I've been using kotlin arrow quite a bit recently, and I've ran into a specific use case that has me stuck.
Let's say I have a collection of some object that I want to convert to another datatype using a convert function. Let's also say that this convert function has an ability to fail-- but instead of throwing an exception, it will just return an Either, where Either.Left() is a failure and Either.Right() is the mapped object. What is the best way to handle this use case? Some sample code below:
val list: Collection<Object> // some collection
val eithers: List<Either<ConvertError, NewObject>> = list.map { convert(it) } // through some logic, convert each object in the collection
val desired: Either<ConvertError, Collection<NewObject>> = eithers.map { ??? }
fun convert(o: Object) : Either<ConvertError, NewObject> { ... }
Essentially, I'd like to call a mapping function on a collection of data, and if any of the mappings respond with a failure, I'd like to have an Either.Left() containing the error. And then otherwise, I'd like the Either.Right() to contain all of the mapped objects.
Any ideas for a clean way to do this? Ideally, I'd like to make a chain of function calls, but have the ability to percolate an error up through the function calls.
You can use Arrow's computation blocks to unwrap Either inside map like so:
import arrow.core.Either
import arrow.core.computations.either
val list: ListObject> // some collection
val eithers: List<Either<ConvertError, NewObject>> = list.map { convert(it) } // through some logic, convert each object in the collection
val desired: Either<ConvertError, Collection<NewObject>> = either.eager {
eithers.map { convert(it).bind() }
}
fun convert(o: Object) : Either<ConvertError, NewObject> { ... }
Here bind() will either unwrap Either into NewObject in the case Either is Right, or it will exit the either.eager block in case it finds Left with ConvertError. Here we're using the eager { } variant since we're assigning it to a val immediately. The main suspend fun either { } block supports suspend functions inside but is itself also a suspend function.
This is an alternative to the traverse operator.
The traverse operation will be simplified in Arrow 0.12.0 to the following:
import arrow.core.traverseEither
eithers.traverseEither(::convert)
The traverse operator is also available in Arrow Fx Coroutines with support for traversing in parallel, and some powerful derivatives of this operation.
import arrow.fx.coroutines.parTraverseEither
eithers.parTraverseEither(Dispatcheres.IO, ::convert)
This is a frequent one, what you're looking for is called traverse. It's like map, except it collects the results following the aggregation rules of the content.
So, list.k().traverse(Either.applicative()) { convert(it) } will return Either.Left is any of the operations return Left, and Right<List< otherwise.
How about arrow.core.IterableKt#sequenceEither?
val desired: Either<ConvertError, Collection<NewObject>> = eithers.sequenceEither()
Related
I am using a functional programming style to solve the Leetcode easy question, Count the Number of Consistent Strings. The premise of this question is simple: count the amount of values for which the predicate of "all values are in another set" holds.
I was able to do this pretty concisely like so:
class Solution {
fun countConsistentStrings(allowed: String, words: Array<String>): Int {
val permitted = allowed.toSet()
return words.count{it.all{it in permitted}}
}
}
I know that Java streams are lazy, but have read Kotlin is only lazy when asSequence is used and are otherwise eager.
For reductions to a boolean based on a predicate using any, none, or all, it makes the most sense to me that this should be done lazily (e.g. a single false in all should evaluate the whole expression to false and stop evaluating the predicate for other elements).
Are these operations implemented this way, or are they still done eagerly like other operations in Kotlin. If so, there a way to do them lazily?
No, those methods are not lazy.
First, bear in mind that there are multiple methods with each of those names: two defined on Sequence, two defined on each of thirteen types of array, two on Map, and one on Iterable. It's clear you're interested only in those defined on Sequence, as those other types don't support laziness.
So, let's look at the docs! The docs for Sequence.any(), for Sequence.none(), and for Sequence.all() methods all say:
The operation is terminal.
To confirm what this means, the docs for the kotlin.sequences package, say:
If the sequence operation returns another sequence, which is produced lazily, it's called intermediate, and otherwise the operation is terminal.
