↓モナド？

More generally, by modeling both `throws` and `async` as effects on function types, we can eventually provide common abstraction tools to abstract over both effects in protocols and generic code

https://gist.github.com/lattner/429b9070918248274f25b714dcfc7619#rethrows-could-be-generalized-to-support-potentially-async-operations

Concrete proposal for async semantics in Swift