From: Ben Giddings <bg-rubytalk@...>
Date: 2009-08-21T01:25:29+09:00
Subject: Re: extending ruby - handling errors

On Thursday 20 August 2009 11:49:03 Jason Lillywhite wrote:
> I want to raise an ArgumentError, "Function only takes numeric objects."
> if a non-numeric argument is passed to the existing Ruby method,
> Numeric.ceil.
...
> irb(main):009:0> "foo".ceil
> NoMethodError: undefined method `ceil' for "foo":String
>         from (irb):9

Right, because "foo" is a String, and Strings aren't numerics.  If you want 
all non-numerics to recognize that "ceil" is a valid call, but only for 
Numerics, you'd need to provide "ceil" to all objects, then override it for 
numerics:

irb(main):001:0> class Object
irb(main):002:1>   def ceil
irb(main):003:2>     raise ArgumentError, "not a numeric!"
irb(main):004:2>   end
irb(main):005:1> end
=> nil
irb(main):006:0> "Foo".ceil
ArgumentError: not a numeric
...
irb(main):005:0> (3.14).ceil
=> 4

This is probably the wrong approach though.  You shouldn't load up "Object" so 
it knows about random other methods that it's subclasses define, you should 
just catch NoMethodError in an appropriate spot.  For example:

def calculate_bounding_box(width, height)
  begin
     width_bound  = width.ceil
     height_bound = height.ceil
  rescue NoMethodError
    raise ArgumentError, "width and height must be numeric"
  end
  width_bound * height_bound
end


Ben