From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-08-08T07:30:10+09:00
Subject: Re: How to react on nil or wrong object-type as parameter

Eric Hodel wrote:
> On Aug 6, 2006, at 7:55 AM, Daniel Schierbeck wrote:
> 
>> Yochen Gutmann wrote:
>>> Hi,
>>> I was just wondering: how do I react on a method-invocation with a 
>>> nil-object instead of eg. String? How do I test it? And how can I 
>>> assure that it is the right kind of object?
>>> class AClass
>>>   def a_method aString
>>>      puts aString+aString
>>>      puts aString.reverse
>>>    end
>>> end
>>> an_object = AClass.new
>>> an_object.a_method "hello"  # works
>>> an_object.a_method          3#-> undefined method
>>> an_object.a_method nil      #-> undefined method
>>
>> 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.


Cheers,
Daniel