From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2013-01-17T02:25:08+09:00
Subject: Re: commutative operations

On Wed, Jan 16, 2013 at 6:15 PM, Nokan Emiro <uzleepito@gmail.com> wrote:
> Hi there,
>
> How would you solve the issue below?  I'm looking for an elegant
> solution for this one.  I usually bump into this when I create my own
> types.
>
> I have a class that extends Ruby's standard type set with my own
> type.  This new type is something that is supposed to be able to
> work together with the already existing, built-in classes as close as
> possible.  I mean it is a valid thing to for instance add a Fixnum
> to it, multiply it by a Rational number (which isn't zero, of course), etc.
> The class can do this, because it's :+, :*, :/, :- methods are
> aware of these types.  So, if x is an instance of my class, x + 1 works
> fine, just like x * 2, and all the others.
>
> The problem is that while I write programs, I always think about + as
> a commutative operation, so I unconsciously expect that not only
> x + 1, but the expression 1 + x gives the same result, which is of
> course not true, because 1 + x calls Fixnum#+, and that method
> does not know anything about my class.
>
> How could I make at least Numeric classes behave this way?  What
> I want is that if a Numeric instance method receives an object as a
> parameter that is an instance of my class, it should call the same
> method on the argument with itself (I mean 'self') as the parameter
> in case of commutative operatins, like + and *.  The problem is a
> bit more difficult in case of order-dependant operators, like / and -...
>

Take a look at this article:

http://blog.rubybestpractices.com/posts/rklemme/019-Complete_Numeric_Class.html

specifically the part about type coercion and operator overloading.

Jesus.