Practical Meta Programming in Ruby
I really recommend watching this talk. It's about metaprogramming in Ruby but the majority of things are applicable to our loved Objective-C, since both languages take advantage of the Smalltalk message passing system.
I really like Swift safeiness but I hope that we don't lose this powerful tool. In fact I've been doing some things in Swift that resulted to be more difficult that doing it in Objective-C with a little of runtime programming.
I have completed a second major release of a library that fully conjugates Classical Latin verbs in each of their 133 froms * 5 standard paradigms. Owing to the irregularity of human communication, modeling the provision of unambiguous answers (return values) to ambiguously asked things (flexible / incomplete method calls) might have required hundreds, if not thousands, of method definitions or static values entered in a database.