I wrote the following code, which appears to work
import kotlin.coroutines.experimental.buildSequence
fun concatElemSeq(elem : String, seq : Sequence<String>) = buildSequence {
seq.forEach { yield(elem+it) }
}
fun concatSeqSeq( seq1 : Sequence<String>, seq2 : Sequence<String> ) = buildSequence {
seq1.forEach { yieldAll(concatElemSeq(it, seq2)) }
}
fun main(args: Array<String>) {
val seq = buildSequence {
val l = listOf("A", "B", "C")
l.forEach { yield(it) }
}
concatSeqSeq(seq, seq).forEach { println(it) }
}
Only one instance of seq is created and namely it is passed into functions, I checked in debugger. Nevertheless, code produces all combinations of charaters, which means, that it has copies of this sequence at various positions.
How is this possible?
UPDATE
I tried the same with Iterator and it didn't work. How to distinguish classes like Sequence, which restore their state, from classes like Iterator, which don't?
How to write my own class, which restores state on re-entry like Sequence?
Basically sequence is not iterator, it's more like Iterable.
It even has iterator() method inside, which can be defined in several ways - return one iterator or new instance on each call.
Related
I have an Observable source where several items are emitted. For each item, I need to execute a long process in the background and start each process only after the previous one is completed. So, I get the desired behavior using these operators:
'0, 1, 2, 3' are observable items, 'A, B, C' are process results. I start the processing of '0' manually using startWith to start this processing circle. The problem is that 'A, B, C' are emitted by BehaviorSubject (in other places I need the latest calculation result), so if I have already processed the old items, the BehaviorSubject returns the old result and then I get the following:
So how can I skip that old result 'X'? If I use skip(1) then the result 'A' is skipped if there is no processing before. Or maybe there is another more appropriate way to reach my goal?
A bit of code to bring specifics, what am I doing:
val decodedChannelsSource =
BehaviorSubject.create<List<ChannelData>>()
fun startTransmitting(channels: List<ChannelData>, frameSize: Int, frameCount: Int) {
transmitSubscription?.dispose()
transmitSubscription = Observable
.create<List<ChannelData>> {
for (i in 1 until frameCount) {
val chls = channels.map { c ->
c.apply { frameData = UserDataProvider.generateData(frameSize) }
}
it.onNext(chls)
}
}
.zipWith(decodedChannelsSource) { trans, rec -> trans }
.startWith(channels.map { c ->
c.apply { frameData = UserDataProvider.generateData(frameSize) }
})
.subscribeOn(Schedulers.io())
.subscribe{ processChannels(it) }
}
fun processChannels(channels: List<ChannelData>) {
... // do calculations with channels in the background
decodedChannelsSource.onNext(channels)
}
Beginner in Kotlin here.
I try to create and populate objects by reflection in a program. I cannot find the equivalent functionality in pure kotlin so my solution resembles the code below which works fine, but requires the use of dirty references like java.lang.String::class.java and intelliJ, understandably, doesn't seem to like this. Is there a simpler way that I am missing to do this?
val jclass = myObject::class.java
val setters = jclass.declaredMethods.filter { it.name.startsWith("set") }
for (s in setters) {
val paramType = s.parameterTypes.first()
val data = when(paramType) {
java.lang.Integer::class.java -> foo
java.lang.Double::class.java -> bar
java.lang.String::class.java -> baz
}
s.invoke(myObject, data)
}
You can use Kotlin reflection, which requires you to add kotlin-reflect as a dependency to your project.
Here you can find kotlin-reflect for Kotlin 1.0.5, or pick another version if you use different Kotlin version.
After that, you can rewrite your code as follows:
val properties = myObject.javaClass.kotlin.memberProperties
for (p in properties.filterIsInstance<KMutableProperty<*>>()) {
val data = when (p.returnType.javaType) {
Int::class.javaPrimitiveType,
Int::class.javaObjectType -> foo
Double::class.javaPrimitiveType,
Double::class.javaObjectType -> bar
String::class.java -> baz
else -> null
}
if (data != null)
p.setter.call(myObject, data)
}
Some details:
Despite using Kotlin reflection, this approach works with Java classes as well, their fields and accessors will be seen as properties, as described here.
Just like with Java reflection, memberProperties returns public properties of this type and all its supertypes. To get all the properties declared in the type (including the private ones, but not those from the supertypes), use declaredMemberProperties instead.
.filterIsInstance<KMutableProperty<*> returns only the mutable properties, so that you can use their p.setter later. If you need to iterate over the getters of all the properties, remove it.
In the when block, I compared p.returnType.javaType to Int::class.javaPrimitiveType and Int::class.javaObjectType, because what's Int in Kotlin can be mapped to either Java int or java.lang.Integer depending on its usage. In Kotlin 1.1, it will be enough to check p.returnType.classifier == Int::class.
