From: Eric Hodel <drbrain@...7.net>
Date: 2007-03-02T04:49:10+09:00
Subject: Re: Why does String have to_str and Integer have to_int?

On Mar 1, 2007, at 06:45, tsela.cg@gmail.com wrote:
> On Mar 1, 10:41 am, Brian Candler <B.Cand...@pobox.com> wrote:
>> On Thu, Mar 01, 2007 at 05:38:50PM +0900, Nanostuff wrote:
>>> That's pretty much my question :) Thanks.
>>
>> So you can write
>>   a += b.to_i
>> or
>>   puts c   # which calls an implied #to_s
>>
>> without getting an error if the argument already *is* an integer or a
>> string.
>>
>> Now, as to why there are both #to_i and #to_int, and #to_s and  
>> #to_str, I
>> have no idea :-)
>
> That's because those pairs of methods have different roles. The
> difference is subtle, but important:
> - #to_i and #to_s are conversion methods. They are used when one wants
> a *representation* of an object as an integer or a string. Nothing
> more, nothing less.
> - #to_int and #to_str are duck-typing methods, basically. In other
> words, they are methods that indicate that an object is Integer-like
> or String-like. The difference is a semantic one rather than a
> syntactic one. By supplying #to_str for instance, an object is
> agreeing to be used when a string would normally be, and react like a
> string in string contexts.
>
> If you check the Core API on http://www.ruby-doc.org, you'll see for
> instance that about a hundred classes implement #to_s, but only three
> implement #to_str. That's because of their semantic difference: Float
> can easily implement #to_s, since it's absolutely normal for numbers
> to have a string representation. However, it doesn't implement
> #to_str, because if it did it would basically be like saying that
> Floats are like strings, which they are of course not. On the other
> hand, Float implements both #to_i and #to_int because Floats can, as
> fellow numbers, be used where Integers would be. They are Integer-like
> enough.
>
> Basically, look at #to_i and #to_s as giving representations, while
> #to_int and #to_str are the sign that an object has a contract to
> behave like an Integer or a String in corresponding contexts.

Or

#to_s means an object *has a* String representation

and

#to_str means on object *is a* String representation