This is a question about Scala continuations. Can resets be nested? If they can: what are nested resets useful for ? Is there any example of nested resets?
Yes, resets can be nested, and, yes, it can be useful. As an example, I recently prototyped an API for the scalagwt project that would allow GWT developers to write asynchronous RPCs (remote procedure calls) in a direct style (as opposed to the callback-passing style that is used in GWT for Java). For example:
field1 = "starting up..." // 1
field2 = "starting up..." // 2
async { // (reset)
val x = service.someAsyncMethod() // 3 (shift)
field1 = x // 5
async { // (reset)
val y = service.anotherAsyncMethod() // 6 (shift)
field2 = y // 8
}
field2 = "waiting..." // 7
}
field1 = "waiting..." // 4
The comments indicate the order of execution. Here, the async method performs a reset, and each service call performs a shift (you can see the implementation on my github fork, specifically Async.scala).
Note how the nested async changes the control flow. Without it, the line field2 = "waiting" would not be executed until after successful completion of the second RPC.
When an RPC is made, the implementation captures the continuation up to the inner-most async boundary, and suspends it for execution upon successful completion of the RPC. Thus, the nested async block allows control to flow immediately to the line after it as soon as the second RPC is made. Without that nested block, on the other hand, the continuation would extend all the way to the end of the outer async block, in which case all the code within the outer async would block on each and every RPC.
reset forms an abstraction so that code outside is not affected by the fact that the code inside is implemented with continuation magic. So if you're writing code with reset and shift, it can call other code which may or may not be implemented with reset and shift as well. In this sense they can be nested.
Related
Let's say we have the following component script
let x = 0;
function increase() {
x += 1;
}
$: console.log(x)
$: if (x == 4) {
x = 0;
}
https://svelte.dev/repl/2bca979673114afea9fc6b37434653a3?version=3.29.0
Intuitively I would expect from cotinually calling increase, is to have the values 0,1,2,3,4,1,2,3,4,... logged to the console. But this is not the case. Instead, the second reactive statement runs first, and 4 is never logged; it is 0. Switching the order of the reactive statements has no effect.
How do you make the logging statement execute before the other one and what is the reason that it runs second, in the first place?
The order in which reactive statements that depends on the same variables are run is not specified / guaranteed by Svelte. You have to build your components with the expectation that they can be run in any order, and make sure that your code doesn't depend on any specific order.
You must not rely on a given order you observe in a specific case either -- it can change following a change in Svelte's internal implementation (e.g. new optimisations), or when the code of your own component changes (causing a different compiler output, that can change the order of reactive blocks for hard-to-anticipate reasons).
The reasons why this happens are highly technical (i.e. I don't really know), but they boil down to ease of implementation / optimization of the compiler. Reactive statements are a kind of async operations, that typically can't guarantee order of execution.
If you really need 2 operations to happen in a specific order then, for peace of mind, you need them in the same reactive block.
$: {
console.log(x) // op 1
if (x == 4) x = 0 // op 2
}
So, I was trying to learn about Continuation. I came across with the following saying (link):
Say you're in the kitchen in front of the refrigerator, thinking about a sandwich. You take a continuation right there and stick it in your pocket. Then you get some turkey and bread out of the refrigerator and make yourself a sandwich, which is now sitting on the counter. You invoke the continuation in your pocket, and you find yourself standing in front of the refrigerator again, thinking about a sandwich. But fortunately, there's a sandwich on the counter, and all the materials used to make it are gone. So you eat it. :-) — Luke Palmer
Also, I saw a program in Scala:
var k1 : (Unit => Sandwich) = null
reset {
shift { k : Unit => Sandwich) => k1 = k }
makeSandwich
}
val x = k1()
I don't really know the syntax of Scala (looks similar to Java and C mixed together) but I would like to understand the concept of Continuation.
Firstly, I tried to run this program (by adding it into main). But it fails, I think that it has a syntax error due to the ) near Sandwich but I'm not sure. I removed it but it still does not compile.
How to create a fully compiled example that shows the concept of the story above?
How this example shows the concept of Continuation.
