From: Jan Svitok <jan.svitok@...>
Date: 2007-02-22T07:35:27+09:00
Subject: Re: Please illuminate how stupid I am re: initialization blocks.

On 2/21/07, Michael Judge <mjudge@surveycomplete.com> wrote:
> I seem to use blocks much differently from other people and that
> frightens me a little.  (Maybe I'm being really dumb and nobody wants
> to say anything.)
>
> Before I get into what I do, let me define how normal people call a
> block passed in to a method.  They use "yield".  And that's cool,
> that's what yield is intended for.  Their methods tend to look like
> this when they're later put into practical use:
>
>    create_table :users do |t|
>      t.column :first, :string
>      t.column :last, :string
>    end
>
> If I had programmed create_table though, I would have used
> instance_eval, and you'd use create_table like this:
>
> create_table :users do
>    column :first, :string
>    column :last, :string
> end
>
> Am I being really dumb?  Is there something horrifying that I'm not
> grasping?
>
> Thanks for your help!

Hi,

the difference is in the scope: with yield you'll have access to
variables outside the block, while with instance_eval you won't IIRC.
Search the archive for instance_eval scope, e.g. in a thread named
'can there be a "with" construction?' somebody mentioned that 'self'
changes when using instance_eval (self is the receiver of
instance_eval) and doesn't with yield (stays as was outside the
block).