From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-08-09T02:35:12+09:00
Subject: Re: How to react on nil or wrong object-type as parameter

Eric Hodel wrote:
> On Aug 8, 2006, at 2:45 AM, Daniel Schierbeck wrote:
>> Eric Hodel wrote:
>>> On Aug 7, 2006, at 3:30 PM, Daniel Schierbeck wrote:
>>>> Eric Hodel wrote:
>>>>> On Aug 6, 2006, at 7:55 AM, Daniel Schierbeck wrote:
>>>>>> I'd do it like this:
>>>>>>
>>>>>>   class SomeClass
>>>>>>     def foo(obj)
>>>>>>       str = obj.to_str
>>>>> Don't call to_str, call to_s.
>>>>>>       puts str + str # or `str * 2'
>>>>>>       puts str.reverse
>>>>>>     end
>>>>>>   end
>>>>>>
>>>>>> Though I know some very smart people on this list disagree with me.
>>>>> Only because you call to_str.  This is not the reason it exists.
>>>>
>>>> I favor using #to_str when I want to treat an object like a string. 
>>>> Practically all classes implement #to_s, so it's not really a type 
>>>> restriction. We're trying to solve two different problems -- I want 
>>>> to let the caller know if he sent the wrong kind of object as an 
>>>> argument. That's type checking. If you want to convert any object 
>>>> received to a string, then we're talking type conversion, which I 
>>>> think should be left to the caller in many cases. But that's just my 
>>>> humble opinion, of course.
>>> #to_str is not for type conversion.  It exists for classes that duck 
>>> type completely to String and is called when a C string is needed by 
>>> a method in String to improve performance.  In other words, #to_str 
>>> should only be implemented on objects that are already "Strings".
>>
>> Exactly my point. I think we're misunderstanding each other here; if 
>> you have an object that is a representation of a string, say
>>
>>   # Bad example, but hey...
>>   class Name
>>     attr_accessor :first, :last
>>
>>     def initialize(first, last)
>>       @first, @last = first, last
>>     end
>>   end
>>
>> Then it's okay for it to have #to_str, e.g.
>>
>>     def to_str
>>       "#{first} #{second}"
>>     end
>>
>> My own logic is this: if an object is a natural string, but with 
>> *more* information, then it can have #to_str. Otherwise, stick to #to_s.
> 
> A Name is not a String.  You don't #each it, you don't #chop it, its 
> never #empty?, ...

That's why you use the return value of #to_str. If only String had 
#to_str, then why have it in the first place? Why not just check the class?

I'm not saying this is how everybody should do it, I'm just saying that 
it's a very smart way to handle types -- duck typing being a very 
similar way.


Cheers,
Daniel