From: Lars Haugseth <ruby-talk@...>
Date: 2012-06-08T16:12:24+09:00
Subject: Re: Inexplicable Argument Error

On 08.06.2012 02:14, I��aki Baz Castillo wrote:
> 2012/6/6 Bartosz Dziewo��ski<matma.rex@gmail.com>:
>
> Ruby methods ending in "=" are special:
>
> def foo=(var)
>    return 42
> end
>
> foo="HELLO"
> =>  "HELLO"
>
> It returns "HELLO" instead of 42. So it's clear that the parses treats
> those methods in a "different" way.

The method is never called in your example. Instead, you assign a value 
to a local variable named "foo".

   def foo=(var)
     puts "foo was called with '#{var}'."
     42
   end

Assignment to local variable, method is never called:

   foo="hello"
   => "hello"

   foo
   => "hello"

Use explicit receiver to ensure we call the method:

   self.foo="hello"
   foo was called with 'hello'.
   => "hello"

Call method by using Object#send:

   send(:foo=, "hello")
   foo was called with 'hello'.
   => 42

Notice how the two ways to call the method give different results.

-- 
Lars Haugseth