In one of the edifying Kotlin tutorials for lambda functions, the folowing code snippet was used:
max(s : String, { a, b -> b.length > a.length })
To learn how to do it, I coded it in two different ways. However, neither of them is working and the follwoing error is generated:
Annotation is required on each parameter
I tried to code it as a function.
fun max(s : String, { a, b -> b.length > a.length }) : String {
return ""
}
I have two questions:
How are the parameter a and b passed?
How do I write the above max function in Kotlin correctly
My guess is that you mean:
fun max(s: String, predicate: (a: String, b: String) -> Boolean) : String {
return "" // don’t know how to do this, what is the required function?
}
Related
I am trying to learn function as parameter. My example could be false but in purpose of learning I try to do code below.
I am getting
Type mismatch. Required: (Int, String) → String Found: () → String
What I try is to create "showNameAndAge" with two parameter(String and Int) required and pass it parameter in myMessage Func.
fun main() {
myMessage(message = "Hello",1){
showNameAndAge(2,"it")
}
}
val showNameAndAge:(Int,String)->String={a,b-> "hello $a and $b" }
fun myMessage(message:String,a:Int,funAsParameter2:(Int,String)->String){ //trailing lamda
println("$message + $a ${funAsParameter2(a,message)}")
}
Your code is almost correct, but you need to provide names for parameters received inside the lambda:
myMessage(message = "Hello",1){ p1, p2 ->
showNameAndAge(2,"it")
}
If you don't use these arguments inside lambda, you can use _ as their names:
myMessage(message = "Hello",1){ _, _ ->
This is the function declaration for rememberCoilPainter:
#Composable
fun rememberCoilPainter(
request: Any?,
imageLoader: ImageLoader = CoilPainterDefaults.defaultImageLoader(),
shouldRefetchOnSizeChange: ShouldRefetchOnSizeChange = ShouldRefetchOnSizeChange { _, _ -> false },
requestBuilder: (ImageRequest.Builder.(size: IntSize) -> ImageRequest.Builder)? = null,
fadeIn: Boolean = false,
fadeInDurationMs: Int = LoadPainterDefaults.FadeInTransitionDuration,
#DrawableRes previewPlaceholder: Int = 0,
): LoadPainter<Any> {
}
The line of code I am having difficulty understanding is:
requestBuilder: (ImageRequest.Builder.(size: IntSize) -> ImageRequest.Builder)? = null
A dot appears after Builder followed by (size: IntSize)
This is the first time I've seen this construct in Kotlin and am not sure how to interpret it. This is a lambda. Normally the dot after an object refers to a sub component of a class or a package. But the ( ) after the dot isn't clear.
How do I implement the requestBuilder parameter?
This is a function with receiver type as described here: https://kotlinlang.org/docs/lambdas.html#function-types
Function types can optionally have an additional receiver type, which is specified before a dot in the notation: the type A.(B) -> C represents functions that can be called on a receiver object of A with a parameter of B and return a value of C. Function literals with receiver are often used along with these types.
It could be tricky to understand at first, but this is like you are providing a function/lambda that is a method of ImageRequest.Builder. Or in other words: your lambda receives one additional parameter of type ImageRequest.Builder and it is available in the lambda as this.
You can provide requestBuilder as any other lambda, but note that inside it you will have access to properties and methods of ImageRequest.Builder object that was provided to you.
What you are looking at is a "function literal with receiver". Speaking generically, a type A.(B) -> C represents a function that can be called on a receiver object of A with a parameter of B and return a value of C. Or in your example:
requestBuilder: (ImageRequest.Builder.(size: IntSize) -> ImageRequest.Builder)?
We have a function requestBuilder which can be called on a ImageRequest.Builder with a parameter size: IntSize and returns another ImageRequest.Builder.
Calling this function is just like calling any other function with a lambda as a parameter. The difference: You have access to ImageRequest.Builder as this inside your lambda block.
Hope the following example helps understand lambdas with receiver type:
data class Person(val name: String)
fun getPrefixSafely(
prefixLength: Int,
person: Person?,
getPrefix: Person.(Int) -> String): String
{
if (person?.name?.length ?: 0 < prefixLength) return ""
return person?.getPrefix(prefixLength).orEmpty()
}
// Here is how getPrefixSafely can be called
getPrefixSafely(
prefixLength = 2,
person = Person("name"),
getPrefix = { x -> this.name.take(x) }
)
How do I implement the requestBuilder parameter?
Hope this part of the code snippet answers the above:
getPrefix = { x -> this.name.take(x) }
PS: These lambdas with receiver types are similar to extension functions IMO.
I am playing with kotlin language and I tried the following code:
data class D( val n: Int, val s: String )
val lst = listOf( D(1,"one"), D(2, "two" ) )
val res = lst.reduce { acc:String, d:D -> acc + ", " + d.toString() }
The last statement causes the following errors:
Expected parameter of type String
Expected parameter of type String
Type mismatch: inferred type is D but String was expected
while this version of the last statement works:
val res = lst.map { e -> e.toString() }.reduce { acc, el -> acc + ", " + el }
I do not understand why the first version does not work. The formal definition of the reduce function, found here, is the following:
inline fun <S, T : S> Iterable<T>.reduce(
operation: (acc: S, T) -> S
): S
But this seems in contrast with the following sentence, on the same page:
Accumulates value starting with the first element and applying
operation from left to right to current accumulator value and each
element.
