I am struggling to understand a concept in kotlin.
Looking at this
val lambda = { greeting: String, name: String ->
if(greeting.length < 3) return // error: return not allowed here
println("$greeting $name")
}
This would result in an error. Rewriting the lambda expression to this
val lambda = greet# { greeting: String, name: String ->
if(greeting.length < 3) return#greet
println("$greeting $name")
}
It works. And it works because "... return from our lambda to the outer, calling function..."
What I am struggling to understand is, what the outer calling function is. There is no other function, calling lambda. Whats going on under the hood here?
You're right that there is no other function calling lambda right now. What it means is that, when you do decide to call lambda, it will return to the function where you are doing that.
For example, if you have:
fun someFunction() {
lambda()
// some other code...
}
Then that return#greet will return to someFunction. someFunction is the "outer, calling function".
Just saying that might not be very meaningful, until you compare this to return without a label. Let's make up a situation where return and return#label are both allowed:
fun main() {
aMoreMeaningfulExample foo#{
if (it == 3) return#foo
println(it)
}
}
inline fun aMoreMeaningfulExample(lambda: (Int) -> Unit) {
for (x in 1..5) {
lambda(x)
}
println("Done!")
}
Here, return#foo returns to the "outer, calling function" of the lambda, which is aMoreMeaningfulExample. This means that println(it) will be skipped when it is 3. After returning to aMoreMeaningfulExample, the for loop there will still continue as normal, so you will see the rest of the numbers, and "Done!" being printed.
On the other hand, if you used return, it would return to whatever called main, in other words, terminating the program. You will only see "1" and "2" being printed.
Related
this custom function call's a lambda block when null.
I expected the function definition below to enforce the same return type. : T
Is their a way to enforce that be block returns type T ?
inline fun <T> T?.whenNull(block: () -> T): T {
if (this == null) {
return block() //accepts any return type
} else {
return this
}
}
fun main() {
val x : Int? = null
println(x ?: 42)
println(x.whenNull { 42 })
println(x.whenNull { "why is kotlin not enforcing return of the same type?" })
}
T in the second whenAll call is being inferred as Any. Imagine that all occurrences of T are replaced Any, the call would be valid, wouldn't you agree? Int and String are both subtypes of Any, after all.
inline fun Any?.whenNull(block: () -> Any): Any {
if (this == null) {
return block()
} else {
return this
}
}
fun main() {
println(x.whenNull { "why is kotlin not enforcing return of the same type?" })
}
Basically, the Kotlin compiler is "trying too hard" here to make your code compile, and infers an unexpected type for your type parameter.
There exists an internal annotation #kotlin.internal.InputTypesOnly that would prevent your code from compiling if the type inferred is not mentioned in one of the input types (parameter types, receiver type, etc) of the function.
In this case, the input type is just Int?, and T is inferred to be Any, so it would be make your code not compile as expected. Unfortunately though, this annotation is internal, and you cannot use it :( KT-13198 is the ticket about making it public.
Interestingly, when passing this to the block the type is preserved and works as expected.
inline fun <T> T?.whenNullAlt(block: (T?) -> T): T {
if (this == null) {
return block(this) // "this" is superfluous, but enforces same return type
} else {
return this
}
}
fun main() {
val x : Int? = null
println(x.whenNullAlt { 42 })
println(x.whenNullAlt { "does not compile" })
}
As other answers pointed out, this is technically correct from Kotlin viewpoint because it can infer T to Any and it would compile.
However, while technically correct, this is a known problem in Kotlin language and is going to be fixed some day. That noted, let's try to understand why your answers's code works while your questions' doesn't.
The reason for this is that lambdas are always inferred last: imagine it as being a queue on what parts of the expression need to have their types inferred and anything inside a lambda is always at the end of the queue, no matter where the lambda is in the expression. So, when going over your example, it infers everything else, decides that the type of this should be Int?, than goes to the lambda, sees a String return type and merges them into Any being very proud of itself.
In the other example, however, the lambda is passed an external fact about it's parameter — the already inferred Int from the receiver (lambda is always the last to get the info, remember?). That way the inference inside the lambda fails because the argument and the result type are in disagreement.