In the link above there was the following saying: "Not a perfect analogy in Scala because makeSandwich is not executed the first time through (unlike in Scheme)". What does it mean?
Since you seem to be more interested in the concept of the "continuation" rather than specific code, let's forget about that code for a moment (especially because it is quite old and I don't really like those examples because IMHO you can't understand them correctly unless you already know what a continuation is).
Note: this is a very long answer with some attempts to describe what a continuations is and why it is useful. There are some examples in Scala-like pseudo-code none of which can actually be compiled and run (there is just one compilable example at the very end and it references another example from the middle of the answer). Expect to spend a significant amount of time just reading this answer.
Intro to continuations
Probably the first thing you should do to understand a continuation is to forget about how modern compilers for most of the imperative languages work and how most of the modern CPUs work and particularly the idea of the call stack. This is actually implementation details (although quite popular and quite useful in practice).
Assume you have a CPU that can execute some sequence of instructions. Now you want to have a high level languages that support the idea of methods that can call each other. The obvious problem you face is that the CPU needs some "forward only" sequence of commands but you want some way to "return" results from a sub-program to the caller. Conceptually it means that you need to have some way to store somewhere before the call all the state of the caller method that is required for it to continue to run after the result of the sub-program is computed, pass it to the sub-program and then ask the sub-program at the end to continue execution from that stored state. This stored state is exactly a continuation. In most of the modern environments those continuations are stored on the call stack and often there are some assembly instructions specifically designed to help handling it (like call and return). But again this is just implementation details. Potentially they might be stored in an arbitrary way and it will still work.
So now let's re-iterate this idea: a continuation is a state of the program at some point that is enough to continue its execution from that point, typically with no additional input or some small known input (like a return value of the called method). Running a continuation is different from a method call in that usually continuation never explicitly returns execution control back to the caller, it can only pass it to another continuation. Potentially you can create such a state yourself, but in practice for the feature to be useful you need some support from the compiler to build continuations automatically or emulate it in some other way (this is why the Scala code you see requires a compiler plugin).
Asynchronous calls
Now there is an obvious question: why continuations are useful at all? Actually there are a few more scenarios besides the simple "return" case. One such scenario is asynchronous programming. Actually if you do some asynchronous call and provide a callback to handle the result, this can be seen as passing a continuation. Unfortunately most of the modern languages do not support automatic continuations so you have to grab all the relevant state yourself. Another problem appears if you have some logic that needs a sequence of many async calls. And if some of the calls are conditional, you easily get to the callbacks hell. The way continuations help you avoid it is by allowing you build a method with effectively inverted control flow. With typical call it is the caller that knows the callee and expects to get a result back in a synchronous way. With continuations you can write a method with several "entry points" (or "return to points") for different stages of the processing logic that you can just pass to some other method and that method can still return to exactly that position.
Consider following example (in pseudo-code that is Scala-like but is actually far from the real Scala in many details):
def someBusinessLogic() = {
val userInput = getIntFromUser()
val firstServiceRes = requestService1(userInput)
val secondServiceRes = if (firstServiceRes % 2 == 0) requestService2v1(userInput) else requestService2v2(userInput)
showToUser(combineUserInputAndResults(userInput,secondServiceRes))
}
If all those calls a synchronous blocking calls, this code is easy. But assume all those get and request calls are asynchronous. How to re-write the code? The moment you put the logic in callbacks you loose the clarity of the sequential code. And here is where continuations might help you:
def someBusinessLogicCont() = {
// the method entry point
val userInput
getIntFromUserAsync(cont1, captureContinuationExpecting(entry1, userInput))
// entry/return point after user input
entry1:
val firstServiceRes
requestService1Async(userInput, captureContinuationExpecting(entry2, firstServiceRes))
// entry/return point after the first request to the service
entry2:
val secondServiceRes
if (firstServiceRes % 2 == 0) {
requestService2v1Async(userInput, captureContinuationExpecting(entry3, secondServiceRes))
// entry/return point after the second request to the service v1
entry3:
} else {
requestService2v2Async(userInput, captureContinuationExpecting(entry4, secondServiceRes))
// entry/return point after the second request to the service v2
entry4:
}
showToUser(combineUserInputAndResults(userInput, secondServiceRes))
}
It is hard to capture the idea in a pseudo-code. What I mean is that all those Async method never return. The only way to continue execution of the someBusinessLogicCont is to call the continuation passed into the "async" method. The captureContinuationExpecting(label, variable) call is supposed to create a continuation of the current method at the label with the input (return) value bound to the variable. With such a re-write you still has a sequential-looking business logic even with all those asynchronous calls. So now for a getIntFromUserAsync the second argument looks like just another asynchronous (i.e. never-returning) method that just requires one integer argument. Let's call this type Continuation[T]
trait Continuation[T] {
def continue(value: T):Nothing
}
Logically Continuation[T] looks like a function T => Unit or rather T => Nothing where Nothing as the return type signifies that the call actually never returns (note, in actual Scala implementation such calls do return, so no Nothing there, but I think conceptually it is easy to think about no-return continuations).
