I agree, but I still wouldn't call a nursery an abstraction; its an indirection, and a mutable one at that. I also disagree with the goto analogy; concurrency a

I agree, but I still wouldn't call a nursery an abstraction; its an indirection, and a mutable one at that. I also disagree with the goto analogy; concurrency and control flow are two distinct things, they have much more differences than similarities. Implementing a promise as a monad will yield all the same benefits while also keeping the ability to compose and be immutable; and then you have an abstraction and the result is simple. I agree restrictions make the code better, this just isn't one of these cases to me.