So those methods are not lazy; when executed, they cause the sequence to be evaluated as far as is needed to produce the required value. (However, they don't evaluate it any further than is needed, which may be what you're asking. After all, that's the point of using Sequences!)
(In fact, you can see from their types that there's no way they could be lazy: each of them returns a Boolean value, which is either true or false. To support lazy evaluation, they'd need to return a Future or similar object with a getter that could be called to produce a final result. But a Boolean already is that final result.)
I think you overinterpret what lazily and eagerly mean. Like "eagerly" means to always do everything in the most inefficient way possible.
Lazy collections (Streams API, sequences) try to postpone calculating their contents until necessary. On the other hand regular collections perform operations immediately when requested. But that doesn't mean if we ask a regular collection for its first element, it will iterate over all of them for no reason.
As a matter of fact, these functions are implemented in almost exactly the same way for both iterables and sequences. The difference is in other transformations and operators. Below is an example for any():
public inline fun <T> Iterable<T>.any(predicate: (T) -> Boolean): Boolean {
if (this is Collection && isEmpty()) return false
for (element in this) if (predicate(element)) return true
return false
}
public inline fun <T> Sequence<T>.any(predicate: (T) -> Boolean): Boolean {
for (element in this) if (predicate(element)) return true
return false
}
The docs don't explicitly say, but this is easy enough to test.
class A : Iterable<String>, Iterator<String> {
public override fun iterator(): Iterator<String> {
return this
}
public override fun hasNext(): Boolean {
return true
}
public override fun next(): String {
return "test"
}
}
fun main(args: Array<String>) {
val a = A()
println(a.any { x -> x == "test" })
println(a.none { x -> x == "test" })
println(a.all { x -> x != "test" })
}
Here, A is a silly iterable class that just produces "test" forever and never runs out. Then we use any, none, and all to check whether it produces "test" or not. It's an infinite iterable, so if any of these three functions wanted to try to exhaust it, the program would hang forever. But you can run this yourself, and you'll see a true and two false's. The program terminates. So each of those three functions stopped when it found, respectively, a match, a non-match, and a non-match.
Try it online!
In short, Sequences in Kotlin are somewhat similar to Java Streams in that they have many lazy functions that simply return another Sequence.
However, all, any and none are immediate regardless of whether you call them on a Sequence or an Iterable. You can tell they are not lazy because they return a Boolean instead of another Sequence, and in their documentation, they are described as terminal, which means they evaluate and iterate the Sequence immediately.
I have simple three functions returning arrow-kt data types
fun validate(input): Validated<Error, Input> = ...
fun fetch(input): Option<Error, InputEntity> = ...
fun performAction(inputEntity): Either<Error, Output> = ...
And want to chain something like this (can use any available function instead of map)
validate(input)
.map{fetch(it)}
.map{performAction(it)}
Only solution I could come up with is to replace Validated and Option with Either and chain using flatMap. Is there any better functional way to make it work without updating the existing functions?
👋 What #pablisco described is correct, but you can keep it simpler by using some syntax extensions we provide to convert from one type to the other. Note that both options are correct, but Monad Transformers can be a bit convoluted and too powerful, and they're also prone to get removed from Arrow soon, once we finally figure out our delimited continuations style completely. But that is out of scope here. Here is how you could solve it by using the extensions I mentioned:
import arrow.core.*
import arrow.core.extensions.fx
sealed class Error {
object Error1 : Error()
object Error2 : Error()
}
data class InputEntity(val input: String)
data class Output(val input: InputEntity)
fun validate(input: String): Validated<Error, InputEntity> = InputEntity(input).valid()
fun fetch(input: String): Option<InputEntity> = InputEntity(input).some()
fun performAction(inputModel: InputEntity): Either<Error, Output> = Output(inputModel).right()
fun main() {
val input = "Some input"
Either.fx<Error, Output> {
val validatedInput = !validate(input).toEither()
val fetched = !fetch(validatedInput.input).toEither { Error.Error1 /* map errors here */ }
!performAction(fetched)
}
}
Hope it was useful 👍
What you are looking for is called a Monad Transformer. In Arrow, you may have seen them already, they end with a T at the end. Like OptionT or EitherT.
