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I went ahead and brain dumped the most obvious multi-threaded solution I could come up with, to get some performance facts to go with all the rumors. It comes out roughly 5x slower than using conditions/restarts. Just in case someone was wondering where we are atm...

> Coroutines are a hack that is inferior to [...] real threads

I don't agree. Sometimes you don't want to pay the price of the OS context switch.

Anyway, in the context of the ANSI spec, threads were out of scope, but they could have done coroutines anyway.

> ... and delimited continuations

But CL doesn't have these, even de facto. Yes, there's CL-Cont, but it's pretty slow.

I guess you could implement generators and cooperative scheduling (which are useful abstractions with their own strengths, not inferior alternatives to threads) using threads. I never tried myself, but it feels backwards to me; like using 'loop' to iterate a list.