WTFRuby

gistfile1.rb

1.9.3p0 :001 > case nil.class
1.9.3p0 :002?>   when NilClass
1.9.3p0 :003?>     p 'yay'
1.9.3p0 :004?>   else
1.9.3p0 :005 >     p 'wtf'
1.9.3p0 :006?>   end
"wtf"
 => "wtf" 
1.9.3p0 :007 > nil.class
 => NilClass 
1.9.3p0 :008 > nil.class == NilClass
 => true 
1.9.3p0 :009 > 'WTF?!!'
 => "WTF?!!" 

Ruby case statements use === to make comparisons, which is aliased to is_a? at a class level. So what you're asking is "is NilClass an instance of NilClass?" hence the false return value.

>> case nil
>>   when NilClass
>>     "yay"
>>   else
?>     "wtf"
>>   end
=> "yay"

Very true, this stemmed from a couple of hours yakshaving working on a custom xml serializer for a godawful protocol. using case obj and then when Array wouldn't ever catch the array (because it was actually an AR::CollectionProxy, thanks Rails! But then AR::Relation or AR::CollectionProxy didn't work either). I had to switch to case; when obj.is_a?(Array) and it magically worked. This was just (what I thought was) an additional wtf I found along the way. Ta for the clarification :)