From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-08-07T03:50:12+09:00
Subject: Re: How to react on nil or wrong object-type as parameter

Yochen Gutmann wrote:
>>    def foo(obj)
>>      unless obj.respond_to? :to_str
>>        raise ArgumentError, "argument must be a string"
>>      end
>>
>>      str = obj.to_str
>>
>>      ...
>>    end
> I see, but if it is a long method I might end up with
> 
> unless obj.respond_to? :to_str,:another_method, :yet_another_method, 
> :and_again_another_method

Yes, that's true. But then again, all objects that respond to those 
messages can be used as arguments to your method, thus increasing the 
flexibility of your application dramatically.

Oftentimes, it isn't necessary to use #respond_to?. Simply call the 
methods on the received argument -- if an exception is raised, you can 
easily debug. e.g.

    nil.foo
=> test.rb:11: undefined method `foo' for nil:NilClass (NoMethodError)

Here you get the line number the error occurred on (11), the kind of 
exception raised (NoMethodError), the message not responded to (#foo), 
and what kind of object the message was sent to (nil).

> Another way would be to wrap a begin/rescue around the whole method-body 
> to capture all possibly arrising ArgumentErrors. Am I right with this?

You could do it like this:

   def foo(obj)
     foo.bar!
   rescue NoMethodError => error
     puts "argument must respond to #{error.name}"
   end

Although this has the downside that it'll rescue *all* such exceptions, 
even those caused by your own mistakes.


Cheers,
Daniel