From: furufuru@... (Ryo Furue)
Date: 2001-11-20T20:51:53+09:00
Subject: [ruby-talk:25907] String#== : Why not error with different type?

Hi there,

I think it would be nice if String#== raised error if the argument
isn't a String.  What was the reason why the designer(s) of the String
class decided the equality method should return false as

 	a = "2"
	b = 2
	a == b # => false

?  I wish the last line raised an error....

I have two reasons why I think comparison of objects with different
types *should* be an error.
1) We naturally think that 'a == b' => true means that objects a and b
have the same value.  Then, most of us intuitvely think that 'a == b'
=> false means that they have different values.  But, the actual
implementation of String#== confuses us because 'a == b' => false can
mean two very different things: that the values are different or that
a and b are of different types.  Of course, you could argue that 'a ==
b' => true means that a and b are of the same type and have the same
value while 'a == b' => false means that 'a == b' is not true. 
Logical.  But, it's more natural and intuitive to say that a Sting and
a Fixnum can't be compared.

2) Not only it's more natural but also it's less prone to errors.  In
fact, I spent hard time to track down a bug which resulted from this:

 	# at one place 
 	a = file0.gets
	#...
	# at another
	b = file1.gets.to_i
	#...
	# later
	if (a == b) .... ## BUG!!!

The datafiles contained a bunch of integers.  But, since my purpose
was to merge two datasets into one, I actually didn't need to convert
strings to integers (but, it was equally OK if I converted).  I didn't
take care much because I knew Ruby is usually strict in type mismatch.
(3 + "4" raises an error, for example.)

What do you think?

Thank you,
Ryo