If You need to get property getter/setter, there is a couple of built-in constructions for it YourClass::propertyName
have a look at example bellow
fun main(args: Array<String>) {
val myObject = Cat("Tom", 3, 35)
println(Cat::age.getter.call(myObject)) // will print 3
Cat::age.setter.call(myObject, 45)
print(myObject) // will print Cat(name=Tom, age=45, height=35)
}
data class Cat(var name : String, var age : Int, val height : Int)
but sometimes you don't know class exactly(working with generics) or need to get list of properties, then use val <T : Any> KClass<T>.declaredMemberProperties: Collection<KProperty1<T, *>> it will return all properties, some of them can be mutable(var) and some immutable(val), you can find out immutability by checking belonging to KMutableProperty<*> (by filtering with is operator or using convenience methods such as filterIsInstance<KMutableProperty<*>>)
about your code snippet
I absolutely agree with hotkey, but now it is better to use myObject::class.declaredMemberProperties instead of myObject.javaClass.kotlin.memberProperties
because the second one is deprecated
https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/java-class.html
data class Cat(var name : String, var age : Int, val height : Int)
#JvmStatic
fun main(args: Array<String>) {
val myObject = Cat("Tom", 3, 35)
val properties = myObject::class.declaredMemberProperties
for (p in properties.filterIsInstance<KMutableProperty<*>>()) {
val data = when (p.returnType.javaType) {
Int::class.javaPrimitiveType,
Int::class.javaObjectType -> 5
String::class.java -> "Rob"
else -> null
}
if (data != null)
p.setter.call(myObject, data)
}
println(myObject)
// it will print Cat(name=Rob, age=5, height=35),
// because height isn't var(immutable)
}
in general, I would approach similar problems with such construction in mind
val myObject = Cat("Tom", 3, 35)
Cat::class.declaredMemberProperties
//if we want only public ones
.filter{ it.visibility == KVisibility.PUBLIC }
// We only want strings
.filter{ it.returnType.isSubtypeOf(String::class.starProjectedType) }
.filterIsInstance<KMutableProperty<*>>()
.forEach { prop ->
prop.setter.call(myObject, "Rob")
}
println(myObject)
//it will print Cat(name=Rob, age=3, height=35),
//because name is only eligible in this case
I want to specify a function based on a string. I'm getting strings out of a map, in the example below they are the values in function while interating ove the map. Now for example, when the string value function == "networkInfo", I would like to "treat" that value as a real functions' name. It's hard to explain, but I think you guys will know what I mean.
My goal is to remove the switch statement and directly call c.AddFunc(spec, func() { networkInfo() }) where networkInfo is the function name, extracted from string function. I know this is possible, but I don't know how :(. Help is appreciated!
// ScheduleCronjobs starts the scheduler
func ScheduleCronjobs() {
tasks := props.P.GetStringMapString("tasks")
log.Infof("number of tasks: %d", len(tasks))
if len(tasks) != 0 {
c := cron.New()
// for each task, initialize
for function, spec := range tasks {
switch function {
case "networkInfo":
c.AddFunc(spec, func() { networkInfo() })
case "bla":
c.AddFunc(spec, func() { bla() })
default:
log.Errorf("unknown task: %q", function)
}
}
c.Start()
}
// after initialization, send out confirmation message
slack.SendMessage("tasks initialized", props.P.GetString("channel"))
}
Why not something like:
taskDefs := map[string]func(){
"networkInfo": networkInfo,
"bla": bla,
}
for function, spec := range tasks {
if fn, ok := taskDefs[function]; ok {
c.AddFunc(spec, func() { fn() }) // not sure if you need the enclosing func
} else {
log.Errorf("unknown task: %q", function)
}
}
If you do need varying signatures of your funcs then you'd actually need reflection, but if the types of the funcs are all the same, then using this map approach might be a simpler solution, without the overhead of reflection.
The only way I've found to find functions by name in a package is by actually parsing the source files. This repo is an example of finding funcs and storing them in a map with the name as the key.
The Go linker will silently drop unreferenced funcs, so if the only way you're referencing the func is through reflection it would break. That is why the map approach I suggest is superior; it let's the linker know the func is being used.
In our current application we have a need to traverse down a tree and capture all operators on a specific device (and child devices). A device could have child devices with also specific operators on it.
As i am new to the use of recursion in Groovy i am wondering if i am doing things right..?
