From: Chris White <cwprogram@...>
Date: 2011-08-15T01:26:51+09:00
Subject: Re: How to use "case" to match class names? (=== not so funny)

> Hi, easy example of what I need:
> 
> -------------------
>  klass = String
> 
>  case klass
>  when String
>    puts "I'm String class"
>  else
>    puts "I'm nothing..."
>  end
> -------------------
> 
> It produces "I'm nothing...". I understand why:
> 
> "case" matches the given object using "===", and:
> 
>  String === String
>  => false

Correct, because triple equals checks to see if the value is an instance or subclass of a class, not if it's the actual class itself

> 
> while:
> 
>  String === "a new string"
>  => true

Which is why this works. One way to potentially get around this is to have an object that simple holds a class and overrides the === operator to make case work properly:

class ClassCheck
  def initialize(klass)
    @holder = klass
  end

  def ===(other)
    @holder == other
  end
end

klass = String

case klass
  when ClassCheck.new(String)
    puts "String"
  else
    puts "Nothing"
end

You could also modify it to accept a variable number of arguments to check against multiple classes in a single when statement:

class ClassCheck
  def initialize(*klass)
    @holder = klass
  end

  def ===(other)
    @holder.each { | klass |
      return true if klass == other
    }
    false
  end
end

klass = String

case klass
  when ClassCheck.new(Object, Numeric, String)
    puts "One of these"
  else
    puts "Nothing"
end

This form works with a single class as well.

Regards,
Chris White
http://www.twitter.com/cwgem