[latch]

I think having a thread pool on top of some evented IO isn't _that_ uncommon.

You might have a thread pool doing some very specific thing. You can do your own threadpool which wont use the Io interface. But if one of the tasks in the threadpool wanted to read a file, I guess you'd have to pass in the blocking Io implementation.

[throwawaymaths]

one of the io interfaces provided is a standard threadpool io. and if it was really important, you could write your own io interface that selects between std threadpool and std blocking based off of an option (i am guessing, i don't know, but seems reasonable)