Internal vs external iteration
Another example is a problem of iteration. Iteration can be internal or external. Internal iteration API looks like this:
trait CollectionI[T] {
def forEachInternal(handler: T => Unit): Unit
}
External iteration looks like this:
trait Iterator[T] {
def nextValue(): Option[T]
}
trait CollectionE[T] {
def forEachExternal(): Iterator[T]
}
Note: often Iterator has two method like hasNext and nextValue returning T but it will just make the story a bit more complicated. Here I use a merged nextValue returning Option[T] where the value None means the end of the iteration and Some(value) means the next value.
Assuming the Collection is implemented by something more complicated than an array or a simple list, for example some kind of a tree, there is a conflict here between the implementer of the API and the API user if you use typical imperative language. And the conflict is over the simple question: who controls the stack (i.e. the easy to use state of the program)? The internal iteration is easier for the implementer because he controls the stack and can easily store whatever state is needed to move to the next item but for the API user the things become tricky if she wants to do some aggregation of the stored data because now she has to save the state between the calls to the handler somewhere. Also you need some additional tricks to let the user stop the iteration at some arbitrary place before the end of the data (consider you are trying to implement find via forEach). Conversely the external iteration is easy for the user: she can store all the state necessary to process data in any way in local variables but the API implementer now has to store his state between calls to the nextValue somewhere else. So fundamentally the problem arises because there is only one place to easily store the state of "your" part of the program (the call stack) and two conflicting users for that place. It would be nice if you could just have two different independent places for the state: one for the implementer and another for the user. And continuations provide exactly that. The idea is that we can pass execution control between two methods back and forth using two continuations (one for each part of the program). Let's change the signatures to:
// internal iteration
// continuation of the iterator
type ContIterI[T] = Continuation[(ContCallerI[T], ContCallerLastI)]
// continuation of the caller
type ContCallerI[T] = Continuation[(T, ContIterI[T])]
// last continuation of the caller
type ContCallerLastI = Continuation[Unit]
// external iteration
// continuation of the iterator
type ContIterE[T] = Continuation[ContCallerE[T]]
// continuation of the caller
type ContCallerE[T] = Continuation[(Option[T], ContIterE[T])]
trait Iterator[T] {
def nextValue(cont : ContCallerE[T]): Nothing
}
trait CollectionE[T] {
def forEachExternal(): Iterator[T]
}
trait CollectionI[T] {
def forEachInternal(cont : ContCallerI[T]): Nothing
}
Here ContCallerI[T] type, for example, means that this is a continuation (i.e. a state of the program) the expects two input parameters to continue running: one of type T (the next element) and another of type ContIterI[T] (the continuation to switch back). Now you can see that the new forEachInternal and the new forEachExternal+Iterator have almost the same signatures. The only difference in how the end of the iteration is signaled: in one case it is done by returning None and in other by passing and calling another continuation (ContCallerLastI).
