On Fri, Nov 21, 2008 at 9:23 AM, Robin Green
withNextE seems to allow occurrences' values to depend on occurrences which haven't yet occurred.
Of course, it doesn't actually allow time travel. If the occurrence is not yet computed, presumably it will block until it does.
Still, this is problematic theoretically. It allows "simulations" to be constructed which contain reverse causality (if the simulation makes all the input occurrences available immediately). Is this a case of making the framework a little too powerful?
In the context of an event, we can distinguish between the time of an
occurrence (t), the time when the occurrence's time is computable
(ct), and the time when the occurrence's value is computable (cv).
Running a real-time simulation on an event, we would certainly want it
to be the case that for all occurrences, cv -- Occurs whenever a new piece comes on the screen.
newPiece :: Event Piece The time when the value of these occurrences is computable (cv) could
be when the game starts, since it is just a random sequence of pieces.
This is one case where withNextE could make sense (but where withNextE
. withTimeE_ probably wouldn't).
Consider an implementation of a preview pane. -- Preview of next piece
preview :: Behavior Geometry
preview = fmap toGeo (stepper (firstE newPiece) (withNextE_ newPiece)) David
--
David Sankel