[armedbear-devel] threads, join, clos

David Kirkman dkirkman at ucsd.edu
Sun Apr 25 15:03:58 UTC 2010

On Sat, Apr 24, 2010 at 5:21 PM, Tobias C. Rittweiler <tcr at freebits.de> wrote:
>> Well, in the original patch it just returned the first value, and the second
>> value is the completed/interrupted flag.  That's on obvious error, so I'm
>> attaching a new patch that returns all of the values, plus the completed
>> interrupted flag (for a function returning (values 1 2) thread-join will now
>> return 1, 2, T)
> That strictly couples the caller with its return sites; pain that
> multiple values are usually supposed to obliterate from.

I think that the biggest problem with returning a status code at the
end of multiple values is that if you want to write a wrapper to
change the way the call operates (e.g. if you want to write a
sb-thread-join that throws like the sbcl version) at some point you've
got to convert the multiple values into a list, strip off the last
element, and then convert the list back into multiple values -- which
seems a bit inelegant.

>> > SBCL returns the values as multiple values, and signals an error in case
>> > it couldn't join. I'm not sure what interface I'd like more. On SBCL,
>> > you pretty much always have to wrap HANDLER-CASE around join-thread
>> > which makes it unappropriate to use with MAPC.
>> >
>> > At the project I'm currently working on, I added the following wrapper:
>> >
>> >  (defun join-thread (thread &key timeout
>> >                                  (on-timeout :timeout)
>> >                                  (on-failure :error))
>> >    ...)
>> >
>> > The ON-TIMEOUT/FAILURE arguments are returned as primary results, making
>> > it potentially ambiguous (does :error come from the return value of the
>> > thread or because of a failure?); however the user can specify these
>> > values -- and the user should almost always know in what range a
>> > thread's return value is going to be -- so it's my humble opinion on how
>> > I think JOIN-THREAD should look like. :-)
>> >

This version looks very convenient to use most of the time.  But it's
not easy to write a generic wrapper to change the semantics without
knowing the allowed return values of the underlying function.  Or is
it?  (That's a real question!)

It could be made fully general if we add an interrupted flag to
LispThread, and a function for querying it.  I think that way
thread-join is easy to use most of the time, but if there is
uncertainty to if :error is an allowed return value you can just
query the thread to see if it completed normally.

Whatever we do, we should have a timeout keyword.

I'm happy to change things, if we can agree what to change them to!



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