Here is a naive pseudo-code implementation of a sum of elements in an array of Int using these signatures (an array is used instead of something more complicated to simplify the example):
class ArrayCollection[T](val data:T[]) : CollectionI[T] {
def forEachInternal(cont0 : ContCallerI[T], lastCont: ContCallerLastI): Nothing = {
var contCaller = cont0
for(i <- 0 to data.length) {
val contIter = captureContinuationExpecting(label, contCaller)
contCaller.continue(data(i), contIter)
label:
}
}
}
def sum(arr: ArrayCollection[Int]): Int = {
var sum = 0
val elem:Int
val iterCont:ContIterI[Int]
val contAdd0 = captureContinuationExpecting(labelAdd, elem, iterCont)
val contLast = captureContinuation(labelReturn)
arr.forEachInternal(contAdd0, contLast)
labelAdd:
sum += elem
val contAdd = captureContinuationExpecting(labelAdd, elem, iterCont)
iterCont.continue(contAdd)
// note that the code never execute this line, the only way to jump out of labelAdd is to call contLast
labelReturn:
return sum
}
Note how both implementations of the forEachInternal and of the sum methods look fairly sequential.
Multi-tasking
Cooperative multitasking also known as coroutines is actually very similar to the iterations example. Cooperative multitasking is an idea that the program can voluntarily give up ("yield") its execution control either to the global scheduler or to another known coroutine. Actually the last (re-written) example of sum can be seen as two coroutines working together: one doing iteration and another doing summation. But more generally your code might yield its execution to some scheduler that then will select which other coroutine to run next. And what the scheduler does is manages a bunch of continuations deciding which to continue next.
Preemptive multitasking can be seen as a similar thing but the scheduler is run by some hardware interruption and then the scheduler needs a way to create a continuation of the program being executed just before the interruption from the outside of that program rather than from the inside.
Scala examples
What you see is a really old article that is referring to Scala 2.8 (while current versions are 2.11, 2.12, and soon 2.13). As #igorpcholkin correctly pointed out, you need to use a Scala continuations compiler plugin and library. The sbt compiler plugin page has an example how to enable exactly that plugin (for Scala 2.12 and #igorpcholkin's answer has the magic strings for Scala 2.11):
val continuationsVersion = "1.0.3"
autoCompilerPlugins := true
addCompilerPlugin("org.scala-lang.plugins" % "scala-continuations-plugin_2.12.2" % continuationsVersion)
libraryDependencies += "org.scala-lang.plugins" %% "scala-continuations-library" % continuationsVersion
scalacOptions += "-P:continuations:enable"
The problem is that plugin is semi-abandoned and is not widely used in practice. Also the syntax has changed since the Scala 2.8 times so it is hard to get those examples running even if you fix the obvious syntax bugs like missing ( here and there. The reason of that state is stated on the GitHub as:
You may also be interested in https://github.com/scala/async, which covers the most common use case for the continuations plugin.
What that plugin does is emulates continuations using code-rewriting (I suppose it is really hard to implement true continuations over the JVM execution model). And under such re-writings a natural thing to represent a continuation is some function (typically called k and k1 in those examples).
So now if you managed to read the wall of text above, you can probably interpret the sandwich example correctly. AFAIU that example is an example of using continuation as means to emulate "return". If we re-sate it with more details, it could go like this:
You (your brain) are inside some function that at some points decides that it wants a sandwich. Luckily you have a sub-routine that knows how to make a sandwich. You store your current brain state as a continuation into the pocket and call the sub-routine saying to it that when the job is done, it should continue the continuation from the pocket. Then you make a sandwich according to that sub-routine messing up with your previous brain state. At the end of the sub-routine it runs the continuation from the pocket and you return to the state just before the call of the sub-routine, forget all your state during that sub-routine (i.e. how you made the sandwich) but you can see the changes in the outside world i.e. that the sandwich is made now.
