From: Robert Klemme <shortcutter@...>
Date: 2006-08-09T17:25:18+09:00
Subject: Re: How to react on nil or wrong object-type as parameter


I sort of agree to both of you and I think despite this thread you maybe 
not too far away from each other.

On 09.08.2006 00:19, Eric Hodel wrote:
> Numeric duck types use #coerce because #to_int won't cover all the 
> bases.  See the earlier thread on #to_i vs #to_int, and try:
> 
> 5 + Roman.new(6)
> 
> You'll find that #to_int does not do anything for you.

But *if* you define a class Roman #to_int should be part of it because a 
roman can actually be treated as an integer.  Of course you'll also have 
to implement #coerce and probably several others.

> This is why you shouldn't call or define #to_xxx methods, how and where 
> they are supposed to be used is confusing, convoluted and error-prone.  
> They aren't supposed to be used where you think they are supposed to be 
> used.

I agree that implementing and invoking #to_int and #to_str needs 
considerably more consideration than their counterparts #to_i and #to_s 
- and you'll rarely see them implemented.

The standard lib has several classes which define these methods: you can 
try it out:

ruby -e '%w{to_int to_str}.each {|mn| print "\nMethod ", mn, ":\n"; 
ObjectSpace.each_object(Module) {|cl| p cl if cl.instance
_methods.any? {|m| mn == m}}}'

> I can't think of a legitimate reason to define a RomanNumeral class.  It 
> would be easier to add Integer#to_roman that returns a String and 
> Kernel#Roman that parses a String into an Integer.

IMHO you can take at least two different positions here - which one is 
more appropriate probably depends on the problem to solve.  You can view 
a roman number as another representation of an integer value.  In that 
case Erik's approach is more appropriate.  You can as well view a roman 
number as something specific if it has properties that are not covered 
by standard integer classes.  In that case it deserves a class of its 
own including #to_i, #to_int, #coerce, #eql?, #==, #hash, #<=>, #to_s 
and maybe more.

Kind regards

	robert