From: Robert Klemme <bob.news@...>
Date: 2005-01-13T18:51:20+09:00
Subject: Re: Kernel#eval and class definition


"Joel VanderWerf" <vjoel@PATH.Berkeley.EDU> schrieb im Newsbeitrag
news:41E55294.2040102@path.berkeley.edu...
> Florian Gross wrote:
> > Yukihiro Matsumoto wrote:
> >
> >> |>     def f ; class C ; end ; end
> >> |>     def f ; eval "class C ; end", binding ; end
> >> |
> >> |Well eval "..." is just an expression and "eval" evaluates everyhing
it
> >> |get's that's syntactically correct.  The ruby code in the string
> >> |expression is not syntactically located in method f.  It's parsed at
> >> |runtime while the line above is completely evaluated at compile
time.
> >>
> >> Thanks for explanation.  class definition in the method is prohibited
> >> by syntax just to detect errors.  And class definition in eval is
> >> allowed as a back door.  Besides, scope management also is a reason,
> >> but it's an implementation matter.
> >
> >
> > There's also Class.new which will create an anonymous class. You can
> > assign that to a constant via Module#const_set if you need to have a
> > named class for some reason. That might be a bit better than using
> > eval() depending on the use case.
>
> Better, in the sense that it takes a block (see below). But evaling a
> string to define the body of the class allows you to interpolate.
>
> def f(arg)
>    Class.new do
>      define_method :g do arg end
>    end
> end
>
> puts f("hello").new.g  # ==> hello

You can even use the def syntax:

irb(main):001:0> cl = Class.new do
irb(main):002:1* def foo() "bar" end
irb(main):003:1> end
=> #<Class:0x10186208>
irb(main):004:0> cl.new.foo
=> "bar"

That's better because then you can define methods that deal with blocks,
which you can't with define_method:

irb(main):005:0> cl = Class.new do
irb(main):006:1* def foo() yield "bar" end
irb(main):007:1> end
=> #<Class:0x10172468>
irb(main):008:0> cl.new.foo {|x| p x}
"bar"
=> nil

Kind regards

    robert