From: Robert Klemme <bob.news@...>
Date: 2004-05-02T20:53:59+09:00
Subject: Re: Commutativity of '*'


"Stephan K�mper" <Stephan.Kaemper@Schleswig-Holstein.de> schrieb im
Newsbeitrag news:c72lo9$hi3jp$1@ID-96582.news.uni-berlin.de...
> Hi all,
>
> say I have a new class Foo which provides * as a multiplicative method.
> Now, given the following lines:
>
> def some_operation_using( lft, rgt )
>    return "#{lft} times #{rgt}"
> end
>
> class Foo
>
>   def *( other )
>      # do something
>      res = some_operation_using( other, self )
>      return res
>    end
>
>    def to_s
> return "a Foo thingy."
>    end
>
> end
>
> # That given, the following lines will work
>
> f = Foo.new
> puts f * 42.0
>
> # But this won't. Instead I get:
> # .../lm.rb:25:in `*': Foo can't be coerced into Float (TypeError)
> # from .../lm.rb:25
> puts 42.0 * f
>
> What would be a (or even the) Ruby Way to get the last line to work?
> Some method to coerce a Foo into a Float? Or some hacking of Float
> itself? Like adding that, for example:
>
> class Float
>    alias old_mult *
>    def *( other )
>      if other.class == Foo
>        return other * self
>      else
>        return self.old_mult( other )
>      end
>    end
> end
>
> That doesn't seem to be terribly elegant, I think.
>
> Any other way?

You want coerce.  See my posting in thread "coerce(), what protocol to
implement it" from 30 Apr.
http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/98791

Kind regards

    robert