I still had to use attempt, as without it the Done error is thrown I guess on next tick after call to Bus.kill (error "Done") bus. is this expected behavior of Aff @natefaubion ? I guess i shouldn't throw, as launchAff returns Fiber and i expect Fiber to throw once it's "joined".

Testing read/write/kill...
ok
/home/travis/build/slamdata/purescript-aff-bus/output/Control.Monad.Aff/foreign.js:513
                throw util.fromLeft(step);
                ^
Error: Done
    at Object.exports.error ....

I think you can eliminate the attempt by holding onto the spawned fiber inside the Bus structure and when killing, kill the fiber first.

That's a good idea, it would also make kill block until the fiber is fully cleaned up, tho it means we need to add eff type var to Bus or use unsafeCoerce to make that work.
In PS 0.12 as there is no effect label this could be done safely.

It would be nice to merge this open PRs, and do that later as of PS 0.12 update.