That is, as explained here:
The difference between the two functions is that fold() takes an
initial value and uses it as the accumulated value on the first step,
whereas the first step of reduce() uses the first and the second
elements as operation arguments on the first step.
But, to be able to apply the operation on first and second element, and so on, it seems to me tha the operation shall have both arguments of the base type of the Iterable.
So, what am I missing ?
Reduce is not the right tool here. The best function in this case is joinToString:
listOf(D(1, "one"), D(2, "two"))
.joinToString(", ")
.let { println(it) }
This prints:
D(n=1, s=one), D(n=2, s=two)
reduce is not designed for converting types, it's designed for reducing a collection of elements to a single element of the same type. You don't want to reduce to a single D, you want a string. You could try implementing it with fold, which is like reduce but takes an initial element you want to fold into:
listOf(D(1, "one"), D(2, "two"))
.fold("") { acc, d -> "$acc, $d" }
.let { println(it) }
However, this will add an extra comma:
, D(n=1, s=one), D(n=2, s=two)
Which is exactly why joinToString exists.
You can see the definition to understand why its not working
To make it work, you can simply create an extension function:
fun List<D>.reduce(operation: (acc: String, D) -> String): String {
if (isEmpty())
throw UnsupportedOperationException("Empty list can't be reduced.")
var accumulator = this[0].toString()
for (index in 1..lastIndex) {
accumulator = operation(accumulator, this[index])
}
return accumulator
}
you can use it as:
val res = lst.reduce { acc:String, d:D -> acc + ", " + d.toString() }
or simply:
val res = lst.reduce { acc, d -> "$acc, $d" }
You can modify the function to be more generic if you want to.
TL;DR
Your code acc:String is already a false statement inside this line:
val res = lst.reduce { acc:String, d:D -> acc + ", " + d.toString() }
Because acc can only be D, never a String! Reduce returns the same type as the Iterable it is performed on and lst is Iterable<D>.
Explanation
You already looked up the definition of reduce
inline fun <S, T : S> Iterable<T>.reduce(
operation: (acc: S, T) -> S
): S
so lets try to put your code inside:
lst is of type List<D>
since List extends Iterable, we can write lst : Iterable<D>
reduce will look like this now:
inline fun <D, T : D> Iterable<T>.reduce(
operation: (acc: D, T) -> D //String is literally impossible here, because D is not a String
): S
and written out:
lst<D>.reduce { acc:D, d:D -> }
I have a function timeout(...) (extension function that returns this) which accepts an argument that is either String, Date or Long. What I am trying to do is to make it accept any lambda that also returns one of these three types.
Kotlin finds the below functions ambiguous and can't decide which one to call when I type, for example, timeout { "something" }.
#JvmName("timeoutString")
fun <CR: CachableResponse> CR.timeout(timeLambda: CR.()->String): CR = timeout(timeLambda())
#JvmName("timeoutLong")
fun <CR: CachableResponse> CR.timeout(timeLambda: CR.()->Long): CR = timeout(timeLambda())
#JvmName("timeoutDate")
fun <CR: CachableResponse> CR.timeout(timeLambda: CR.()->Date): CR = timeout(timeLambda())
The error I'm getting is Cannot choose among the following candidates without completing type inference.
Of course one way to work around this, is to have one function instead of three like this:
fun <CR: CachableResponse, Type> CR.timeout(timeLambda: CR.()->Type): CR =
timeLambda().let { when (it) {
is String -> timeout(it)
is Date -> timeout(it)
is Long -> timeout(it)
else -> this
} }
In this case, though, the developer won't have any clue what its lambda will have to return without reading the description or checking the source code.
Is there any more elegant solution?
Actually, you solution is rather elegant.
I would only suggest to inline CR generic parameter and capture when subject in a variable:
fun <Type> CachableResponse.timeout(timeLambda: CachableResponse.() -> Type) =
when (val it = timeLambda()) {
is String -> timeout(it)
is Date -> timeout(it)
is Long -> timeout(it)
else -> this
}
In this case, though, the developer won't have any clue what its lambda will have to return without reading the description or checking the source code.
IDE comes to the rescue:
I'm trying to have this compiling:
val criteriaList = aList.stream().map { dateRange -> {
Criteria.where("KEY").`is`(dateRange) } }.toList().toTypedArray()
Criteria().orOperator(*criteriaList)
But:
Criteria().orOperator(*criteriaList)
Currently does not compile:
Type mismatch.
Required:
Array<(out) Criteria!>!
Found:
Array<(() → Criteria!)!>
Why?
You are mapping your dateRange to a () -> Criteria.
You do not need to wrap what is following after -> with curly braces. Check also the Kotlin reference regarding Lambda expression syntax:
val sum = { x: Int, y: Int -> x + y }
A lambda expression is always surrounded by curly braces [...], the body goes after an -> sign. If the inferred return type of the lambda is not
Unit, the last (or possibly single) expression inside the lambda body is treated as the return value.
So you could just write the following instead:
.map { dateRange -> Criteria.where("KEY").`is`(dateRange) }
Note also that you do not really need to call stream(), but you can directly call map on it (except it wouldn't be a real List in the first place).
So your code could probably be simplified to something like:
val criteriaList = aList.map { dateRange -> Criteria.where("KEY").`is`(dateRange) }
.toTypedArray()
or
val criteriaList = aList.map { Criteria.where("KEY").`is`(it) }
.toTypedArray()