There are some good examples here for EitherT:
https://arrow-kt.io/docs/0.10/arrow/mtl/eithert/
And here for OptionT:
https://arrow-kt.io/docs/0.10/arrow/mtl/optiont/
The idea would be that to choose what your final value is going to be (let's say Either) and using an FX block you can then use EitherT to convert the other types to an Either.
I'd like to pass a function reference on a nullable object. To take an Android example, say I want to use Activity#onBackPressed from a fragment that is a child of that actvity.
If I wanted to invoke this function, I could easily do
activity?.onBackPressed()
However, say I wanted to pass that as a reference instead:
val onBackPressedRef = activity::onBackPressed
This gives the familiar null safe error of Only safe or non null assserted calls are allowed...
I can get the error to go away with the following, but using !! is obviously not ideal:
val onBackPressedRef = activity!!::onBackPressed
Attemping activity?::onBackPressed was my first instinct, but this also breaks with several errors, where the interpreter seems confused.
val onBackPressedRef = activity?.let { it::onBackPressed }
This last variation works, but it's a lot more ugly than just using ?::. I checked all the docs I could find, but I feel like I'm missing something. Any ideas?
You are right, there is no ?:: operator in Kotlin.
You have several alternatives:
1. let and run
Thus, you have to use a helper function. Instead of let(), you can also use run(), making the expression a tiny bit shorter:
val onBackPressedRef = activity?.let { it::onBackPressed }
val onBackPressedRef = activity?.run { ::onBackPressed }
But keep in mind that either way, the invocation will be more verbose, too:
onBackPressedRef?.invoke(args)
Thus you should ask yourself, if this is really what you want, or if a no-op function call is also acceptable.
2. Closures
You could use a closure -- this will change semantics however:
val onBackPressedRef = { activity?.onBackPressed() }
Here, onBackPressedRef is not nullable anymore, so you can call it using the () operator, and in case of null activity it will have no effect.
3. Helper function
If function references with nullable objects are something you encounter a lot, you can write your own little abstraction:
// Return type: () -> Unit
fun <T> funcRef(obj: T?, function: T.() -> Unit) = { obj?.function() }
This trades a different syntax for a non-null function variable:
// activity can be null
val onBackPressedRef = funcRef(activity, Activity::onBackPressed)
// Callable directly
onBackPressedRef()
The Code A is from https://github.com/mycwcgr/camera/blob/master/CameraXBasic/app/src/main/java/com/android/example/cameraxbasic/fragments/CameraFragment.kt
It's a little difficult to understand the also syntax for me, so I convert the Code A to the Code B.
I think that the Code B is the same as the Code A, right?
Code A
private fun bindCameraUseCases() {
val metrics = DisplayMetrics().also { viewFinder.display.getRealMetrics(it) }
...
}
Code B
private fun bindCameraUseCases() {
val metrics = DisplayMetrics()
viewFinder.display.getRealMetrics(metrics)
}
Yes, it is. What the also { } extension function does is perform actions defined in its block with the caller object as a parameter and return the caller.
val list = mutableListOf<Int>().also {
// the newly created empty `MutableList` is a parameter in this lambda
// and can be referred using the `it` identifier
it.add(1)
}
// is equivalent to
val list = mutableListOf<Int>()
list.add(1)
In this case, yes: your Code A and Code B do the same thing. Within the lambda, it refers to the object that also was called on (the newly-created DisplayMetrics instance); and that's also what's returned.
And to answer your implied question: yes, in this case using also probably doesn't have much benefit!