To provide at least one compilable example with the current version of the scala-continuations, here is a simplified version of my asynchronous example:
case class RemoteService(private val readData: Array[Int]) {
private var readPos = -1
def asyncRead(callback: Int => Unit): Unit = {
readPos += 1
callback(readData(readPos))
}
}
def readAsyncUsage(rs1: RemoteService, rs2: RemoteService): Unit = {
import scala.util.continuations._
reset {
val read1 = shift(rs1.asyncRead)
val read2 = if (read1 % 2 == 0) shift(rs1.asyncRead) else shift(rs2.asyncRead)
println(s"read1 = $read1, read2 = $read2")
}
}
def readExample(): Unit = {
// this prints 1-42
readAsyncUsage(RemoteService(Array(1, 2)), RemoteService(Array(42)))
// this prints 2-1
readAsyncUsage(RemoteService(Array(2, 1)), RemoteService(Array(42)))
}
Here remote calls are emulated (mocked) with a fixed data provided in arrays. Note how readAsyncUsage looks like a totally sequential code despite the non-trivial logic of which remote service to call in the second read depending on the result of the first read.
For full example you need prepare Scala compiler to use continuations and also use a special compiler plugin and library.
The simplest way is a create a new sbt.project in IntellijIDEA with the following files: build.sbt - in the root of the project, CTest.scala - inside main/src.
Here is contents of both files:
build.sbt:
name := "ContinuationSandwich"
version := "0.1"
scalaVersion := "2.11.6"
autoCompilerPlugins := true
addCompilerPlugin(
"org.scala-lang.plugins" % "scala-continuations-plugin_2.11.6" % "1.0.2")
libraryDependencies +=
"org.scala-lang.plugins" %% "scala-continuations-library" % "1.0.2"
scalacOptions += "-P:continuations:enable"
CTest.scala:
import scala.util.continuations._
object CTest extends App {
case class Sandwich()
def makeSandwich = {
println("Making sandwich")
new Sandwich
}
var k1 : (Unit => Sandwich) = null
reset {
shift { k : (Unit => Sandwich) => k1 = k }
makeSandwich
}
val x = k1()
}
What the code above essentially does is calling makeSandwich function (in a convoluted manner). So execution result would be just printing "Making sandwich" into console. The same result would be achieved without continuations:
object CTest extends App {
case class Sandwich()
def makeSandwich = {
println("Making sandwich")
new Sandwich
}
val x = makeSandwich
}
So what's the point? My understanding is that we want to "prepare a sandwich", ignoring the fact that we may be not ready for that. We mark a point of time where we want to return to after all necessary conditions are met (i.e. we have all necessary ingredients ready). After we fetch all ingredients we can return to the mark and "prepare a sandwich", "forgetting that we were unable to do that in past". Continuations allow us to "mark point of time in past" and return to that point.
Now step by step. k1 is a variable to hold a pointer to a function which should allow to "create sandwich". We know it because k1 is declared so: (Unit => Sandwich).
However initially the variable is not initialized (k1 = null, "there are no ingredients to make a sandwich yet"). So we can't call the function preparing sandwich using that variable yet.
So we mark a point of execution where we want to return to (or point of time in past we want to return to) using "reset" statement.
makeSandwich is another pointer to a function which actually allows to make a sandwich. It's the last statement of "reset block" and hence it is passed to "shift" (function) as argument (shift { k : (Unit => Sandwich).... Inside shift we assign that argument to k1 variable k1 = k thus making k1 ready to be called as a function. After that we return to execution point marked by reset. The next statement is execution of function pointed to by k1 variable which is now properly initialized so finally we call makeSandwich which prints "Making sandwich" to a console. It also returns an instance of sandwich class which is assigned to x variable.
Not sure, probably it means that makeSandwich is not called inside reset block but just afterwards when we call it as k1().
I was looking over some Scala server code and I saw thins async/await block:
async {
while (cancellationToken.nonCancelled) {
val (request, exchange) = await(listener.nextRequest)
respond(exchange, cancellationToken, handler(request))
}
}
How can this be correct syntax?
As I understand it:
For every execution of the while loop
Thread 1 will execute the code from the while loop except the one in the await clause.
Thread 2 will go in the await clause.
But then Thread 1 will have val (request, exchange) uninstantiated in case Thread 2 doesn't finish computing.
These values will be passed to the respond and handler methods uninstantiated.
So how can you have an assignment in two different threads?
So how can you have an assignment in two different threads?
async-await's main goal is to allow you to do asynchronous programming in a synchronous fashion.