So I'm making regexes for collections (all quantifiers are possessive). It looks like this (keep in mind the example is overly simplified for readability's sake):
val mayBeAPerson: Boolean = "totally not a person"
.toList()
.matches { // this: PatternScope
one { it.isUpperCase() } // execution of lambda could end after this method
moreThan(0) { it.isLetter() }
one { it == ' ' }
lessThan(2) { // this: PatternScope
one { it.isUpperCase() }
one { it == '.' }
one { it == ' ' }
}
one { it.isUpperCase() }
moreThan(0) { it.isLetter() }
}
As you can see, execution of lambda passed to matches could end after first one, as the predicate passed to it doesn't match first character in List. And it indeed does end. However, my solution is the opposite of elegant, as it uses throwing an exception in one and catching it in matches.
fun List<Char>.matches(build: PatternScope.() -> Unit) = try {
val scope = PatternScope(iterator())
scope.build() // may throw MatchFailed
!scope.iterator.hasNext()
} catch (_: MatchFailed) {
false
}
class PatternScope(private val iterator: Iterator<Char>) {
inline fun one(predicate: (element: Char) -> Boolean) {
if (!iterator.hasNext() || !predicate(iterator.next())) {
throw MatchFailed("match failed")
}
}
.
. etc
.
}
It totally works, but I can't help but wonder: is there a better way? I do know throwing exceptions like this is just a fancy GOTO, and I could wrap all the methods of PatternScope in ifs, like this:
class PatternScope(private val iterator: Iterator<Char>) {
private var matchFailed = false
inline fun one(predicate: (element: Char) -> Boolean) {
if (!matchFailed) {
if (!iterator.hasNext() || !predicate(iterator.next())) {
matchFailed = true
}
}
}
inline fun moreThan(n: Int, predicate: (element: Char) -> Boolean) {
if (!matchFailed) {
// logic
}
}
.
. etc
.
}
Is it more elegant though? Now I'm invoking all the functions in lambda passed to matches, and I like it even less to be honest.
My real question is: is there even better way to do it? Some magic solution to return from lambda I don't even have real access to? Some non-local returns, but from functions lambda hasn't even see yet?
Can I return from lambda by invoking some function inside its body?
Edit
Just to clarify, let's say we have a lambda:
val lambda: () -> Unit = {
someMethod() // this should return from lambda (in some cases)
someOtherMethod() // this shouldn't be invoked
}
How should the body of someMethod look like, so that someOtherMethod does not even execute when the lambda is invoked? Is there any other way but making someMethod throw an exception and wrapping lambda in try-catch block like this:
try {
lambda() // throws
} catch (_: SomeThrowableIdk) { }
I don't see a better way, but please prove me wrong.
I assume you're actually using #PublishedApi since you have a private iterator and public inline functions that access it.
Since Kotlin doesn't have checked exceptions, it is against Kotlin convention to throw Exceptions for things that are not actually errors in the program (bugs). Your first approach feels a little hacky for this reason. Since your API has public inline functions, there's no way to totally encapsulate the exceptions. You could switch to non-inline functions and storing the steps in a collection to be run internally, but this is surely more runtime overhead than the inline functions or your second approach with if statements.
Your second approach is more like a typical builder, so I don't see the problem with it. Since your functions are inline, it's not like your compiled code has a bunch of unnecessary function calls. Just if statements. You could however add a helper function to clean up your code at all the sub-functions, though I'm not sure if this can extend to the complexity of your actual class:
class PatternScope(#PublishedApi internal val iterator: Iterator<Char>) {
#PublishedApi internal var matchFailed = false
#PublishedApi internal inline fun nextRequire(require: () -> Boolean) {
matchFailed = matchFailed || !require()
}
inline fun one(predicate: (element: Char) -> Boolean) = nextRequire {
iterator.hasNext() && predicate(iterator.next())
}
}
There's no way to do what you described in your edit. Non-local returns work only with lambdas. To support something like what you describe, Kotlin would need syntax for a special kind of function that has the ability to return from the function that calls it. A function like this would have to have a new kind of signature that also declares the return type of the type of function that is permitted to call it. There simply is no such syntax or function type like that in Kotlin.
