From: Rick DeNatale <rick.denatale@...>
Date: 2009-03-02T23:52:16+09:00
Subject: Re: What does this statement do?

--001636163d193ef5d2046423fb50
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit

On Mon, Mar 2, 2009 at 9:31 AM, Stanislaw Wozniak <stan@wozniak.com> wrote:

> Ok, this is the example I want to get to work. Method set_field is never
> triggered because local variable set_field is being set.
>
> class A
>
>  def self.set_field=(value)
>      puts "Trying to set field value"
>  end
>
>  def method_missing(name, *args)
>  self.class.method(name).call args
>  end
>   def self.method_missing(name, *args)
>   new.method(name).call args
>  end
>
> end
>
> class B < A
>
>   def my_method(value)
>      set_field = value
>   end
>
> end
>
> B.my_method
>

Yes this is a well-known ruby newbie gotcha.

Because the Ruby syntax allows simple names to refer to either a local
variable or a method invocation, there are some times when things get
ambiguous.

In the case of a name on the right hand side of an assignment, or equivalent
settings, the ruby compiler treats the name as a local variable if the name
has already been assigned a value, and as a method call with an implied
receiver of self otherwise.

In the case of a name on the right hand side, it treats the name as a local
variable, and assigns it a value.

You HAVE to explicitly give a receiver of self when invoking an attribute
setter method in ruby. So in my_method you have to use self.set_field =
value.

And this is why, for those who like to mark methods as private.


class Foo

private
def a=(value)
     ...
   end

   def b
     ....
   end

   public
   def c
      self.a= 2  # This is okay
      self.b       # This triggers a NoMethodError "private method 'b'
called for ...
   end
end
-- 
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

--001636163d193ef5d2046423fb50--