It's more useful in the middle of a complex expression or return value, e.g.:
private fun getMetrics()
= DisplayMetrics().also{ println("Created metrics: $it") }
instead of:
private fun getMetrics(): DisplayMetrics {
val metrics = DisplayMetrics()
println("Created metrics: $metrics")
return metrics
}
Here it avoids an explicit local value, references to it, and an explicit return; once you're used to the idiom, it's simpler to read as well — especially when it's used for something like logging that's not part of the main program logic.
Kotlin's scoping functions (also, apply, let, run, with) can be a big confusing, but this page explains them fairly well.
I wish to have a good example for each function run, let, apply, also, with
I have read this article but still lack of an example
All these functions are used for switching the scope of the current function / the variable. They are used to keep things that belong together in one place (mostly initializations).
Here are some examples:
run - returns anything you want and re-scopes the variable it's used on to this
val password: Password = PasswordGenerator().run {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
generate()
}
The password generator is now rescoped as this and we can therefore set seed, hash and hashRepetitions without using a variable.
generate() will return an instance of Password.
apply is similar, but it will return this:
val generator = PasswordGenerator().apply {
seed = "someString"
hash = {s -> someHash(s)}
hashRepetitions = 1000
}
val pasword = generator.generate()
That's particularly useful as a replacement for the Builder pattern, and if you want to re-use certain configurations.
let - mostly used to avoid null checks, but can also be used as a replacement for run. The difference is, that this will still be the same as before and you access the re-scoped variable using it:
val fruitBasket = ...
apple?.let {
println("adding a ${it.color} apple!")
fruitBasket.add(it)
}
The code above will add the apple to the basket only if it's not null. Also notice that it is now not optional anymore so you won't run into a NullPointerException here (aka. you don't need to use ?. to access its attributes)
also - use it when you want to use apply, but don't want to shadow this
class FruitBasket {
private var weight = 0
fun addFrom(appleTree: AppleTree) {
val apple = appleTree.pick().also { apple ->
this.weight += apple.weight
add(apple)
}
...
}
...
fun add(fruit: Fruit) = ...
}
Using apply here would shadow this, so that this.weight would refer to the apple, and not to the fruit basket.
Note: I shamelessly took the examples from my blog
There are a few more articles like here, and here that are worth to take a look.
I think it is down to when you need a shorter, more concise within a few lines, and to avoid branching or conditional statement checking (such as if not null, then do this).
I love this simple chart, so I linked it here. You can see it from this as written by Sebastiano Gottardo.
Please also look at the chart accompanying my explanation below.
Concept
I think it as a role playing way inside your code block when you call those functions + whether you want yourself back (to chain call functions, or set to result variable, etc).
Above is what I think.
Concept Example
Let's see examples for all of them here
1.) myComputer.apply { } means you want to act as a main actor (you want to think that you're computer), and you want yourself back (computer) so you can do
var crashedComputer = myComputer.apply {
// you're the computer, you yourself install the apps
// note: installFancyApps is one of methods of computer
installFancyApps()
}.crash()
Yup, you yourself just install the apps, crash yourself, and saved yourself as reference to allow others to see and do something with it.
2.) myComputer.also {} means you're completely sure you aren't computer, you're outsider that wants to do something with it, and also wants it computer as a returned result.
var crashedComputer = myComputer.also {
// now your grandpa does something with it
myGrandpa.installVirusOn(it)
}.crash()
3.) with(myComputer) { } means you're main actor (computer), and you don't want yourself as a result back.
with(myComputer) {
// you're the computer, you yourself install the apps
installFancyApps()
}
4.) myComputer.run { } means you're main actor (computer), and you don't want yourself as a result back.
myComputer.run {
// you're the computer, you yourself install the apps
installFancyApps()
}
but it's different from with { } in a very subtle sense that you can chain call run { } like the following
myComputer.run {
installFancyApps()
}.run {
// computer object isn't passed through here. So you cannot call installFancyApps() here again.
println("woop!")
}
This is due to run {} is extension function, but with { } is not. So you call run { } and this inside the code block will be reflected to the caller type of object. You can see this for an excellent explanation for the difference between run {} and with {}.
