His issue seems to be the potential for modifying or referencing mutable state where in the yield block (or map or flatMap or whatever). It can be a problem.

His issue seems to be the potential for modifying or referencing mutable state where in the yield block (or map or flatMap or whatever). It can be a problem. In Akka actors, referencing sender() from a future will be unpredictable because sender() could've changed in the mean time. I think there are three strong solutions which address this problem: 1) Immutable state. Solves this problem completely but accidental capture remains an issue. 2) Hiding mutable state within actors. I hesitate to present this as a general solution though since it really requires going all-in with actors (I don't consider that to be a bad thing, necessarily). 3) Projects like Scala Spores [1] aim to tackle this at the compiler by capturing mutable references and executing async closures in an immutable environment and prohibiting accidental capturing. IOW, turning accidental capture into a compiler error. I'm excited about this one. [1] https://speakerdeck.com/heathermiller/spores-distributable-functions-in-scala https://speakerdeck.com/heathermiller/spores-distributable-f...