Is it possible to create a Signal from a List? Essentially what I want is something with the signature List a -> Signal a. I know that a Signal represents a time-varying value and so something like this doesn't actually make any sense (i.e. I can't think of a reason to use it in production code).
I could see applications of it for testing though. For example, imagine some function which depended on the past values of a Signal (via foldp for instance) and you wanted to make assertions about the state of the system given the signal had received values x, y, and z.
Note that there wouldn't have to be anything special about the Signal denoting that it only would ever receive a fixed number of values. I'm thinking of it more like: in production you have a Signal of mouse clicks, and you want to test that from a given starting position, after a given set of clicks, the system should be in some other known state. I know you could simulate this by calling the function a fixed number of times and feeding the results back in with new values, I'm just wondering if it is possible.
I guess it's possible. You use a time-based signal, and map the values from the list over it:
import Time
import Graphics.Element exposing (show)
list = [1..10]
signalFromList : List a -> Signal a
signalFromList list =
let
(Just h) =
List.head list
time =
Time.every Time.second
maybeFlatMap =
flip Maybe.andThen
lists =
Signal.foldp (always <| maybeFlatMap List.tail) (Just list) time
in
Signal.filterMap (maybeFlatMap List.head) h lists
main = Signal.map show <| signalFromList list
However!
It shouldn't be hard to do testing without signals. If you have a foldp somewhere, in a test you can use List.foldl over a list [x,y,z] instead. That should give you the ability to look at the state of your program after inputs x, y, z.
I don't think there is any way it can be done synchronously in pure elm (Apanatshka's answer illustrates well how to set up a sequence of events across time and why it's a bad idea). If we look at how most Signals are defined, we'll see they all head down into a native package at some point.
The question then becomes: can we do this natively?
f : List a -> Signal a
I often think of (Signal a) as 'an a that changes over time'. Here we provide a List of as, and want the function to make it change over time for us.
Before we go any further, I recommend a quick look at Native/Signal.js: https://github.com/elm-lang/core/blob/master/src/Native/Signal.js
Let's say we get down to native land with our List of as. We want something a bit like Signal.constant, but with some extra behaviour that 'sends' each a afterwards. When can we do the sending, though? I am guessing we can't do it during the signal construction function, as we're still building the signal graph. This leaves us a couple of other options:
something heinous with setTimeout, scheduling the sending of each 'a' at an appropriate point in the future
engineering a hook into the elm runtime so that we can run an arbitrary callback at the point when the signal graph is fully constructed
To me at least, the former sounds error prone, and I hope the latter doesn't exist (and never does)!
For testing, your suggestion of using a List fold to mimic the behaviour of foldp would be the way I would go.
Related
I know that you can turn on a vehicle signal (for example, the left indicator) in traci using:
traci.vehicle.setSignals(vehID, int)
where the integer related to the specific signal can be found using the following link (https://sumo.dlr.de/docs/TraCI/Vehicle_Signalling.html#signaling), but is there a way of turning off a specific signal that would be otherwise turned on by the program (i.e., a setSignalOff)?
I think that there is a function in the underlying C++ code (switchOffSignal() in MSVehicle.h) but there doesn't appear to be a traci command that turns off a specific signal.
I appreciate that it is (generally) a pleasant visual aesthetic and has no impact on vehicle behaviour, but it would be very useful for what I am trying to do!
Switching off signals should work from traci. By using sometihng like traci.vehicle.setSignals("ego", 0), I can switch them off. Be aware that this will be reset after the step, so you may have to do that in every timestep.
So, Michael is right in that:
traci.vehicle.setSignals("ego", 0)
should turn off all signals (although the signals still appeared on for me visually, which confused me initially).
To turn off individual signals but keep the others on you need to:
For all the "on" signals find the value of 2^n, where n is the bit integer (which can be found using the following link: https://sumo.dlr.de/docs/TraCI/Vehicle_Signalling.html)
Sum all these 2^n values (let's call this variable x) and use this value in the setSignals function: traci.vehicle.setSignals("ego", x).
So for example, if we want the brake light, the right indicator and the high beam on (but all the other signals off) we would do:
RightIndicatorValue = pow(2,0)
BrakeLightValue = pow(2,3)
HighBeamValue = (2,6)
SignalValue = RightIndicatorValue + BrakeLightValue + HighBeamValue
traci.vehicle.setSignals(("ego", SignalValue)
I am new to Functional programming.
The challenge I have is regarding the mental map of how a binary search tree works in Haskell.
In other programs (C,C++) we have something called root. We store it in a variable. We insert elements into it and do balancing etc..
The program takes a break does other things (may be process user inputs, create threads) and then figures out it needs to insert a new element in the already created tree. It knows the root (stored as a variable) and invokes the insert function with the root and the new value.