I have a function that catches recoverable exceptions and returns a fallback
private fun <T> safely(block: () -> T, fallback: T): T {
return try {
block()
} catch(exc: SomeException) {
// Log the exception/do some logic
fallback
}
}
I want to be able to add this to the public methods of my class e.g.
fun doSomething(): List<String> = safely({
val list = mutableListOf<String>("Hello")
fun someCheck1() = false // Some boolean value
fun someCheck2() = true // Some boolean value
do {
if(someCheck2()) {
return arrayListOf<String>("Hello", "World")
}
} while (someCheck1())
return list
}, arrayListOf<String>())
However I get compiler errors 'return' is not allowed here
Yet if I remove the return then my return in the loop no longer works and it gets highlighted in my IDE with warning the expression is never used
How can I maintain this type of return logic within a Lambda?
Playground Example
Try
fun doSomething(): List<String> = safely(
{
val list = mutableListOf<String>("Hello")
fun someCheck1() = false // Some boolean value
fun someCheck2() = true // Some boolean value
do {
if (someCheck2()) {
return#safely arrayListOf<String>("Hello", "World")
}
} while (someCheck1())
list
}
, arrayListOf<String>())
For further reference, check Using return inside a lambda?
Or you can also extract your block into a separate function (i.e. someCheckFunction(): List<String>), and have fun doSomething() = safely({ someCheckFunction() }, arrayListOf()), but I guess you want to maintain lambda code like above.
return arrayListOf<String>("Hello", "World") here tries to return a value from doSomething function rather than from the lambda passed to safely. However, such return is non-local, since it tries to exit from the function that is not on the top of stack, and therefore it is prohibited.
Another option here is to make safely function inline:
inline fun <T> safely(block: () -> T, fallback: T): T { ... }
and then you'll be able to make a non-local return from block lambda function passed to it.
The code in question is as:
fun get(context: Context, s: String): MyObjectDb? {
return context.database.use {
return#use select(MyObjectDb.TABLE_NAME, *MyObjectDb.PROJECTION)
.whereArgs("${MyObjectDb.COLUMN_S} = {s}", "s" to s)
.exec {
return#exec getOne(MyObjectDb::fromCursor)
}
}
}
When I check this for code style (sonar with Kotlin plugin that uses detekt) I get a warning that I should "Restrict the number of return statements in methods."
Is there a way to only return in return#exec or to write the code in more Kotlinized way - without so many returns.
You can omit return when a lambda only contain a single expression. Since your function also only contains a single expression, you can write the function body after a = to omit the return here too. You can therefore shorten your code to this:
fun get(context: Context, s: String): MyObjectDb? = context.database.use {
select(MyObjectDb.TABLE_NAME, *MyObjectDb.PROJECTION)
.whereArgs("${MyObjectDb.COLUMN_S} = {s}", "s" to s)
.exec { getOne(MyObjectDb::fromCursor) }
}
I am using io.vertx.reactivex.kafka.client.producer.KafkaProducer client. The client has a
rxWrite function which returns Single<RecordMetadata>. However I need to log error if any, during write operation. It apparently is not getting executed.
I have written following working example.
test(): Function to test the chaining and logging
fun test(): Single<Int> {
val data = Single.just(ArrayList<String>().apply {
add("Hello")
add("World")
})
data.flattenAsObservable<String> { list -> list }
.flatMap { advertiser ->
//does not work with writeKafka
writeError(advertiser).toObservable().doOnError({ println("Error $data") })
}
.subscribe({ record -> println(record) }, { e -> println("Error2 $e") })
return data.map { it.size }
}
writeKafka: Writes the given given string into Kafka and returns Single
fun writeKafka(param: String): Single<RecordMetadata> {
//null topic to produce IllegalArgumentException()
val record = KafkaProducerRecord.create(null, UUID.randomUUID().toString(), param)
return kafkaProducer.rxWrite(record)
}
writeError: Always return a single with error of same type
fun writeError(param: String): Single<RecordMetadata> {
return Single.error<RecordMetadata>(IllegalArgumentException())
}
So when I call writeKafka It only prints Error2 but if I use writeError it prints both Error and Error2. Looks like the single returned by writeKafka is still waiting for result, but then why even Error2 is printed?
I am pretty newbie in RxJava2, could somebody point out any error in that?
It is important to read and post the stacktrace of errors so that the problem can be isolated.
In this case, looks like you get the IllegalArgumentException from create and you don't get any Single because the relevant Kafka class throws it. return kafkaProducer.rxWrite(record) never executes at all and you practically crash the flatMap. doOnError never gets into play hence only the "Error2" is printed.