From: ara.t.howard@...
Date: 2006-03-20T00:20:15+09:00
Subject: Re: can you enable coercion between types?

On Sun, 19 Mar 2006, Markus wrote:

> On Sat, 2006-03-18 at 17:11, ara.t.howard@noaa.gov wrote:
>
>> try this:
>>
>>      harp:~ > cat a.rb
>>      class Duration
>>        def initialize seconds
>>          @seconds = seconds
>>        end
>>        def coerce other
>>          if other.class == @seconds.class
>>            [@seconds, other]
>>          else
>>            [Float(@seconds), Float(other)]
>>          end
>>        end
>
> There's a gottcha here.  Coerce is used to implement double dispatch and
> so needs to return the coerced values in the opposite order (see Pick
> Axe)--(other,self), not (self,other).
>
> The way you wrote it works fine for addition & multiplication, but
> you'll pull your hair out the first time you try to subtract or divide.

indeed.  good catch.

cheers.
-a
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