So everything in ruby is an object, which means that when using format strings with Objective-C methods like String+stringWithFormat and NSLog, the only formatter that makes sense is "%@". Other formatters generally give unexpected results. E.g. "%d" will print the value of the pointer to the number object. (Or I think that's what it is.) OTOH ruby itself is, by necessity, smarter in its own handling of format strings. Kernel#sprintf and String#% will generally figure out a way to shoehorn whatever object you're passing into the necessary format, e.g.: "%d" % "4" # => "4" "%d" % 4.1 # => "4" Assorted observations: - Newcomers will inevitably run into sad situations trying to use NSLog with %d and getting really weird numbers. - The inconsistency between the Objective-C and Ruby behaviors still feels weird even though I understand why it is that way. - As far as I know, there' no way to specify a non-object argument value (e.g. int) in a ruby method call. - I'm not sure there's a sane solution to this, e.g. if it's safe to translate ruby objects into the necessary native C types on the fly. - I wonder if, even after being painfully aware of this dichotomy, there will still be pitfalls that can't be worked around: - For example, how do I deal with localizedStringWithFormat? Ruby's native methods aren't locale-aware. - Or what about .strings localization files in a mixed Objective-C / MacRuby app? Has this been given any thought? What are the current plans / recommendations?