I used unsafe* function, I'm not pure anymore. 😭

A common pattern: if b then m else return () == when b m

Very Nice!

I considered doing this initially and I will now. Seems like a good idea.

Consider splitting the type parameter v of Coroutine in two seperate types: i for input values and o for output values.

This doesn't only yield a more expressive result, but also provides more guidance as of which elements need to be plugged in where - especially in the implemetation of >>>.
Another cool thing is that all code - except the type annotations - can stay the same.

Does the GADT enforce that there isn't a literal on the left hand side of an operator? That's neat!