Cocoa and Cocoa Touch were designed with imperative programming in mind though, so this is what the creators of Swift had to work with. I think the main use cas

Cocoa and Cocoa Touch were designed with imperative programming in mind though, so this is what the creators of Swift had to work with. I think the main use case for mutable captured values is mutable self, in particular the following, extremely common pattern: dispatch_async(my_background_queue) { do_some_io() let result = do_expensive_computation() dispatch_async(dispatch_get_main_queue()) { self.value = result update_ui() } }