From: vjoel@...
Date: 2001-10-24T07:52:10+09:00
Subject: [ruby-talk:23114] Re: Bruce Eckel's opinion of Ruby

Paul Brannan wrote:
> module Foo
>   class Array < Array
>     def []=(foo, bar)
>       super(foo, bar)
>       puts 'foo'
>     end
>   end
> 
>   def self.foo
>     a = Array.new
>     a[0] = 42
>   end
> 
>   foo
> end
> 
> a = Array.new
> a[0] = 42

I think I'm going to frame that and put it on my wall!

This trick can also be used to dynamically type-check/convert, as a way
of emulating parametric types:

class Float
  class Array < Array
    def []=(i, j)
      super i, Float(j)
    end
  end
end

af = Float::Array.new

af[0] = 1
af[1] = 2.3
af[2] = "1.2e17"

p af
  => [1.0, 2.3, 1.2e+17]

af[3] = :foo
  => TypeError

You'd have to override a few other methods, such as <<. But it wouldn't
be too hard to automate generating x::Array for x in a list of classes.
Since the :: syntax accepts a variable on the left, you can choose the
"type parameter" dynamically:

x = 3.1
a = x.type::Array.new
a << x

Heartfelt thanks to the unnamed conference participant who came up with
this idea.

--
Joel VanderWerf                          California PATH, UC Berkeley
mailto:vjoel@path.berkeley.edu                     Ph. (510) 231-9446
http://www.path.berkeley.edu                       FAX (510) 231-9512