Re: [Haskell-cafe] Yampa vs. Reactive
On 21 Dec 2008, at 13:10, Henrik Nilsson wrote: Hi Tom, In reactive, one doesn't. All behaviors and events have the same absolute 0 value for time. Right. I believe the possibility of starting behaviors later is quite important. And from what Conal wrote in a related mail, I take it that this is recognized, and that this capability is something that is being considered for reactive? Yep, it is indeed. Thanks for this series of emails by the way. It's helped clarify in my head exactly what problems Yampa solved, and exactly which of them Reactive does or doesn't solve. Thanks Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Tom, In reactive, one doesn't. All behaviors and events have the same absolute 0 value for time. Right. I believe the possibility of starting behaviors later is quite important. And from what Conal wrote in a related mail, I take it that this is recognized, and that this capability is something that is being considered for reactive? Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham n...@cs.nott.ac.uk This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Henrik, On 19 Dec 2008, at 02:05, Henrik Nilsson wrote: Hi Tom, I'm not sure why mapping the function is not satisfactory -- It would create a new Behavior, who's internals contain only the two elements from the list -- that would expose to the garbage collector that the second element has no reference from this behavior any more, and thus the whole behavior could be collected. We must be talking at cross purposes here: there is no way that deleting the *output* from one of the behaviours from a list of outputs would cause the underlying behavior whose output no longer is observed to be garbage collected. After all, that list of three numbers is just a normal value: why should removing one of its elements, so to speak, affect the producer of the list? But if we have a list of running behaviors or signals, and that list is changed, then yes, of course we get the desired behavior (this is what Yampa does). So maybe that's what you mean? I'm afraid not, rereading what I said, I really didn't explain what I was talking about well. A Behavior in reactive is not just a continuous function of time. It is a series of steps, each of which carries a function of time. One such behavior might look like this: (+5) - 5 , (+6) - 10 , integral That is to say, this behavior starts off being a function that adds 5 to the current time. At 5 seconds, it steps, and the value changes to a function that adds 6 to time. At this point, the function that adds 5 to time can be garbage collected, along with the step. At 10 seconds, it becomes the integral of time, and the (+6) function, along with the next step is GCed. To come back to your example, I'd expect the behavior to look like this (using named functions only so that I can refer to them): i1 t = integral t i2 t = integral (2 * t) i3 t = integral (3 * t) f t = [i1 t, i2 t, i3 t)] g t = [i1 t, i3 t] f - 2, g After 2 seconds, both f, and the first step may be garbage collected. As g does not have any reference to i2 t, it too can be garbage collected. I hope that answers you more clearly. That's a yes. My first answer to how to implement the resetting counter would be someting along the lines of this, but I'm not certain it's dead right: e = (1+) $ mouseClick e' = (const 0) $ some event b = accumB 0 (e `mappend` e') i.e. b is the behavior got by adding 1 every time the mouse click event occurs, but resetting to 0 whenever some event occurs. Hmm. Looks somewhat complicated to me. Anyway, it doesn't really answer the fundamental question: how does one start a behavior/signal function at a particular point in time? In reactive, one doesn't. All behaviors and events have the same absolute 0 value for time. One can however simulate such a behavior, by using a `switcher`, or accumB. In practice, having potentially large numbers of behaviors running but not changing until a certain event is hit is not a major problem. This is not a problem because reactive knows that the current step contains a constant value, not a real function of time, because of this, no changes are pushed, and no work is done, until the event hits. I believe Conal is however working on semantics for relative time based behaviors/events though. Thanks Tom Davie ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
Nice to see this discussion, and I just want to comment on the applicative v.s. arrow style. The example Henrik gave is z - sf2 sf1 - x which models a composition, and is in general the strength of a combinator approach. But the strength of Applicative, in my opinion, is not composition but currying: f * x * y where f can have the type Behavior a - Behavior b - Behavior c. I don't think there is an exact match in arrows. One could, however, require sf to be of type SF (a, b) c, and write z - sf - (x, y) The tupling may seem an extra burden, but it's an inherent design choice of arrows, which builds data structure on top of products, and people can't run away from it when writing arrow programs. -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
Hi guys, Thanks for the comments and lively discussion. After reading these posts, a few papers, and Conal's blogs, I'm going to try Reactive because it is newer and thus likely to incorporate most of the good things from past FRP sytems including Yampa, actively being developed, and mature enough to start using now. I'll see you on the Reactive mailing list. Cheers, Tony ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