So far so good in other languages. But how do I mimic such a thing in Haskell, i.e.
I see functions implementing converting a list to a Binary Tree, inserting a value etc.. That's all good
I want this functionality to be part of a bigger program and so i need to know what the root is so that i can use it to insert it again. Is that possible? If so how?
Note: Is it not possible at all because data structures are immutable and so we cannot use the root at all to insert something. in such a case how is the above situation handled in Haskell?
It all happens in the same way, really, except that instead of mutating the existing tree variable we derive a new tree from it and remember that new tree instead of the old one.
For example, a sketch in C++ of the process you describe might look like:
int main(void) {
Tree<string> root;
while (true) {
string next;
cin >> next;
if (next == "quit") exit(0);
root.insert(next);
doSomethingWith(root);
}
}
A variable, a read action, and loop with a mutate step. In haskell, we do the same thing, but using recursion for looping and a recursion variable instead of mutating a local.
main = loop Empty
where loop t = do
next <- getLine
when (next /= "quit") $ do
let t' = insert next t
doSomethingWith t'
loop t'
If you need doSomethingWith to be able to "mutate" t as well as read it, you can lift your program into State:
main = loop Empty
where loop t = do
next <- getLine
when (next /= "quit") $ do
loop (execState doSomethingWith (insert next t))
Writing an example with a BST would take too much time but I give you an analogous example using lists.
Let's invent a updateListN which updates the n-th element in a list.
updateListN :: Int -> a -> [a] -> [a]
updateListN i n l = take (i - 1) l ++ n : drop i l
Now for our program:
list = [1,2,3,4,5,6,7,8,9,10] -- The big data structure we might want to use multiple times
main = do
-- only for shows
print $ updateListN 3 30 list -- [1,2,30,4,5,6,7,8,9,10]
print $ updateListN 8 80 list -- [1,2,3,4,5,6,7,80,9,10]
-- now some illustrative complicated processing
let list' = foldr (\i l -> updateListN i (i*10) l) list list
-- list' = [10,20,30,40,50,60,70,80,90,100]
-- Our crazily complicated illustrative algorithm still needs `list`
print $ zipWith (-) list' list
-- [9,18,27,36,45,54,63,72,81,90]
See how we "updated" list but it was still available? Most data structures in Haskell are persistent, so updates are non-destructive. As long as we have a reference of the old data around we can use it.
As for your comment:
My program is trying the following a) Convert a list to a Binary Search Tree b) do some I/O operation c) Ask for a user input to insert a new value in the created Binary Search Tree d) Insert it into the already created list. This is what the program intends to do. Not sure how to get this done in Haskell (or) is am i stuck in the old mindset. Any ideas/hints welcome.
We can sketch a program:
data BST
readInt :: IO Int; readInt = undefined
toBST :: [Int] -> BST; toBST = undefined
printBST :: BST -> IO (); printBST = undefined
loop :: [Int] -> IO ()
loop list = do
int <- readInt
let newList = int : list
let bst = toBST newList
printBST bst
loop newList
main = loop []
"do balancing" ... "It knows the root" nope. After re-balancing the root is new. The function balance_bst must return the new root.
Same in Haskell, but also with insert_bst. It too will return the new root, and you will use that new root from that point forward.
Even if the new root's value is the same, in Haskell it's a new root, since one of its children has changed.
See ''How to "think functional"'' here.
Even in C++ (or other imperative languages), it would usually be considered a poor idea to have a single global variable holding the root of the binary search tree.
Instead code that needs access to a tree should normally be parameterised on the particular tree it operates on. That's a fancy way of saying: it should be a function/method/procedure that takes the tree as an argument.
So if you're doing that, then it doesn't take much imagination to figure out how several different sections of code (or one section, on several occasions) could get access to different versions of an immutable tree. Instead of passing the same tree to each of these functions (with modifications in between), you just pass a different tree each time.
It's only a little more work to imagine what your code needs to do to "modify" an immutable tree. Obviously you won't produce a new version of the tree by directly mutating it, you'll instead produce a new value (probably by calling methods on the class implementing the tree for you, but if necessary by manually assembling new nodes yourself), and then you'll return it so your caller can pass it on - by returning it to its own caller, by giving it to another function, or even calling you again.
Putting that all together, you can have your whole program manipulate (successive versions of) this binary tree without ever having it stored in a global variable that is "the" tree. An early function (possibly even main) creates the first version of the tree, passes it to the first thing that uses it, gets back a new version of the tree and passes it to the next user, and so on. And each user of the tree can call other subfunctions as needed, with possibly many of new versions of the tree produced internally before it gets returned to the top level.
