From: Joel VanderWerf <vjoel@...>
Date: 2005-01-13T01:38:55+09:00
Subject: Re: Kernel#eval and class definition

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