The example Henrik gave [...] models a composition, and is in general the strength of a combinator approach. But the strength of Applicative, in my opinion, is not composition but currying [...] Well put, Paul. I really do like the semantic model of Yampa. Signal transformers model interactive behaviors, where the behaviors/signals of classic FRP model non-interactive behaviors. (See http://conal.net/blog/posts/why-classic-frp-does-not-fit-interactive-behavior/.) I also like currying. As long as we use not just the arrow abstraction but also *arrow notation*, I don't know how we'll ever be able to get an efficient implementation, in which portions of computed signals get recomputed only when necessary. And probably the Arrow abstraction itself is a bit too restrictive, given that it disallows any conditions on its type arguments. So I've been noodling some about formulations of signal functions that don't fit into the standard arrow discipline. Regards, - Conal On Fri, Dec 19, 2008 at 6:31 AM, Paul L nine...@gmail.com wrote: Nice to see this discussion, and I just want to comment on the applicative v.s. arrow style. The example Henrik gave is z - sf2 sf1 - x which models a composition, and is in general the strength of a combinator approach. But the strength of Applicative, in my opinion, is not composition but currying: f * x * y where f can have the type Behavior a - Behavior b - Behavior c. I don't think there is an exact match in arrows. One could, however, require sf to be of type SF (a, b) c, and write z - sf - (x, y) The tupling may seem an extra burden, but it's an inherent design choice of arrows, which builds data structure on top of products, and people can't run away from it when writing arrow programs. -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Tony, I'm glad for your interest in Reactive. Yes, it is certainly being developed actively. To start you off with realistic expectations, I'd like you to know that Reactive currently has one or more sneaky laziness bugs that block serious application at the moment. While the Reactive implementation is almost entirely pure (free of IO), it has some quite subtle aspects, and it's taking a while to get it solid. There's also a new higher-level programming model on the way, as hinted at in my blog. Welcome! - Conal 2008/12/19 Tony Hannan tonyhann...@gmail.com Hi guys, Thanks for the comments and lively discussion. After reading these posts, a few papers, and Conal's blogs, I'm going to try Reactive because it is newer and thus likely to incorporate most of the good things from past FRP sytems including Yampa, actively being developed, and mature enough to start using now. I'll see you on the Reactive mailing list. Cheers, Tony ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
On 12/19/08, Conal Elliott co...@conal.net wrote: As long as we use not just the arrow abstraction but also *arrow notation*, I don't know how we'll ever be able to get an efficient implementation, in which portions of computed signals get recomputed only when necessary. And probably the Arrow abstraction itself is a bit too restrictive, given that it disallows any conditions on its type arguments. So I've been noodling some about formulations of signal functions that don't fit into the standard arrow discipline. Paul (Hudak) and I recently worked on a notion called Causal Commutative Arrows, which actually gave a very good optimization result for Yampa like arrows. One notable feature is that all programs normalize, regardless of whether they were originally written using the Arrow combinators or translated from Arrow notations. I recently give a talk at NEPLS on this, the slides are here: http://www.cs.yale.edu/homes/hl293/download/NEPLS-talk.pdf Due to the use of arrow laws, our technique remains fully abstract without committing to any concrete representation of arrows or signals/streams. The re-computation problem is another issue though. I fully agree with Henrik's comment on push v.s. pull. But if one really wants to avoid re-computation at all efforts, here is one possibility: pass :: SF a b - SF (Maybe a) (Maybe b) It'll only invoke the given SF when the input is Just something, and do nothing otherwise. Coupled with hold, it shall lead to efficient implementation that avoids re-computation when inputs don't change. Intuitively it's like selectively turning on/off part of a circuit according to inputs, which naturally falls in the ArrowChoice class. Also one has to extract the implicit time from the inplementation and make it an explicit input in order for this to be semantically sound. -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Tom,
I'll have an attempt at addressing the questions, although I freely
admit that I'm not as into Reactive as Conal is yet, so he may come
and correct me in a minute.
[...]
Reactive has explicitly parameterized inputs. In your robot example
I would expect something along the lines of
data RobotInputs =
RI {lightSensor :: Behavior Colour;
bumbSwitch :: Event ()} -- A couple of example robot sensors
robotBehavior :: RobotInputs - Behavior Robot
robotBehavior sensors = a behovior that combines the light sensor and
the bumb switch to stay in the light, but not keep driving into
things.