Note that I haven't actually described any special features of Haskell here. You can do all of this in just about any programming language, including C++. This is what people mean when they say that learning other types of programming makes them better programmers even in imperative languages they already knew. You can see that your habits of thought are drastically more limited than they need to be; you could not imagine how you could deal with a structure "changing" over the course of your program without having a single variable holding a structure that is mutated, when in fact that is just a small part of the tools that even C++ gives you for approaching the problem. If you can only imagine this one way of dealing with it then you'll never notice when other ways would be more helpful.
Haskell also has a variety of tools it can bring to this problem that are less common in imperative languages, such as (but not limited to):
Using the State monad to automate and hide much of the boilerplate of passing around successive versions of the tree.
Function arguments allow a function to be given an unknown "tree-consumer" function, to which it can give a tree, without any one place both having the tree and knowing which function it's passing it to.
Lazy evaluation sometimes negates the need to even have successive versions of the tree; if the modifications are expanding branches of the tree as you discover they are needed (like a move-tree for a game, say), then you could alternatively generate "the whole tree" up front even if it's infinite, and rely on lazy evaluation to limit how much work is done generating the tree to exactly the amount you need to look at.
Haskell does in fact have mutable variables, it just doesn't have functions that can access mutable variables without exposing in their type that they might have side effects. So if you really want to structure your program exactly as you would in C++ you can; it just won't really "feel like" you're writing Haskell, won't help you learn Haskell properly, and won't allow you to benefit from many of the useful features of Haskell's type system.
I've been working on the HM reference software for a while, to improve something in the intra prediction part. Now a new intra prediction algorithm is added to the code and I let the encoder choose between my algorithm and the default algorithm of HM (according to the RDCost of course).
What I need now, is to signal a flag for each PU, so that the decoder will be able to perform the same algorithm as the encoder decides in the rate distortion loop.
I want to know what exactly should I do to properly add this one bit flag to the stream, without breaking anything in the code.
Assuming that I want to use a CABAC context model to keep the track of my flag's statistics, what else should I do:
adding a new context model like ContextModel3DBuffer m_cCUIntraAlgorithmSCModel to the TEncSbac.h file.
properly initializing the model (both at encoder and decoder side) by looking at how the HM initialezes other context models.
calling the function m_pcBinIf->encodeBin(myFlag, cCUIntraAlgorithmSCModel) and m_pcTDecBinIfdecodeBin(myFlag, cCUIntraAlgorithmSCModel) at the encoder side and decoder side, respectively.
I take these three steps but apparently it breaks something.
PS: Even an equiprobable signaling (i.e. without using CABAC contexts) will be useful. I just want to send this flag peacefully!
Thanks in advance.
I could solve this problem finally. It was a bug in the CABAC context initialization.
But I want to share this experience as many people may want to do the same thing.
The three steps that I explained are essentially necessary to add a new syntax element, but one might be very careful with the followings:
In the beginning, you need to decide either you want to use a separate context model for your syntax element? Or you want to use an existing one? In case of CABAC separation, you should define a ContextModel3DBuffer and the best way to do that is: finding a similar syntax element in the code; then duplicating its ``ContextModel3DBuffer'' definition and ALL of its occurences in the code. This way assures that you are considering everything.
Encoding of each syntax elements happens in two different places: first, in the RDO loop to make a "decision", and second, during the actual encoding phase and when the decisions are being encoded (e.g. encodeCtu function).
The order of encoding/decoding syntaxt elements should be the same at the encoder/decoder sides. For example if your new syntax element is encoded after splitFlag and before predMode at the encoder side, you should decode it exactly between splitFlag and predMode at the decoder side.
The context model is implemented as a 3D matrix in order to let track the statistics of syntaxt elements separately for different block sizes, componenets etc. This means that when you want to call the function encodeBin, you may make sure that a correct index is being used. I've made stupid mistakes in this part!
Apart from the above remarks, I found a the function getState very useful for debugging. This function returns the state of your CABAC context model in an arbitrary place of the code when you have access to it. It is very useful to compare the state at the same place of the encoder and the decoder when you have a mismatch. For example, it happens a lot that you encode a 1 but you decode a 0. In this case, you need to check the state of your CABAC context before encoding and decoding. They should be the same. If they are not the same, track back the error to find the first place of mismatch.
I hope it was helpful.
How can I parameterise one signal based on another signal?
e.g. Suppose I wanted to modify the fps based on the x-position of the mouse. The types are:
Mouse.x : Signal Int
fps : number -> Signal Time
How could I make Elm understand something along the lines of this pseudocode:
fps (Mouse.x) : Signal Time
Obviously, lift doesn't work in this case. I think the result would be Signal (Signal Time) (but I'm still quite new to Elm).
