[cells-devel] I think it's a bug [was SOME vs The Ephemerals]

[Ken Tilton]

If you have been following the drama, i broke Lars's chat log with this rule
(somewhat revised):

  (chat-log:accessor chat-log
        :initform (c? (concatenate 'string
                            (or .cache "")
                            (some 'speech-of (participants-of self)))))

SOME stops at the first non-nil speech, establishing no dependencies on the
speech of any participant farther down the "participants-of" list.

This is a problem, because speech-of is an ephemeral. It gets set to nil
/without/ propagating after any non-nil value gets set and propagated. That
is what ephemerals are all about, and I think that much is solid.......
maybe not. We'll come back to this. Anyway, if someone farther down the list
now says something, the rule does not run because it has no dependency on
their speech. And the log misses an entry.

Now I could just call this a user error ("Hey, Kenny! No dependency, no
re-calc!") and wipe my hands of the whole mess, but I have been doing Cells
for ten years and I had to debug in the internals for the first time in a
lonnng time to figure this one out, so I think an unwritten law is being
violated: normal, healthy Lisp code should transparently maintain
dependencies automatically.

deep background: if there is no way around this, so be it, we have an entry
for a "gotchas" FAQ. But one thing that ruins supercool libraries is exactly
this sort of violation of the principle of least surprise. So I want to see
if the above code can be made to work.

What I was thinking (until just now) was that any time a rule encountered an
ephemeral, just after the ephemeral got reset, the rule should be rerun just
to establish dependencies, not for its value. The code would be simple
(famous last words): after resetting an ephemeral, iterate over all
dependents of the ephemeral and call their rule, ignoring the return value.
Weird, but I bet it works.

The alternative is to implement the "reset ephemeral step" actually as a
proper datapulse in which the ephemeral is set to NIL. Full propagation,
including observers of the ephemeral. Boy, that sure weakens the
ephemerality metaphor. :)

Thoughts? Meanwhile, I will canvas real-world uses of ephemeral cells for
inspiration.

cheers, kenneth
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