This looks exactly like Classical FRP.
And if it is like Classical FRP behind the scenes, it nicely
exemplifies the problem.
In Classical FRP, Behavior is actually what I would call a signal
function. When started (switched into), they map the system input
signal from that point in time to a signal of some particular type.
So, the record RobotInputs is just a record of lifted projection
functions that selects some particular parts of the overall system
input. Behind the scenes, all Behaviors are connected to the one
and only system input.
data UIInputs = UI {mousePoint :: Behavior Point; mouseClick :: Event
(); ...}
world :: UIInputs - Behavior World
world = interpret mouse and produce a world with barriers, robots and
lights in it
Fine, of course, assuming that all behaviours share the same kind
of system input, in this case UI input.
But what if I want my reactive library to interface to different kinds
of systems? The robot code should clearly work regardless of whether
we are running it on a real hardware platform, or in a simulated
setting where the system input comes form the GUI. In Classical FRP,
this was not easily possible, because all combinators at some level
need to refer to some particular system input type which is hardwired
into the definitions.
Had Haskell had ML-style parameterized modules,
that would likely have offered a solution: the libraries could
have been parameterized on the system input, and then one could obtain
say robot code for running on real hardware or in a simulated
setting by simply applying the robot module to the right kind of
system input.
An alternative is to parameterize the behaviour type explicitly on
the system input type:
Behavior sysinput a
This design eventually evolved into Arrowized FRP and Yampa.
So, from your examples, it is not clear to what extent Reactive
as addressed this point. Just writing functions that maps behaviours
to behaviours does not say very much.
On a more philosophical note, I always found it a bit odd that if
I wanted to write a function that mapped a signal of, say, type a,
which we can think of as
type Signal a = Time - a
to another signal, of type b say, in Classical FRP, I'd have to write
a function of type
Behavior a - Behavior b
which really is a function of type
(Signal SystemInput - Signal a) - (Signal SystemInput - Signal b)
I find this unsatisfying, as my mapping from a signal of type a to
a signal of type b is completely independent from the system
input (or the function wouldn't have a polymorphic type).
* A clear separation between signals, signal functions, and ordinary
functions and values, yet the ability to easily integrate all
kinds of computations.
I agree and disagree here (that'll be the matter of taste creeping
in). I agree that in Reactive you often spend a lot of keystrokes
lifting pure values into either an Event or a Behavior. Having said
that I'd argue that Yampa requires us to do this too -- it merely
enforces the style in which we do it (we must do it with arrows).
Yes, there is lifting in Yampa, but the arrow syntax mostly does it for
the programmer, which in practice (in my experience) translates to a lot
less effort, and, in my opinion, leads to clearer code as it is easy to
maintain a distinction between signals and static values. After all, why
should I want to live a constant to a signal, if all I'm going to do
with it is to apply one and the same function to it over and over?
(I'm not worried about efficiency here, that can be fixed: it's
a philosophical point.)
Also, form practical experience when programming with Classical FRP,
we often lifted entire libraries we wanted to use to avoid having
to write explicit lifts all the time. Tedious, but OK, doable.
However, quite often we then discovered that actually, we needed the
unlifted version of the library too, leading to name clashes and
thus extra noise to do the need to disambiguate, be it by qualified
input or naming the lifted versions differently.
Not a show stopper by any means, but a tedious extra level of concerns.
The arrow framework offer clear guidance in this case which translates
to convenient coding practice: just use whatever library
you need and let the arrow syntax take care of liftings where
necessary.
My personal opinion on this one is that I
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Henrik,
On 18 Dec 2008, at 14:26, Henrik Nilsson wrote:
Hi Tom,
I'll have an attempt at addressing the questions, although I freely
admit that I'm not as into Reactive as Conal is yet, so he may
come and correct me in a minute.
[...]
Reactive has explicitly parameterized inputs. In your robot
example I would expect something along the lines of
data RobotInputs =
RI {lightSensor :: Behavior Colour;
bumbSwitch :: Event ()} -- A couple of example robot sensors
robotBehavior :: RobotInputs - Behavior Robot
robotBehavior sensors = a behovior that combines the light sensor
and the bumb switch to stay in the light, but not keep driving into
things.
This looks exactly like Classical FRP.
