[ScottBurson]

Coroutines are not quite threads, but then, they don't require a scheduler. They're also not quite firstclass continuations, but then, they're simpler to implement, simpler to use, and don't have the same implications for the rest of the language and implementation design. Still, they can do some of the things you might otherwise use threads or firstclass continuations for; and as reikonomusha points out, those things can be quite difficult to do otherwise in a clean way.

I'm sure some of the implementors (though not Symbolics, of course) would have been unenthusiastic about their inclusion, but still I wish it had been done.

[ohyes]

Coroutines and first-class continuations make the implementation of non-local transfer of control much more complicated.

Their inclusion would have made staples like unwind-protect, conditions, and underlying them try/catch, much more difficult to implement, if not nearly impossible to get right.

I personally think that those language features are worth it and that this particular trade-off was well made.