From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-07-20T00:52:04+09:00
Subject: Re: Instance Eval of a Proc

On Wed, 20 Jul 2005, Gavin Kistner wrote:

> Why can I not use a Proc created from a method in #instance_eval? (Or rather, 
> am I making a mistake, or is there a way to do this?)
>
> Related: are first-class functions planned for 2.0? The whole Proc/ 
> block/method situation needs unifying and simplifying, IMHO.
>
>
> [Slim:~/Desktop] gavinkis% cat instance_eval_proc.rb
>   class Foo
>     def self.show_name( *args )
>       puts "Foo.show_name says that @name is '#@name' for #{self.inspect}"
>     end
>     def initialize( name )
>       @name = name
>     end
>   end
>
>   f = Foo.new( "Jim" )
>
>   my_lambda = lambda{
>     puts "my_lambda says that @name is '#@name' for #{self.inspect}"
>   }
>   p my_lambda
>   f.instance_eval( &my_lambda )
>
>   my_proc = Proc.new{
>     puts "my_proc says that @name is '#@name' for #{self.inspect}"
>   }
>   p my_proc
>   f.instance_eval( &my_proc )
>
>   meth_lambda = Foo.method( :show_name ).to_proc
>   p meth_lambda
>   f.instance_eval( &meth_lambda )

just because you are call meth_lambda in instance_eval doesn't mean @name will
be defined.  imagine that you'd written this instead

   f.instance_eval{ Foo::show_name }

of course this is exactly what's happening - you've used the class Foo to look
up a class method that prints an uninitialized class variable.  the fact that
this whole business gets wrapped in another proc
(Foo.method(:show_name).to_proc) still doesn't initialize it nor bind the
instance's @name to the class @name somehow.  probably doing something like

   f.instance_eval do
     f.class.instance_eval{ @name = 'Jim' }
     meth_lambda.call
   end

might though.

another approach would be (i think) to 'install' the class method as an
instance one using define_method or something.

cheers.

-a
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