From: furufuru@... (Ryo Furue)
Date: 2001-11-24T16:38:28+09:00
Subject: [ruby-talk:26298] Re: String#== : Why not error with different type?

matz@ruby-lang.org (Yukihiro Matsumoto) wrote in message news:<1006526284.656538.15432.nullmailer@ev.netlab.jp>...
> Hi,
> 
> In message "[ruby-talk:25907] String#== : Why not error with different type?"
>     on 01/11/20, Ryo Furue <furufuru@ccsr.u-tokyo.ac.jp> writes:
> 
> |I think it would be nice if String#== raised error if the argument
> |isn't a String.
> 
> No.  It makes something harder in dynamic typed languages like Ruby.
> For example, where ary is a non homogeneous array.
> 
>   ary.select{|x| x == "foo"}
> 
> should be protected like
> 
>   ary.select{|x| begin x == "foo" rescue TypeError; false; end}

Your point taken.  However, it's different from my point, I'm afraid.
In my opnion, if you really want a 'false' when the type of the
argument is different, you should explicitly say so (as somebody
pointed out in this thread):

   ary.select{|x| x.eql?("foo")}

and I wish we left '==' to compare values only.  Of course, you could
argue that if we really want to compare values only, we should
explicitly say so:

   ary.select{|x| (x <=> "foo") == 0}

Then, the problem is: What is the most "natural" (intuitive)
definition of '==' ?  I'd like to argue that most programmers think
(or "feel") they are comparing "values" in their mindset when they say
'=='; they don't (usually) think they are comparing values *and*
types.  So, if you really need to mean the less common thing, you
should use the less usual operator 'eql?', and you should leave the
more common '==' for value comparisons only.

And I think this (what I think is) more natural definition would be
less error-prone, as I described in the original post.  The operator
=='s silently returning false for a different type can cause a subtle
bug.  When you write

   ary.select{|x| x == "foo"}

the reader of your code will wonder: "Does the author of the code
really mean that ary can be heterogeneous and s/he doesn't want to
'select' non-String component?  Or does s/he mean that ary should be
homogeneous and want to select only "foo"s?"  When you use a
heterogeneous array, you must be more careful than with a homogeneous
one.  A heterogeneous array is inherently more complex than a
homogenous one, and can be more confusing. So, *I* would make my
intention clearer by using 'eql?' instead of '=='.

By the way, the pickaxe book's table 7.1 (p.81) *seems* to contradict
actual definitions:

Operator      Meaning
==            Test for equal value.
....
eql?          True if the receiver and argument have both the
              same type and equal values. 1==1.0 returns true,
              but 1.eql?(1.0) is false.

In fact, this doesn't contradict anything: it doesn't say what happens
if == is given an argument of a different type.  But, this *seems* to
support my argument (that is, use eql? when you want to compare values
and types; use == when you want to compare values).

Thank you for your response,
Ryo