I believe that he's referring to the fact that Scala's Set is not a monad, so it "shouldn't" have a .flatMap operation. Scala has other containers that have a

I believe that he's referring to the fact that Scala's Set is not a monad, so it "shouldn't" have a .flatMap operation. Scala has other containers that have a .flatMap operation that are not real monads, such as Future and Try. That can lead to surprising behavior on for/yeld loops. Disclaimer: I do work in Scala and enjoy the language.