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When I try to build today, I get this:

Unhandled UNDEFINED-FUNCTION in thread #<SB-THREAD:THREAD "main thread" RUNNING {1004477613}>: The function BLACKBIRD::PROMISE-VALUES is undefined.

http://report.quicklisp.org/2014-12-09/failure-report/green-threads.html has more context.

There have been a lot of updates to cl-async-future/blackbird recently. Maybe something got out of sync?

On QuickLoad of :GREEN-THREADS in LWM 7.1, I end up with the result:

CL-USER 2 > (ql:quickload :green-threads)
To load "green-threads":
Load 1 ASDF system:
green-threads
; Loading "green-threads"
[package blackbird-base]..........................
[package blackbird-syntax]........................
[package blackbird-util]..........................
[package blackbird]...............................
[package cl-async-future].........................
[package green-threads]..

The following function is undefined:
BLACKBIRD::PROMISE-VALUES which is referenced by GREEN-THREADS:FUTURE-VALUES

Looks like either:
(A) PROMISE-VALUES needs to be exported by BLACKBIRD-BASE and BLACKBIRD (and hence, CL-ASYNC), or else
(B) One should not be referencing an internal symbol from BLACKBIRD in GREEN-THREADS:FUTURE-VALUES.

I can't tell which of these alternatives is the correct one. From browsing the Blackbird docs, there is never any mention of directly referencing the PROMISE-VALUES, which leads me to think there are alternative ways of getting a promise value.

And from reviewing the Green-Threads docs, there doesn't seem to be a case presented where the need for FUTURE-VALUES is manifest.

I'm just getting started perusing the sources to make sense of all of this stuff.

So the issue seems to be that:

1. PROMISE-VALUES is an internal accessor function defined in BLACKBIRD-BASE, not exported
2. Package BLACKBIRD uses BLACKBIRD-BASE, and does not gain access to the internal symbols of BLACKBIRD-BASE
3. Hence no internal symbol BLACKBIRD::PROMISE-VALUES

So, since GREEN-THREADS is the client referring to BLACKBIRD internal symbols, the corrected form for GT:FUTURE-VALUES is:

(defun future-values (future)
"Returns the values given to the future when it
was completed."
(values-list (blackbird-base::promise-values future)))

It appears that Green Threads are intended to cycle amongst themselves on a single system thread, using THREAD-YIELD to pass control to the next available green thread.

But there is nothing to prevent multiple instances of the same green threads running concurrently on multiple cores in a multi-core SMP system.

This can become inherently unsafe unless the client programmer is fully aware of this situation. There is no warning of this in the current Green Threads documentation.