From: Robert Klemme <shortcutter@...>
Date: 2008-08-09T17:28:09+09:00
Subject: Re: Works in irb but not elsewhere

On 08.08.2008 23:01, cardboard42@gmail.com wrote:
> I think what's happening is that yield is calling the block passed to
> the caller of instance_eval so method is being called on self in that
> block's closure. Since it was executed at the top level that would be
> Object. Object.method takes 1 parameter, thus causing the error you're
> seeing. The reason the code I posted works because it takes the block
> passed to create and passes it directly to instance_eval as it's
> block. This means that method will be called on self in instance_eval,
> where it is set to the object instance_eval is called on, in this case
> the result of A.new.
> 
> Sorry if my explanation is hard to understand, but I'm positive this
> is not a bug in ruby. instance_eval { yield } simply has very
> different semantics from instance_eval &block.

Absolutely correct, there is no bug - at least not in Ruby.  You can 
also see it from this:

$ ruby <<XXX
 > class A
 >   def method
 >     puts "a"
 >   end
 >   def A.create
 >     A.new.instance_eval { yield }
 >   end
 > end
 >
 > A.create{ p self; method}
 >
 > XXX
main
-:10:in `method': wrong number of arguments (0 for 1) (ArgumentError)
         from -:10
         from -:6:in `create'
         from -:6:in `instance_eval'
         from -:6:in `create'
         from -:10

Note the output of "p self": it's main.  And this is the case because 
self is not changed for the block invoked via "yield".

Note also that this does not work in IRB as well:

$ irb
irb(main):001:0> class A
irb(main):002:1>   def method
irb(main):003:2>     puts "a"
irb(main):004:2>   end
irb(main):005:1>   def A.create
irb(main):006:2>     A.new.instance_eval { yield }
irb(main):007:2>   end
irb(main):008:1> end
=> nil
irb(main):009:0>
irb(main):010:0* A.create{ p self; method}
main
ArgumentError: wrong number of arguments (0 for 1)
         from (irb):10:in `method'
         from (irb):10
         from (irb):6:in `create'
         from (irb):6:in `instance_eval'
         from (irb):6:in `create'
         from (irb):10
irb(main):011:0>


Kind regards

	robert