Thanks!
Preamble
fps Mouse.x
Results in a type-error that fps requires an Int, not a Signal Int.
lift fps Mouse.x : Signal (Signal Int)
You are correct there. As CheatX's answers mentions, you cannot using these "nested signals" in Elm.
Answer to your question
It seems like you're asking for something that doesn't exist yet in the Standard Libraries. If I understand your question correctly you would like a time (or fps) signal of which the timing can be changed dynamically. Something like:
dynamicFps : Signal Int -> Signal Time
Using the built-in functions like lift does not give you the ability to construct such a function yourself from a function of type Int -> Signal Time.
I think you have three options here:
Ask to have this function added to the Time library on the mailing-list. (The feature request instructions are a little bloated for a request of such a function so you can skip stuff that's not applicable)
Work around the problem, either from within Elm or in JavaScript, using Ports to connect to Elm.
Find a way to not need a dynamically changing Time signal.
I advise option 1. Option 3 is sad, you should be able to what you asked in Elm. Option 2 is perhaps not a good idea if you're new to Elm. Option 1 is not a lot of work, and the folks on the mailing-list don't bite ;)
To elaborate on option 2, should you want to go for that:
If you specify an outgoing port for Signal Int and an incoming port for Signal Time you can write your own dynamic time function in JavaScript. See http://elm-lang.org/learn/Ports.elm
If you want to do this from within Elm, it'll take an uglier hack:
dynamicFps frames =
let start = (0,0)
time = every millisecond -- this strains your program enormously
input = (,) <~ frames ~ time
step (frameTime,now) (oldDelta,old) =
let delta = now - old
in if (oldDelta,old) == (0,0)
then (frameTime,now) -- this is to skip the (0,0) start
else if delta * frameTime >= second
then (delta,now)
else (0,old)
in dropIf ((==) 0) 0 <| fst <~ foldp step start input
Basically, you remember an absolute timestamp, ask for the new time as fast as you can, and check if the time between the remembered time and now is big enough to fit the timeframe you want. If so you send out that time delta (fps gives time deltas) and remember now as the new timestamp. Because foldp sends out everything it is to remember, you get both the new delta and the new time. So using fst <~ you keep only the delta. But the input time is (likely) much faster than the timeframe you want so you also get a lot of (0,old) from foldp. That's why there is a dropIf ((==) 0).
Nested signals are explicitly forbidden by the Elm's type system [part 3.2 of this paper].
As far as I understand FRP, nested signals are only useful when some kind of flattering provided (monadic 'join' function for example). And that operation is hard to be implemented without keeping an entire signal history.
I would like to know if anyone knows how to perform a cross-correlation between two audio signals on iOS.
I would like to align the FFT windows that I get at the receiver (I am receiving the signal from the mic) with the ones at the transmitter (which is playing the audio track), i.e. make sure that the first sample of each window (besides a "sync" period) at the transmitter will also be the first window at the receiver.
I injected in every chunk of the transmitted audio a known waveform (in the frequency domain). I want estimate the delay through cross-correlation between the known waveform and the received signal (over several consecutive chunks), but I don't know how to do it.
It looks like there is the method vDSP_convD to do it, but I have no idea how to use it and whether I first have to perform the real FFT of the samples (probably yes, because I have to pass double[]).
void vDSP_convD (
const double __vDSP_signal[],
vDSP_Stride __vDSP_signalStride,
const double __vDSP_filter[],
vDSP_Stride __vDSP_strideFilter,
double __vDSP_result[],
vDSP_Stride __vDSP_strideResult,
vDSP_Length __vDSP_lenResult,
vDSP_Length __vDSP_lenFilter
)
The vDSP_convD() function calculates the convolution of the two input vectors to produce a result vector. It’s unlikely that you want to convolve in the frequency domain, since you are looking for a time-domain result — though you might, if you have FFTs already for some other reason, choose to multiply them together rather than convolving the time-domain sequences (but in that case, to get your result, you will need to perform an inverse DFT to get back to the time domain again).
Assuming, of course, I understand you correctly.
Then once you have the result from vDSP_convD(), you would want to look for the highest value, which will tell you where the signals are most strongly correlated. You might also need to cope with the case where the input signal does not contain sufficient of your reference signal, and in that case you may wish to (for example) ignore values in the result vector below a certain level.
Cross-correlation is the solution, yes. But there are many obstacles you need to handle. If you get samples from the audio files, they contain padding which cross-correlation function does not like. It is also very inefficient to perform correlation with all those samples - it takes a huge amount of time. I have made a sample code which demonstrates time shift of two audio files. If you are interested in the sample, look at my Github Project.