Type Alias Ask 

pub type Ask = Arc<dyn Fn(Session) -> Pin<Box<dyn Future<Output = Sent> + Send>> + Send + Sync + 'static>;

Send one session and come back with its answers, or with why it did not.

Boxed rather than generic: the server hands this to spawned tasks and to the eval runner, and one Arc is easier to pass around than a type parameter threaded through every function.

Aliased Type

pub struct Ask { /* private fields */ }