[Haskell-cafe] Improving event management in elerea

Patai Gergely patai_gergely at fastmail.fm
Thu Aug 13 17:31:07 EDT 2009

Hello Jean,

If I got you right, you basically want to have a signal that changes its
value by a predetermined transform function whenever there is an edge
detected on a bool signal. In that case, it would be easier to define it
in terms of transfer:

accumIf :: a -> (a -> a) -> Signal Bool -> SignalMonad (Signal a)
accumIf v0 f b = transfer v0 g b
  where g _dt c x0 = if c then f x0 else x0

event :: a -> Signal Bool -> (a -> a) -> SignalMonad (Signal a)
event v0 b f = accumIf v0 f =<< edge b

I even factored out the edge functionality. Of course you also have to
pass playerX0 to this function as is, without the pure wrapping.

Also, your flat function is practically equivalent to (return . pure),
why did you define it using transfer?

Another thing to note is that you might want to define playerX as an
integral of a velocity derived from lpress and rpress, so you wouldn't
need to press and release the buttons repeatedly, and the speed would be
the same regardless of sampling rate, since integral takes the time step
into account.

> but I still can't answer my first one (couldn't this function
> -or its improvement- be part of the library ?)
Sure, anything is possible. Elerea is a new library, and it deliberately
exposes only some low-level constructs. In particular transfer, which
basically lets you fall back on pure state transformer functions. My
idea was that as more and more code is written in it, we can extract
recurring patterns and add them to the library. But at the moment it is
not clear what the useful patterns are, and I'd rather not start
guessing and littering the library with useless combinators in advance.

For instance, the accumIf function is a conditional version of stateful,
and it might be generally useful. Whether it's the best name, that's
another question.


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