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Wed Mar 3 11:23:41 CET 2010

The Actor model is not composable

I recently ran into this in practice (in Java): a system with a lot of
message passing suddenly needed a sequential behaviour.  This then
lead me to remove all synchronous message passing and use semaphores
instead.

It talks about something I've tried to hint at earlier:

   * FP allows for ``exponential'' expressivity: since everything can
     compose with everything else, the total number of expressable
     behaviours grows very fast.

   * Stateful programming allow for ``linear'' expressivity: you can
     add stuff, but in general it can't be combined as-is with other
     code.

This is of course a black&white picture, but it seems to be true in
spirit: it's harder to reach exponential expressivity in stateful
languages exactly because of the inertia present in state -- hidden
assumptions as mentioned in [1].

[1] http://pchiusano.blogspot.com/2010/01/actors-are-not-good-concurrency-model.html



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