[Haskell-cafe] ANN: Elerea, another FRP library

[Wolfgang Jeltsch]

Am Mittwoch, 22. April 2009 16:00 schrieb Patai Gergely:
> This also means that if you want to restart a signal without external
> dependencies using a latcher, you have to inject some bogus dependency
> to prevent memoisation. If the new signal depends on some others,
> latching should behave intuitively.

So does this mean that whether a signal is started at the beginning or at 
switching time depends on what dependencies the signal has?

> > Implementing the first semantics is difficult since the system
> > would have to know what signals will be used later. I think
> > this is impossible in general because of undecidability
> > issues. (Grapefruit's approach is to force the user to specify
> > what signals are used later.)
>
> Do you have a compact use case that demonstrates this problem?

Maybe it’s not directly undecidability.

What I have in mind is the following: Say, you have a complicated function
f :: DTime -> Bool and two signals s1 and s2. Then you form the following 
signal: (\t -> if f t then s1 else s2) <$> time. In order to know what 
signals should be started at the beginning, you would have to know whether f 
will ever yield False or True so that you know which of s1 and s2 will be 
needed at some point.

> > Implementing the second semantics would require a single signal
> > having possibly different values when started at different
> > times. This in turn would disallow caching of signal values in
> > mutable variables.
>
> Or at least it would require deep copying some initial snapshot at every
> restart. But this only applies to completely self-contained signals,
> since anything that depends on the outer world cannot be restarted by
> definition.

Why not? You could have a signal which always yields the last key press. This 
clearly depends on the outer world. Then you construct another signal out of 
this which counts how often a certain key was pressed. If this latter signal 
is evaluated several times, it could start counting from different times on, 
couldn’t it?

Best wishes,
Wolfgang