object)

I've poked around, but I don't get instance_eval at all.

I do know what it means to evaluate code in the context of a *class*
object.  Ie, I know what *this* does:

    class Foo
      # We're now in the context of the object Foo.
      C = 1
      class Bar
  def bar ; "bar" ; end
      end
      def foo ; "foo" ; end
    end
    p Foo::C  ->  1
    p Foo::Bar.new.bar  ->  "bar"
    p Foo.new.foo  ->  "foo"

Now let's try this again with "instance_eval":

    class Foo
    end
    Foo.instance_eval do
      # We're now supposedly in the context of the object Foo.
      C = 1
      class Bar
  def bar ; "bar" ; end
      end
      def foo ; "foo" ; end
    end
    p Foo::C
    p Foo::Bar.new.bar
    p Foo.new.foo

Nothing works as I expected.  The result is:

tmp.rb:26: warning: toplevel constant C referenced by Foo::C
1
tmp.rb:27: warning: toplevel constant Bar referenced by Foo::Bar
"bar"
tmp.rb:28: undefined method `foo' for #<Foo:0x401c25b0> (NoMethodError)

Evidentally, C and Bar scoped lexically, rather than being in the
context of Foo.  And the foo definition ended up in the context of
"class <<Foo ... end".

So, hat in hand, I ask for a description of "instance_eval".
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