And if it is like Classical FRP behind the scenes, it nicely
exemplifies the problem.
In Classical FRP, Behavior is actually what I would call a signal
function. When started (switched into), they map the system input
signal from that point in time to a signal of some particular type.
So, the record RobotInputs is just a record of lifted projection
functions that selects some particular parts of the overall system
input. Behind the scenes, all Behaviors are connected to the one
and only system input.
I don't think this is really true. Behaviors and Events do not reveal
in their type definitions any relation to any system that they may or
may not exist in. A Behavior can exist wether or not it is being run
by a particular legacy adapter (a piece of code to adapt it to work as
expected on a legacy, imperative computer). I can define an Event e =
(+1) $ atTimes [0,10..] and use it as a Haskell construct without
needing any system at all to run it within. Similarly I can define a
Behavior b = accumB 0 e that depends on this event, completely
independant of any system, or definition of what basic events and
behaviors I get to interact with it.
data UIInputs = UI {mousePoint :: Behavior Point; mouseClick ::
Event
(); ...}
world :: UIInputs - Behavior World
world = interpret mouse and produce a world with barriers, robots
and lights in it
Fine, of course, assuming that all behaviours share the same kind
of system input, in this case UI input.
But what if I want my reactive library to interface to different kinds
of systems? The robot code should clearly work regardless of whether
we are running it on a real hardware platform, or in a simulated
setting where the system input comes form the GUI. In Classical FRP,
this was not easily possible, because all combinators at some level
need to refer to some particular system input type which is hardwired
into the definitions.
There are no hardwired definitions of what inputs I'm allowed to use
or not use. If I would like my reactive program to run on a legacy
robot which uses imperative IO, then I may write a legacy adapter
around it to take those IO actions and translate them into Events and
Behaviors that I can use. One such legacy adapter exists, called
reactive-glut, which ties glut's IO actions into reactive events one
can use. I could easily imagine several others, for example one that
interacts with robot hardware and presents the record above to the
behaviors it's adapting, or another still which works much like the
interact function, but instead of taking a String - String, takes
an Event Char - Event Char.
Had Haskell had ML-style parameterized modules,
that would likely have offered a solution: the libraries could
have been parameterized on the system input, and then one could obtain
say robot code for running on real hardware or in a simulated
setting by simply applying the robot module to the right kind of
system input.
An alternative is to parameterize the behaviour type explicitly on
the system input type:
Behavior sysinput a
This design eventually evolved into Arrowized FRP and Yampa.
So, from your examples, it is not clear to what extent Reactive
as addressed this point. Just writing functions that maps behaviours
to behaviours does not say very much.
On a more philosophical note, I always found it a bit odd that if
I wanted to write a function that mapped a signal of, say, type a,
which we can think of as
type Signal a = Time - a
to another signal, of type b say, in Classical FRP, I'd have to
write
a function of type
Behavior a - Behavior b
which really is a function of type
(Signal SystemInput - Signal a) - (Signal SystemInput - Signal
b)
I find this unsatisfying, as my mapping from a signal of type a to
a signal of type b is completely independent from the system
input (or the function wouldn't have a polymorphic type).
Yes, certainly that would be unsatisfactory. But I don't agree about
the type of the function -- this really is a (Time - a) - (Time -
a). It may be though that the argument (Time - a) is a system input
from our legacy adapter, or an internal part of our program.
