From: ara.t.howard@...
Date: 2006-12-13T03:58:19+09:00
Subject: Re: Assignment method strangeness

On Wed, 13 Dec 2006, Ryan Williams wrote:

> Assignment methods are weird, and I wish they acted more like regular
> methods.  It seems like they could be, but the parser doesn't properly parse
> the usage.  Let me show you what I mean.
>
> Assignment methods can't take more than one argument:
>
> class Test
> def d=(a,b)
>   puts "A: #{a}, B: #{b}"
> end
> end
>
> test.d = 1, 2      #=>ArgumentError: wrong number of arguments (1 for 2)
> test.d = [1, 2]    #=> ArgumentError: wrong number of arguments (1 for 2)
> test.d = *[1, 2]   #=> ArgumentError: wrong number of arguments (1 for 2)
>
> Assignment methods can't take blocks:
>
> class Test
> def t=(t, &block)
>   @t = t
>   block.call
> end
> end
>
> test = Test.new
> test.t = 5 { puts "block" }   #=> SyntaxError: compile error
>                                     #=> parse error, unexpected '{',
> expecting $
>
>
> You can't seem to replace the single required argument with a block:
>
> class Test
> def v=(&block)
>   block.call
> end
> end
>
> test = Test.new
> test.v ={ puts "block" }   #=> Exception: wrong number of arguments (1 for
> 0)
>
>
> You can, of course, get around these restrictions with #send
>
> test.send(:d=, 1, 2)     #=> A: 1, B: 2
>
> Though you can define these methods all you want, it seems as though the
> parser that does the syntactic sugaring at the call location makes some very
> narrow assumptions about the nature of the assignment method.  Is there any
> way around this?
>
> -RYaN

   harp:~ > cat a.rb
   require 'rubygems'
   require 'attributes'

   class Test
     attribute 't'
   end

   test = Test.new

   test.t 42
   p test.t

   harp:~ > ruby a.rb
   42

it looks really nice with classes:

     harp:~ > cat a.rb
     require 'rubygems'
     require 'attributes'

     class Base
       class << self
         attribute 'a' => 4
         attribute 'b' => 2
         attribute 'c'     # no default value
       end
     end

     class Derived < Base
       c 'forty-two'
     end


     p [Derived.a, Derived.b]
     p Derived.c


     harp:~ > ruby a.rb
     [4, 2]
     "forty-two"

if you don't want to install the gem just download and cut and paste the code:
it's on 42 lines long!

-a
-- 
if you find yourself slandering anybody, first imagine that your mouth is
filled with excrement.  it will break you of the habit quickly enough. - the
dalai lama