From: "Mikael Høilund" <mikael@...>
Date: 2008-08-09T05:36:57+09:00
Subject: Re: Works in irb but not elsewhere


On Aug 8, 2008, at 22:16, Patrick Li wrote:

> Thanks for all your comments:
>
> I think there's a bug in how ruby treats block parameters.
>
> ie. the following works
> def myMethod &block
>  A.new.instance_eval &block
> end
>
> but this doesn't
> def myMethod
>  A.new.instance_eval{yield}
> end
>

For reasons I cannot quite understand at the moment, the block for  
instance_eval is executed in the scope of the receiver, but yield  
inside that block will cause the block given to the *caller* to  
execute, not raise a LocalJumpError as one would expect (since there's  
no block to yield inside the object). That block is executed at its  
original scope:

 >> @foo = :outside
=> :outside
 >> class Klass
 >>   def initialize
 >>     @foo = :inside
 >>   end
 >> end
 >> Klass.new.instance_eval { @foo }
=> :inside # As expected
 >> def lol; Klass.new.instance_eval { yield } end
 >> lol do puts @foo end
=> :outside # o.O

I might be completely missing something here, but I would expect that  
last line to raise that darn LocalJumpError. This shows that the  
“current block” inside the instance_eval is indeed what's passed to  
the caller.

…
 >> def lol; Klass.new.instance_eval { Proc.new } end # Proc.new with  
no block returns the current block, cf. http://bit.ly/37NtlX
 >> my_proc = proc { :test }
=> #<Proc:0x0004fb28@(irb):15>
 >> lol &my_proc
=> #<Proc:0x0004fb28@(irb):15>
 >> lol(&my_proc) == my_proc
=> true

This might be a bug.

-- 
# Mikael Høilund
def method_missing(m, a=0) a +
m.to_s[/[a-z]+/].size * 2; end
p What is the meaning of life?