* A clear separation between signals,
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Tom, I don't think this is really true. Behaviors and Events do not reveal in their type definitions any relation to any system that they may or may not exist in. OK. So how does e.g. mousePoint :: Behavior Point get at the mouse input? unsafePerformIO? I.e. it is conceptually a global signal? I'm not sure I understand you clearly. If I wish to apply a constant function to a signal, can I not just use fmap? The question is why I would want to (conceptually). I'm just saying I find it good and useful to be able to easily mix static values and computations and signals and computations on signals. You would certainly need to ask Conal on this point, but I have no reason to suspect that b' = [1,2,3,4,5] `stepper` listE [(1,[])] would not deallocate the first list once it had taken its step. It's not the lists that concern me, nor getting rid of a collection of behaviors all at once. The problem is if we ant to run a collection of behaviors in parallel, all potentially accumulating internal state, how do we add/delete individual behaviors to/from that collection, without disturbing the others? For the sake of argument, say we have the following list of behaviours: [integral time, integral (2 * time), integral (3 * time)] We turn them into a single behavior with a list output in order to run them. After one second the output is thus [1,2,3] Now, we want to delete the second behavior, but continue to run the other two, so that the output at time 2 is [2,6] Simply mapping postprocessing that just drops the second element from the output isn't a satisfactory solution. let n :: Behavior Int n = behaviour that counts left mouse button clicks in n `until` some event -= n I'm not sure I got the syntax right. But the idea is that we output the number of left mouse button clicks, and then at some point, we switch to a behavior that again output the number of left mouse button clicks, notionally the same one n. The question is, after the switch, do we observe a count that continues from where the old one left off, i.e. is there a single *shared* instance of n that is merely being *observed* from within the two branches of the switch, or is the counting behavior n restarted (from 0) after the switch? Yes, we really do get a shared n -- without doing that we certainly would see a large space/time leak. Interesting, although I don't see why not sharing would imply a space/time leak: if the behavior is simply restarted, there is no catchup computation to do, nor any old input to hang onto, so there is neither a time nor a space-leak? Anyway, let's explore this example a bit further. Suppose lbp is the signal of left button presses, and that we can count them by count lbp Then the question is if let n :: Behavior Int n = count lbp in n `until` some event -= n means the same as (count lbp) `until` some event -= (count lbp) If no, then Reactive is not referentially transparent, as we manifestly cannot reason equationally. If yes, the question is how to express a counting that starts over after the switch (which sometimes is what is needed). Yep, such Behaviors are seperated in Reactive only by the method you create them with. I may use the `stepper` function to create a behavior that increases in steps based on an event occurring, or I may use fmap over time to create a continuously varying Behavior. But the question was not about events vs continuous signals. The question is, what is a behavior conceptually, and when is it started? E.g. in the example above, at what point do the various instances of count lbp start counting? Or are the various instances of count lbp actually only one? Or if you prefer, are beahviours really signals, that conceptually start running all at once at a common time 0 when the system starts? The answers regarding input behaviors like mousePosition, that n is shared, and the need to do catchup computations all seem to indicate this. But if so, that leaves open an important question on expressivity, examplified by how to start counting from the time of a switch above, and makes if virtually impossible to avoid time and space leaks in general, at least in an embedded setting. After all, something like count lbp can be compiled into a function that potentially may be invoked at some point. And as long as this possibility exists, the system needs to hang on to the entire history of mouse clicks so that they can be coounted at some future point if necessary. These are all questions that go back to classical FRP, which we didn't find any good answers to back then, and which also were part of the motivation for moving to AFRP/Yampa. If Reactive has come up with better answers, that would be very exciting indeed! Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham n...@cs.nott.ac.uk This message has been checked
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Henrik, On 18 Dec 2008, at 19:06, Henrik Nilsson wrote: Hi Tom, I don't think this is really true. Behaviors and Events do not reveal in their type definitions any relation to any system that they may or may not exist in. OK. So how does e.g. mousePoint :: Behavior Point get at the mouse input? unsafePerformIO? I.e. it is conceptually a global signal? main = adapter doSomeStuff -- Note here that different adapters provide different UIs. adapter :: (Behavior UI - Behavior SomethingFixedThatYouKnowHowToInterpret) - IO () adapter f = set up the system behaviors, pass them into f, grab the outputs, and do the something to render. doSomeStuff :: Behavior UI - Behavior SomethingFixedThatYouKnowHowToInterpret I'm not sure I understand you clearly. If I wish to apply a constant function to a signal, can I not just use fmap? The question is why I would want to (conceptually). I'm just saying I find it good and useful to be able to easily mix static values and computations and signals and computations on signals. Yep, I can see that, I think we need to agree to disagree on this front, I would prefer to use fmap, or $, while you prefer arrow syntax. You would certainly need to ask Conal on this point, but I have no reason to suspect that b' = [1,2,3,4,5] `stepper` listE [(1,[])] would not deallocate the first list once it had taken its step. It's not the lists that concern me, nor getting rid of a collection of behaviors all at once. The problem is if we ant to run a collection of behaviors in parallel, all potentially accumulating internal state, how do we add/delete individual behaviors to/from that collection, without disturbing the others? For the sake of argument, say we have the following list of behaviours: [integral time, integral (2 * time), integral (3 * time)] We turn them into a single behavior with a list output in order to run them. After