[cells-cvs] CVS cells

ktilton ktilton at common-lisp.net
Tue Jun 6 04:54:11 UTC 2006

Update of /project/cells/cvsroot/cells
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--- /project/cells/cvsroot/cells/cells-manifesto.txt	2006/06/06 04:54:11	NONE
+++ /project/cells/cvsroot/cells/cells-manifesto.txt	2006/06/06 04:54:11	1.1

                                Cells In A Nutshell

The Cells library as it stands is all about doing interesting things with slots of CLOS instances.
Nothing says a global variable could not be mediated by a Cell, and indeed one Cells user is 
known to have experimented with that. Also, some work was done on having slots of DEFSTRUCTs mediated
by Cells. But for the rest of this exposition let's just talk about CLOS slots and instances.

The Cells library allows the programmer to specify at make-instance time that a slot of an instance
be mediated for the life of that instance by one of:

   -- a so-called "input" Cell;
   -- a "ruled" Cell; or
   -- no Cell at all.

Note that "slot of an instance" is not the same as "slot of a class". A vital feature of the Cells library
is that different instances may do different things Cells-wise with the same slot.

A slot mediated by an input Cell may be assigned new values at runtime. It is an error to assign a new
value to a slot of an instance not mediated by any Cell. Ruled Cells come with an instance-specific 
rule in the form of an anonymous function of two variables, the instance owning the slot and the prior value
(if any) computed by the rule. These rules consist of arbitrarily complex Common Lisp code, and are invoked
immediately after instance initialization or, if they are declared lazy, when their slot readers are invoked. 

When a rule runs, any dynamic read (either expressly in the rule source or during the execution of 
some function invoked by the rule) of a slot of any instance mediated by a Cell of any type establishes a
runtime dependency of the ruled cell on the slot of the instance that was read. Note then that thanks
to code branching, dependencies can vary after every rule invocation.

When application code assigns a new value to an input Cell (a quick way of saying an instance slot mediated by
an input Cell) -- typically by code polling OS events or a socket or an input device -- a cascade of recalculation
ensues to bring direct and indirect ruled dependents current with the new value assigned to the input Cell.

To allow the emergent data animation model to operate usefully on the world outside the model--if only to
update the screen--programmers may specify so-called observer callbacks dispatched according to: slot name, 
instance, new value, old value, and whether the old value actually existed (false only on the first go).

Finally, to make it possible for such a declarative model to talk intelligibly to imperative systems such as
Tcl/Tk which sometimes requires a precise sequence of commands for something to work at all, a mechanism exists by
which client code can (a) queue tasks for execution after a data change has fully propagated and (b) process
those tasks with a client-supplied handler. Tasks are queued with arbitrary keying data which can be used by 
the handler to sort or compress the queued tasks.

                                          Data Integrity

When application code assigns to some input cell X, the Cells engine guarantees:

  - recomputation exactly once of all and only state affected by the change to X, directly or indirectly through 
    some intermediate datapoint. note that if A depends on B, and B depends on X, when B gets recalculated
    it may come up with the same value as before. In this case A is not considered to have been affected
    by the change to X and will not be recomputed.

  - recomputations, when they read other datapoints, must see only values current with the new value of X.
    Example: if A depends on B and X, and B depends on X, when A reads B it must return a value recomputed from
    the new value of X.

  - similarly, client observer callbacks must see only values current with the new value of X; and

  - a corollary: should a client observer SETF a datapoint Y, all the above must
    happen with values current with not just X, but also with the value of Y /prior/
    to the change to Y.

  - Deferred "client" code must see only values current with X and not any values current with some
    subsequent change to Y queued by an observer


Program state guaranteed to be self-consistent, without programmer effort. Dependencies are identified
by the engine, and change propagation happens automatically.

Greater object re-use. Slots of instances can be authored with rules, not just literal values. In a sense,
we get greater reuse by allowing instances to override slot derivations instance by instance. But not slot
expressions, which are still class-oriented.

Natural decomposition of overall application complexity into so many simple rules and slot observers.


Any application that must maintain an interesting, long-lived data model incorporating a stream of unpredictable 
data. Two examples: any GUI application and a RoboCup soccer client.

An application needing to shadow data between two systems. Examples: a Lisp GUI imlemented by thinly wrapping a 
C GUI library, where Lisp-land activity must be propagated to the C GUI, and C GUI events must propagate
to Lisp-land. See the Cells-Gtk or Celtk projects. Also, a persistent CLOS implementation that must echo 
CLOS instance data into, say, SQL tables.

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