From: Richard Dale <Richard_Dale@...>
Date: 2004-08-18T00:30:57+09:00
Subject: Re: instance_eval

lopex wrote:

> Hi,
> 
> I'm wondering why the following code works:
> 
> class X
> end
> 
> x=X.new
> a="some variable"
> 
> 
> p x.instance_eval("a")
> x.instance_eval{p a}
> 
> why the x object knows the a variable ?
Maybe this makes it clearer:

class X
end

x=X.new
a="some variable"

def m1(thing, &block)
 block.call(thing)
end

def m2(thing)
 thing.instance_eval{p a}
end

p x.instance_eval("a")
x.instance_eval{p a}
m1(x) {p a}
m2(x)

The m1() method works because it is passed a block where 'a' is in scope.
Then in m1 the block is called with that complete context, ie including
'a'.

Whereas the m2() method fails because instance_eval() runs within the
context of just 'thing', ie 'x', without its environment.

"some variable"
"some variable"
"some variable"
try_instance_eval.rb:12:in `m2': undefined local variable or method `a' for
#<X:0x3004c0d8> (NameError)

-- Richard