From: GOTO Kentaro <gotoken@...>
Date: 2001-09-28T15:17:00+09:00
Subject: [ruby-talk:21797] Re: Help: how to (re)define the unary minus?

From: furufuru@ccsr.u-tokyo.ac.jp (Ryo Furue)
Subject: [ruby-talk:21795] Help: how to (re)define the unary minus?
Date: Fri, 28 Sep 2001 14:48:36 +0900

> Hi there,
> 
> I'm wondering how one should (re)define the unary minus operator for a
> class.  The following fails:

A way is to define -@. 

> Note that what I want to create is not a complex class; I've used it
> as an obvious example.

If your target is a kind of Numeric, the following may help:

class X < Numeric
  def initialize(x)
    @val = x
  end

  attr_reader :val

  def -(x)
    case x
    when Integer, Float
      X.new(@val - x)
    when X
      X.new(@val - x.val)
    else
      a, b = x.coerce
      a - b
    end
  end

  def coerce(x)
    case x
    when Integer, Float
      [X.new(x), self]
    else
      raise TypeError
    end
  end
end

p -X.new(2) # default -@: 
            #   0 - X.new(2) ==> a,b = X.new(2).coerce(0) ==> a - b