What really happens is that the awaited call executes listener.nextRequest and asynchronously waits for it's completion, it doesn't execute the next line of code until then. This guarantees that if the next line of code is executed, it's values are populated. The assignment should happen where it is visible to the next LOC in the method.
This is possible due to the fact that the async macro actually transforms this code into a state-machine, where the first part is the execution up until the first await, and the next part is everything after.
I'm trying to separate observable into windows (or for my purposes also Buffers are fine) while being able to close windows/buffers at custom location.
E.g. I have an observable which produces integers starting at 1 and moving up. I want to close a window at each number which is divisible by 7. My closing function would need to take in the item as parameter in that case.
There is an overload of Window method:
Window<TSource, TWindowClosing>(IObservable<TSource>, Func<IObservable<TWindowClosing>>)
Either it cant be done using this overload, or I can't wrap my head around it. Documentation describes that it does exactly what I want but does not show an example. Also, it shows an example of non-deterministic closing, which depends on timing when closing observable collection emits items.
The Window operator breaks up an observable sequence into consecutive
non-overlapping windows. The end of the current window and start of
the next window is controlled by an observable sequence which is the
result of the windowClosingSelect function which is passed as an input
parameter to the operator. The operator could be used to group a set
of events into a window. For example, states of a transaction could be
the main sequence being observed. Those states could include:
Preparing, Prepared, Active, and Committed/Aborted. The main sequence
could include all of those states are they occur in that order. The
windowClosingSelect function could return an observable sequence that
only produces a value on the Committed or Abort states. This would
close the window that represented transaction events for a particular
transaction.
I'm thinking something like following would do the job, but I'd have to implement it myself:
Window<TSource, TWindowClosing>(IObservable<TSource>, Func<TSource, bool>)
Is such windowing possible with built-in functions (I know I can build one myself)?
Is it possible to close a window based on emitted item or only non-deterministically, once an item is emitted from windowing observable?
Use the original sequence with a Where clause as your closing sequence. If your source sequence is cold, then make use of Publish and RefCount to make it work correctly.
var source = ...;
var sharedSource = source.Publish().RefCount();
var closingSignal = sharedSource.Where(i => (i % 7) == 0);
var windows = sharedSource.Window(() => closingSignal);
( I've read a lot about async and I wonder what happens if there is a mix of async function call and non-async) and im talking about THREAD pov .
( I know that mix should not be done , but im asking in order to understand the flow better)
suppose I have(cube 1) function which calls async functions which calls asyc function.
(please notice that cube 1 function is not async)
Question : when the control reaches (cube 3) await Task.Delay(100); --
what is really happening as the control reaches cube3 await ? does the control is back(and blocked) at the pink arrow (cube 1 ) or does the control is released and still awaits ( orange arrow)
When control reaches the await statement in Cube3, Task.Delay() is called and immediately returns a Task that will complete after the delay. The rest of CalculateAnswer() is refactored into a continuation that will run when the Task returned by Delay() completes, and CalculateAnswer() immediately returns a Task<int> that will complete when that continuation completes.
CallCalculateAnswer() awaits the Task<int> returned by CalculateAnswer(), so the same logic applies: the code running after the await (i.e. the return statement) is refactored into a continuation that will run when the Task<int> completes, and CallCalculateAnswer() proceeds to return its own Task<int>.
StartChain(), however, does not await the Task<int> produced by CallCalculateAnswer(), so no continuation is generated. Instead, it stores the task into a local variable and returns. Therefore, the orange arrow in your question is the right one.
Note that, in this specific case, since nothing is done with the Task<int> returned by CallCalculateAnswer() and it is only stored in a local variable, it becomes "forgotten" once StartChain() returns: the task will complete (or fail) some time in the future, but its result (the int it produces) will be lost.
Update following comments: The pink arrow in your question implies you're expecting StartChain() to block, waiting for the task returned by CallCalculateAnswer() to complete, because it is not async and does not await that task. This is not what happens, as the usual control flow semantics apply: the method has returned a task, so you get a reference to that task and nothing more.
If you want StartChain() to block, you can use Task.Wait(), Task<T>.Result, or similar on the task returned by CallCalculateAnswer().