one second the output is thus [1,2,3] Now, we want to delete the second behavior, but continue to run the other two, so that the output at time 2 is [2,6] Simply mapping postprocessing that just drops the second element from the output isn't a satisfactory solution. I'm not sure why mapping the function is not satisfactory -- It would create a new Behavior, who's internals contain only the two elements from the list -- that would expose to the garbage collector that the second element has no reference from this behavior any more, and thus the whole behavior could be collected. Yes, we really do get a shared n -- without doing that we certainly would see a large space/time leak. Interesting, although I don't see why not sharing would imply a space/time leak: if the behavior is simply restarted, there is no catchup computation to do, nor any old input to hang onto, so there is neither a time nor a space-leak? Anyway, let's explore this example a bit further. Suppose lbp is the signal of left button presses, and that we can count them by count lbp Then the question is if let n :: Behavior Int n = count lbp in n `until` some event -= n means the same as (count lbp) `until` some event -= (count lbp) If no, then Reactive is not referentially transparent, as we manifestly cannot reason equationally. If yes, the question is how to express a counting that starts over after the switch (which sometimes is what is needed). That's a yes. My first answer to how to implement the resetting counter would be someting along the lines of this, but I'm not certain it's dead right: e = (1+) $ mouseClick e' = (const 0) $ some event b = accumB 0 (e `mappend` e') i.e. b is the behavior got by adding 1 every time the mouse click event occurs, but resetting to 0 whenever some event occurs. Yep, such Behaviors are seperated in Reactive only by the method you create them with. I may use the `stepper` function to create a behavior that increases in steps based on an event occurring, or I may use fmap over time to create a continuously varying Behavior. But the question was not about events vs continuous signals. The question is, what is a behavior conceptually, and when is it started? E.g. in the example above, at what point do the various instances of count lbp start counting? Or are the various instances of count lbp actually only one? They are indeed, only 1. Or if you prefer, are beahviours really signals, that conceptually start running all at once at a common time 0 when the system starts? The answers regarding input behaviors like mousePosition, that n is shared, and the need to do catchup computations all seem to indicate this. But if so, that leaves open an important question on expressivity, examplified by how to start counting from the time of a switch above, and makes if virtually impossible to avoid time and space leaks in general, at least in an embedded setting. After all, something like count lbp can be compiled
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Tom, I'm not sure why mapping the function is not satisfactory -- It would create a new Behavior, who's internals contain only the two elements from the list -- that would expose to the garbage collector that the second element has no reference from this behavior any more, and thus the whole behavior could be collected. We must be talking at cross purposes here: there is no way that deleting the *output* from one of the behaviours from a list of outputs would cause the underlying behavior whose output no longer is observed to be garbage collected. After all, that list of three numbers is just a normal value: why should removing one of its elements, so to speak, affect the producer of the list? But if we have a list of running behaviors or signals, and that list is changed, then yes, of course we get the desired behavior (this is what Yampa does). So maybe that's what you mean? That's a yes. My first answer to how to implement the resetting counter would be someting along the lines of this, but I'm not certain it's dead right: e = (1+) $ mouseClick e' = (const 0) $ some event b = accumB 0 (e `mappend` e') i.e. b is the behavior got by adding 1 every time the mouse click event occurs, but resetting to 0 whenever some event occurs. Hmm. Looks somewhat complicated to me. Anyway, it doesn't really answer the fundamental question: how does one start a behavior/signal function at a particular point in time? I consider the fact that Yampa, through supporting both signals and signal functions, provides simple yet flexible answers to the question when a signal function starts to be one of its key strengths over Classical FRP and maybe then also over Reactive. Best, /Henrik -- Henrik Nilsson School of Computer Science The University of Nottingham n...@cs.nott.ac.uk This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
Hi Tony, Reactive so far has focused mainly on events and functions of time (behaviors/signals), while Yampa on transformations between signals. I'm in the process of building a higher-level interface with some semantic similarity to the arrow/Yampa style. See recent posts at http://conal.net/blog to get some flavor of where I'm going. The post Why classic FRP does not fit interactive behavior in particular mentions part of my motivation for doing something different from both classic FRP and Yampa. - Conal 2008/12/16 Tony Hannan tonyhann...@gmail.com Hello, Can someone describe the advantages and disadvantages of the Yampa library versus the Reactive library for functional reactive programming, or point me to a link. Thanks, Tony P.S. It is hard to google for Yampa and Reactive together because reactive as in function reactive programming always appears with Yampa ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
On 17 Dec 2008, at 03:14, Tony Hannan wrote: Hello, Can someone describe the advantages and disadvantages of the Yampa library versus the Reactive library for functional reactive programming, or point me to a link. Thanks, Tony P.S. It is hard to google for Yampa and Reactive together because reactive as in function reactive programming always appears with Yampa Advantages of Yampa: • Just at the moment, slightly more polished. • (maybe) harder to introduce space/time leaks. Advantages of Reactive: • More functional programming like -- doesn't require you to use arrows everywhere, and supports a nice applicative style. • In very active development. • Active community. Hope that helps -- my personal preference is that Reactive is the one I'd use for any FRP project at the moment. Bob___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Yampa vs. Reactive
I'll have an attempt at addressing the questions, although I freely
admit that I'm not as into Reactive as Conal is yet, so he may come
and correct me in a minute.
