On Mar 2, 10:41 pm, Jeffrey Straszheim <straszheimjeff...@gmail.com>
wrote:
> First off, Clojure is not Common Lisp (to say the least), so I don't think
> we necessarily need to create as stateful a library as the original Cells.
> I've used Cells-like architectures before in GUI apps (although not as a
> separate abstract library), and don't really think it is the best way.
I can certainly imagine a functional interface such that any "cell"
will
return a value that is a function of time (or step) and other cell(s)
at
the same time/step:
(value-at-step d 1000) = (d-func (value-at-step c 1000) (value-at-step
b 1000))
its crucial that all the other cells any cell depends on be at the
same step
as the step we're trying to compute before we do the computation or
we end up with multiple updates for the same time/step in certain
dependency graphs. IOW simply making one cell's value a function
of the value of some other cell(s) and *not* also a function of step/
time
loses for certain dependency graphs.
The "diamond" pattern is the classic example:
a is independent (or draws its value from GUI input, etc.)
b and c both depend on a
d depends on b and c.
a
/ \
b c
\ /
d
If we don't check the time/step/version of both b and c when a change
to either triggers an update to d, we end up updating d twice, once
with a broken value where only one of b or c is at the current step,
and a second time with the correct value.
I mention this because one the clojure cells-alikes had/has this bug.
A functional interface to this sort of library would need to be
stateful
under the covers in order to avoid lots of recomputation with long
dependency chains. Memoization is one simple means - memoize
is in clojure already, and you could keep a functional
interface while avoiding unnecessary recomputation of past state
transitions.
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