From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-07-29T03:45:05+09:00
Subject: Re: Symbols are your friends

Aleks Kissinger wrote:
> If an object _can_ be represented as an integer in some cases (e.g. a
> string, which could be set to something like "12"), it implements
> to_i.
> 
> If an object _is_ a natural representation of an integer, (like roman
> numerals, as the example the Pickaxe gives), it implements to_int.

That very nicely sums it up!

> So this makes sense, though nothing about the naming convention of
> these methods makes this particularly clear.  However, this brings up
> the question:  If something acts exactly like an integer method-wise,
> whats the benefit of explicitly calling to_int?  This seems like a non
> duck-typey thing to do.

In the example with the roman numerals, you might not want to implement 
all the behavior of Numeric or Fixnum -- it would make your class 
unnecessarily complex. Rather just have a #to_int method that returned a 
normal integer, be that a Fixnum or another object responded to the same 
methods. In many cases, you have to return a Fixnum, or some methods 
will complain.

This approach means that you can give your roman numeral objects 
directly as arguments to methods. If they call #to_int on the object 
before dealing with it, it'll behave just as if it's a Fixnum.

   # doesn't really have to be an integer,
   # but what the hell...
   def double(num)
     num.to_int * 2
   end

   double(roman_num)


Cheers,
Daniel