Thanks Daniel, I'll give this some consideration.

A more interesting alternative that could soon come to pass is an
extension to Facets' annotation system. Right now one can easily do:

  require 'facet/annotation'

  class Foo
    attr_accessor :x
    ann :x, :validation => proc { |x| ... }
  end

But this is nothing more than a note. For it to actually have any
effect one would have to manually utilize the note. With then next
version of annotation.rb I plan to add annotation hooks. By defining a
hook you can automatically have it do what you would like when a
particular annotation is define. For instance:

  Annotation.annotation_action( :validation ) { |symbol, validator|
    wrap_method( "#{symbol}=") { |target, x|
      unless validator.call( x )
        raise ArgumentError, "Invalid value provided for #{symbol}"
      else
        target.call(x)
      end
    }
  }

Now when you make the above annotation it will automatically generate
the validating setter as you describe.

The only downside to this kind of thing at the moment is the lack of an
adequate AOP foundation built into Ruby.

But all that is down the road yet. In the mean time I will likely add
your method. Thanks.

T.

P.S. Using this list is fine, especially if you're interested in wider
input from the Ruby community. You can also email the Nitro list
--Facets is a key component of Nitro/Og. And of course you can always
email me directly.