On 17 Dec 2008, at 15:29, Henrik Nilsson wrote:
I have not used Reactive as such, but I did use Classic FRP
extensively, and as far as I know the setup is similar, even
if Reactive has a modern and more principled interface.
Based on my Classic FRP experience (which may be out of date, if so,
correct me), I'd say advantages of Yampa are:
* More modular.
Yampa's signal function type is explicitly parameterized on the
input signal type. In Classic FRP and Reactive (as far as I know),
the system input is implicitly connected to all signal functions
(or behaviours) in the system.
One consequence of this is that it there were issues with reusing
Classical FRP for different kinds of systems inputs, and difficult
to combine systems with different kinds of input. This was what
prompted a parameterization on the type of the system input in the
first place, which eventually led to Arrowized FRP and Yampa.
I don't know what the current story of Reactive is in this respect.
But having parameterized input has been crucial for work on big,
mixed-domain, applications. (For example, a robot simulator with an
interactive editor for setting up the world. The robots were
FRP systems too, but their input is naturally of a different kind
that the overall system input. It also turned out to be very useful
to have an FRP preprocessor for the system input, which then was
composed with the rest of the system using what effectively was
arrow composition (), but called something else at the time.)
I'm not sure how this was set up in a classic FRP system, so I'm
unable to comment on how exactly it's changed. What I will say is
that as far I understand what you're saying, Reactive has explicitly
parameterized inputs. In your robot example I would expect something
along the lines of
data RobotInputs = RI {lightSensor :: Behavior Colour; bumbSwitch ::
Event ()} -- A couple of example robot sensors
robotBehavior :: RobotInputs - Behavior Robot
robotBehavior sensors = a behovior that combines the light sensor and
the bumb switch to stay in the light, but not keep driving into things.
data UIInputs = UI {mousePoint :: Behavior Point; mouseClick :: Event
(); ...}
world :: UIInputs - Behavior World
world = interpret mouse and produce a world with barriers, robots and
lights in it
robotInputs :: World - Behavior Robot - RobotInputs
robotInputs = given a robot in a world, generate the robot's inputs
* A clear separation between signals, signal functions, and ordinary
functions and values, yet the ability to easily integrate all kinds
of computations.
Arguably a matter of taste, and in some ways more a consequence of
the Arrow syntax than Arrows themselves. But in Classical FRP,
one had to do either a lot of explicit lifting (in practice, we
often ended up writing lifting wrappers for entire libraries), or
try to exploit overloading for implicit lifting. The latter is
quite limited though, partly due to how Haskell's type classes
are organized and that language support for overloaded constants
is limited to numerical constants.
In any case, when we switched to arrows and arrow syntax, I found
it liberating to not have to lift everything to signal functions
first, but that I could program both with signals and signal
functions on the one hand, and plain values and functions on the
other, at the same time and fairly seamlessly. And personally,
I also felt this made the programs conceptually clearer and easier
to understand,
My understanding is that Reactive is similar to Classical FRP in
this respect.
I agree and disagree here (that'll be the matter of taste creeping
in). I agree that in Reactive you often spend a lot of keystrokes
lifting pure values into either an Event or a Behavior. Having said
that I'd argue that Yampa requires us to do this too -- it merely
enforces the style in which we do it (we must do it with arrows).
My personal opinion on this one is that I prefer the applicative
interface to the arrow based one, because it feels more like just
writing a functional program.
