Hi all, Code first: cocoa_array = NSArray.new ruby_array = [] puts ruby_array.count # => 0 puts ruby_array.count { true } # => 0 puts ruby_array.count("whatever") # => 0 puts cocoa_array.count # => 0 puts cocoa_array.count { true } # => 0 # unknown: warning: passing a block to an Objective-C method - will be ignored puts cocoa_array.count("whatever") # => wrong number of arguments (1 for 0) (ArgumentError) I originally ran into this issue in the following code, where the fact that we have an NSArray, and not a ruby array ends up concealed by the bajillion of ruby-ish method calls such as compact and map: paths_from_clipboard = NSPasteboard.generalPasteboard.pasteboardItems .map { |pbi | pbi.stringForType('public.file-url') }.compact .map { |url | NSURL.URLWithString(url).path } .map { |path| Pathname.new(path) } Full, pretty and colorful version: http://gist.github.com/602174 What's going on there is that NSArrays (but not NSMutableArrays) will use the vanilla Cocoa's count method. So I'm resorting to doing: Array.new(SomeCocoaClass.withAMethodThatReturnsAnNSArray) I wonder, is this a bug or "works as designed"?