Recursive named effect

[iacore]

I have the following design in mind, but the compiler says

thread.kk(1, 7): error: Type task cannot be declared as a value type since it is recursive.

Code:

named effect task<a>
    ctl join() : (a, thread)
    ctl cancel() : thread
    ctl is_done() : bool

named effect thread
    ctl run(task : () -> a, thread: thread) : task<a> // consume the thread
    // drop the effect to destroy the thread

effect threadspawner
    ctl fork() : thread
 

In some other languages there is "unique" type (with multiplicity of 1) that cannot be cloned. I want the achieve the same with Koka.