From: Chris Pine <nemo@...>
Date: 2002-12-11T14:05:53+09:00
Subject: all the pretty evals...

So, I've been kind of confused about all of the different ways of evaling
stuff; in particular, all the different ways to define methods in classes:

  class Foo
    def bar
      "bar"
    end
  end

  p Foo.new.bar  #  -->  "bar"

...is just like...

  class Foo
    $binding = binding
  end

  eval 'def bar; "bar"; end', $binding

  p Foo.new.bar  #  -->  "bar"

...which is just like...

  class Foo
  end

  Foo.module_eval 'def bar; "bar"; end'

  p Foo.new.bar  #  -->  "bar"

...which is just like...

  class Foo
  end

  Foo.module_eval {define_method("bar"){"bar"}}

  p Foo.new.bar  #  -->  "bar"

So, am I right that those are all basically identical ways of doing the same
thing?  Here's something slightly different:  extending only one object:

  class Foo
  end

  foo = Foo.new

  foo.instance_eval 'def bar; "bar"; end'

  p foo.bar  #  -->  "bar"

...which is maybe the same as...

  class Foo
  end

  foo = Foo.new

  #  Is this the only way to get this class?
  klass = class << foo; self; end

  klass.module_eval 'def bar; "bar"; end'

  p foo.bar  #  -->  "bar"

...however, the following does not work...

  class Foo
  end

  foo = Foo.new

  foo.instance_eval {define_method("bar"){"bar"}}
  NoMethodError: undefined method `define_method' for #<Foo:0x400be61c>
          from (irb):4
          from (irb):4:in `instance_eval'
          from (irb):4:in `instance_eval'
          from (irb):4

...Well, I understand why that didn't work... I guess my question is, why
does...

  foo.instance_eval 'def bar; "bar"; end'

...work?  What does that even mean?  Also, why (in the last working snippet)
does foo.class give Foo, rather than klass?

Just trying to make sense of it all,

Chris