It does work with (fn [] ...) instead of #(...)

I think the hint gets lost with the anonymous function

The upcoming arg-tags feature in 1.12 will provide a different better way to say this

(^[Runnable] Thread. …)

Oh, a vector of parameter types?

Yeah

Btw, why is it that the type hint is needed above in the first place? fns implement Runnable, is that not apparent when compiled?

They also implement Callable which is an overload here

Sorry, that’s in the executor calls, not here

Here it’s because that fn is not typed as a Runnable, it’s just an expression with unknown type

There is some nonobvious complexity here between what that expression is and what it returns when invoked