* Classical FRP lacked a satisfying approach to handle dynamic
collections of reactive entities as needed when programming
typical video games for example. Yampa has a way. One can
argue about how satisfying it is, but at least it fulfills
basic requirements such that allowing logically removed entities to
be truly removed (garbage collected).
I don't know where Reactive stands here.
I reserve judgement at the moment because I haven't explicitly written
a reactive program involving a collection of behaviors, having said
that, I see no reason why removing a value from the list in a Behavior
[a],
Re: [Haskell-cafe] Yampa vs. Reactive
Thomas Davie wrote: Advantages of Yampa: • Just at the moment, slightly more polished. • (maybe) harder to introduce space/time leaks. Advantages of Reactive: • More functional programming like -- doesn't require you to use arrows everywhere, and supports a nice applicative style. • In very active development. • Active community. I have not used Reactive as such, but I did use Classic FRP extensively, and as far as I know the setup is similar, even if Reactive has a modern and more principled interface. Based on my Classic FRP experience (which may be out of date, if so, correct me), I'd say advantages of Yampa are: * More modular. Yampa's signal function type is explicitly parameterized on the input signal type. In Classic FRP and Reactive (as far as I know), the system input is implicitly connected to all signal functions (or behaviours) in the system. One consequence of this is that it there were issues with reusing Classical FRP for different kinds of systems inputs, and difficult to combine systems with different kinds of input. This was what prompted a parameterization on the type of the system input in the first place, which eventually led to Arrowized FRP and Yampa. I don't know what the current story of Reactive is in this respect. But having parameterized input has been crucial for work on big, mixed-domain, applications. (For example, a robot simulator with an interactive editor for setting up the world. The robots were FRP systems too, but their input is naturally of a different kind that the overall system input. It also turned out to be very useful to have an FRP preprocessor for the system input, which then was composed with the rest of the system using what effectively was arrow composition (), but called something else at the time.) * A clear separation between signals, signal functions, and ordinary functions and values, yet the ability to easily integrate all kinds of computations. Arguably a matter of taste, and in some ways more a consequence of the Arrow syntax than Arrows themselves. But in Classical FRP, one had to do either a lot of explicit lifting (in practice, we often ended up writing lifting wrappers for entire libraries), or try to exploit overloading for implicit lifting. The latter is quite limited though, partly due to how Haskell's type classes are organized and that language support for overloaded constants is limited to numerical constants. In any case, when we switched to arrows and arrow syntax, I found it liberating to not have to lift everything to signal functions first, but that I could program both with signals and signal functions on the one hand, and plain values and functions on the other, at the same time and fairly seamlessly. And personally, I also felt this made the programs conceptually clearer and easier to understand, My understanding is that Reactive is similar to Classical FRP in this respect. * Classical FRP lacked a satisfying approach to handle dynamic collections of reactive entities as needed when programming typical video games for example. Yampa has a way. One can argue about how satisfying it is, but at least it fulfills basic requirements such that allowing logically removed entities to be truly removed (garbage collected). I don't know where Reactive stands here. * There was also an issue with Classical FRP having to do with the need to observe the output from one part of the system in another part of the system. This is quite different from parameterizing the second part of the the system on the first, as this approach loses sharing across switches. This led to the development for running in constructs, which effectively made it possible for behaviours to be both signals (i.e. signal functions already applied to the system input), and signal functions (not yet applied to the system input). Yet there was no distinction between these running behaviours (= signals) and normal behaviours (= signal functions) at the type level. The approach also led to semantic difficulties, and, when trying to resolve those, to a very complicated design involving complicated overloading and auxiliary classes. The arrows approach obviated the need for all of this, and I consider that and other distinct advantage. Again, I don't know where Reactive stands here, but it needs to have a good answer to this issue, or it is going to suffer from limited expressivity. Many of the above advantages are matters of opinion (but so are the advantages initially put forward for Reactive above). However, the development of AFRP and Yampa was motivated by fundamental expresssivity limitations of Classical FRP, in at least some ways related to the very way the system was set up. To the extent Reactive
[Haskell-cafe] Yampa vs. Reactive
Hello, Can someone describe the advantages and disadvantages of the Yampa library versus the Reactive library for functional reactive programming, or point me to a link. Thanks, Tony P.S. It is hard to google for Yampa and Reactive together because reactive as in function reactive programming always appears with Yampa ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