Any pointer to help me learn better ways of doing things?
def listOperators(device) {
// list with all operator id's
def results = []
// closure to traverse down the tree
def getAllOperators = { aDevice->
if(aDevice) {
aDevice.operators.each { it ->
results << it.id
}
}
if (aDevice?.children) {
aDevice.children.each { child ->
results << owner.call(child)
}
}
}
// call the closure with the given device
getAllOperators(device)
// return list with unique results
return results.unique()
}
A couple things to note:
Doing the recursive call through owner is not a good idea. The definition of owner changes if the call is nested within another closure. It's error prone and has no advantages over just using the name. When the closure is a local variable, split its up the declaration and definition of the closure so the name is in scope. E.g.:
def getAllOperators
getAllOperators = { ...
You are appending the operators to a result list outside the recursive closure. But you are also appending the result of each recursive call to the same list. Either append to the list or store the results from each recursive call, but not both.
Here's a simpler alternative:
def listOperators(device) {
def results = []
if (device) {
results += device.operators*.id
device.children?.each { child ->
results += listOperators(child)
}
}
results.unique()
}
Can I retrieve a Method via reflection, somehow combine it with a target object, and return it as something that looks like a function in Scala (i.e. you can call it using parenthesis)? The argument list is variable. It doesn't have to be a "first-class" function (I've updated the question), just a syntactic-looking function call, e.g. f(args).
My attempt so far looks something like this (which technically is pseudo-code, just to avoid cluttering up the post with additional definitions):
class method_ref(o: AnyRef, m: java.lang.reflect.Method) {
def apply(args: Any*): some_return_type = {
var oa: Array[Object] = args.toArray.map { _.asInstanceOf[Object] }
println("calling: " + m.toString + " with: " + oa.length)
m.invoke(o, oa: _*) match {
case x: some_return_type => x;
case u => throw new Exception("unknown result" + u);
}
}
}
With the above I was able to get past the compiler errors, but now I have a run-time exception:
Caused by: java.lang.IllegalArgumentException: argument type mismatch
The example usage is something like:
var f = ... some expression returning method_ref ...;
...
var y = f(x) // looks like a function, doesn't it?
UPDATE
Changing the args:Any* to args:AnyRef* actually fixed my run-time problem, so the above approach (with the fix) works fine for what I was trying to accomplish. I think I ran into a more general issue with varargs here.
Sure. Here's some code I wrote that add an interface to a function. It's not exactly what you want, but I think it can be adapted with few changes. The most difficult change is on invoke, where you'll need to change the invoked method by the one obtained through reflection. Also, you'll have to take care that the received method you are processing is apply. Also, instead of f, you'd use the target object. It should probably look something like this:
def invoke(proxy: AnyRef, method: Method, args: Array[AnyRef]) = method match {
case m if /* m is apply */ => target.getClass().getMethod("name", /* parameter type */).invoke(target, args: _*)
case _ => /* ??? */
}
Anyway, here's the code:
import java.lang.reflect.{Proxy, InvocationHandler, Method}
class Handler[T, R](f: Function1[T, R])(implicit fm: Manifest[Function1[T, R]]) extends InvocationHandler {
def invoke(proxy: AnyRef, method: Method, args: Array[AnyRef]) = method.invoke(f, args: _*)
def withInterface[I](implicit m: Manifest[I]) = {
require(m <:< manifest[Function1[T, R]] && m.erasure.isInterface)
Proxy.newProxyInstance(m.erasure.getClassLoader(), Array(m.erasure), this).asInstanceOf[I]
}
}
object Handler {
def apply[T, R](f: Function1[T, R])(implicit fm: Manifest[Function1[T, R]]) = new Handler(f)
}
And use it like this:
trait CostFunction extends Function1[String, Int]
Handler { x: String => x.length } withInterface manifest[CostFunction]
The use of "manifest" there helps with syntax. You could write it like this:
Handler({ x: String => x.length }).withInterface[CostFunction] // or
Handler((_: String).length).withInterface[CostFunction]
One could also drop the manifest and use classOf instead with a few changes.
If you're not looking for a generic invoke that takes the method name--but rather, you want to capture a particular method on a particular object--and you don't want to get too deeply into manifests and such, I think the following is a decent solution:
class MethodFunc[T <: AnyRef](o: Object, m: reflect.Method, tc: Class[T]) {
def apply(oa: Any*): T = {
val result = m.invoke(o, oa.map(_.asInstanceOf[AnyRef]): _*)
if (result.getClass == tc) result.asInstanceOf[T]
else throw new IllegalArgumentException("Unexpected result " + result)
}
}
Let's see it in action:
val s = "Hi there, friend"
val m = s.getClass.getMethods.find(m => {
m.getName == "substring" && m.getParameterTypes.length == 2
}).get
val mf = new MethodFunc(s,m,classOf[String])
scala> mf(3,8)
res10: String = there
The tricky part is getting the correct type for the return value. Here it's left up to you to supply it. For example,if you supply classOf[CharSequence] it will fail because it's not the right class. (Manifests are better for this, but you did ask for simple...though I think "simple to use" is generally better than "simple to code the functionality".)