From: Chris Pine <nemo@...>
Date: 2003-03-27T15:37:40+09:00
Subject: Re: case not calling class' '===' method?

----- Original Message -----
From: "Phil Tomson" <ptkwt@shell1.aracnet.com>

Thanks matz.  Could you explain why it is done this way?
----------------------------

Well, I'm not Matz, but I'll give it a shot...

It works this way because this is how you want it.  :)

In a case statement, you are trying to fit an object into one of many
different categories.  You could try to have every object figure out what
categories it belongs to (terribly difficult), or you could have each
category figure out what belongs in it (easy).

Just look at some examples of case statements:

  x = 5

  case x
  when 4 then ...
  when 5 then ...
  ...
  end

  case x
  when String then ...
  when Fixnum then ...
  ...
  end

  case x
  when 0...3 then ...
  when 3...6 then ...
  ...
  end

Imagine trying to code Fixnum#=== with this in mind:

  class Fixnum
    def === other
      if other.kind_of? Fixnum
        self == other
      elsif other.kind_of? Module
        self.kind_of? other
      elsif other.kind_of? Range
        ...
      ...
      end
    end
  end

You'd almost have to use a case statement!  Seem like a bad idea yet?  If
not, imagine the code in String#=== or Array#===; most of it would be
identical to what is in Fixnum#===.

The current system is definitely the Right Way.

Chris