From: benjohn@...
Date: 2006-05-11T19:02:38+09:00
Subject: Re: plz help with binding

> Ruby guru, please help:
>
> I'm creating variable-less storage, wich works fine:
> #
> def arr_fn() ; arr=[] ; lambda{ arr } ; end
> fa = arr_fn ; fa[] << 10
> fb = arr_fn ; fb[] << 20
> p fa[] #=> [10]
> p fb[] #=> [20]
> # note that fa, fb have different binding for local arr - as expected
>
> Now I'm trying to create instance-var-less storage:
> #
> class C
>     def self.arr_meth( sym )
>         arr = []
>         define_method sym.to_sym, lambda{ arr }
>     end
>     arr_meth :arm
> end
> ca = C.new; ca.arm << 10
> cb = C.new; cb.arm << 20
> p ca.arm #-> [10, 20]
> p cb.arm #-> [10, 20]
> p( ca.arm.eql?( cb.arm ) ) #-> true
> #
> Why ca.arm, cb.arm share same local variable?
> Is it bug or feature?

:) You're mad :) But I get you... I think.

ca.arm and cb.arm share the same local because they're the same function.

In the first example, you're creating a new lambda, that holds a new
array, with each call to "arr_fn".

In the second example, you're only creating the method "arr" inside the
class B once (with the line "arr_meth : arm"). This method (which is
sitting in the class B, not in each instance of class B - the instances
_share_ it), is what you're later calling, and appending to.

:) Totally mad, but thanks, it's a good mind bender.

[I really don't mean to sound insulting, by the way, I think you're
doing something impressively crazy there - it reminds me of "boxes" in
lisp (I think "boxes" is the right term).]