5.) myComputer.let { } means you're outsider that looks at the computer, and want to do something about it without any care for computer instance to be returned back to you again.
myComputer.let {
myGrandpa.installVirusOn(it)
}
The Way to Look At It
I tend to look at also and let as something which is external, outside. Whenever you say these two words, it's like you try to act up on something. let install virus on this computer, and also crash it. So this nails down the part of whether you're an actor or not.
For the result part, it's clearly there. also expresses that it's also another thing, so you still retain the availability of object itself. Thus it returns it as a result.
Everything else associates with this. Additionally run/with clearly doesn't interest in return object-self back. Now you can differentiate all of them.
I think sometimes when we step away from 100% programming/logic-based of examples, then we are in better position to conceptualize things. But that depends right :)
There are 6 different scoping functions:
T.run
T.let
T.apply
T.also
with
run
I prepared a visual note as the below to show the differences :
data class Citizen(var name: String, var age: Int, var residence: String)
Decision depends on your needs. The use cases of different functions overlap, so that you can choose the functions based on the specific conventions used in your project or team.
Although the scope functions are a way of making the code more concise, avoid overusing them: it can decrease your code readability and lead to errors. Avoid nesting scope functions and be careful when chaining them: it's easy to get confused about the current context object and the value of this or it.
Here is another diagram for deciding which one to use from https://medium.com/#elye.project/mastering-kotlin-standard-functions-run-with-let-also-and-apply-9cd334b0ef84
Some conventions are as the following :
Use also for additional actions that don't alter the object, such as logging or printing debug information.
val numbers = mutableListOf("one", "two", "three")
numbers
.also { println("The list elements before adding new one: $it") }
.add("four")
The common case for apply is the object configuration.
val adam = Person("Adam").apply {
age = 32
city = "London"
}
println(adam)
If you need shadowing, use run
fun test() {
var mood = "I am sad"
run {
val mood = "I am happy"
println(mood) // I am happy
}
println(mood) // I am sad
}
If you need to return receiver object itself, use apply or also
let, also, apply, takeIf, takeUnless are extension functions in Kotlin.
To understand these function you have to understand Extension functions and Lambda functions in Kotlin.
Extension Function:
By the use of extension function, we can create a function for a class without inheriting a class.
Kotlin, similar to C# and Gosu, provides the ability to extend a class
with new functionality without having to inherit from the class or use
any type of design pattern such as Decorator. This is done via special
declarations called extensions. Kotlin supports extension functions
and extension properties.
So, to find if only numbers in the String, you can create a method like below without inheriting String class.
fun String.isNumber(): Boolean = this.matches("[0-9]+".toRegex())
you can use the above extension function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber)
which is prints true.
Lambda Functions:
Lambda functions are just like Interface in Java. But in Kotlin, lambda functions can be passed as a parameter in functions.
Example:
fun String.isNumber(block: () -> Unit): Boolean {
return if (this.matches("[0-9]+".toRegex())) {
block()
true
} else false
}
You can see, the block is a lambda function and it is passed as a parameter. You can use the above function like this,
val phoneNumber = "8899665544"
println(phoneNumber.isNumber {
println("Block executed")
})
The above function will print like this,
Block executed
true
I hope, now you got an idea about Extension functions and Lambda functions. Now we can go to Extension functions one by one.
let
public inline fun <T, R> T.let(block: (T) -> R): R = block(this)
Two Types T and R used in the above function.
T.let
T could be any object like String class. so you can invoke this function with any objects.
block: (T) -> R
In parameter of let, you can see the above lambda function. Also, the invoking object is passed as a parameter of the function. So you can use the invoking class object inside the function. then it returns the R (another object).
Example:
val phoneNumber = "8899665544"
val numberAndCount: Pair<Int, Int> = phoneNumber.let { it.toInt() to it.count() }
In above example let takes String as a parameter of its lambda function and it returns Pair in return.
In the same way, other extension function works.
also
public inline fun <T> T.also(block: (T) -> Unit): T { block(this); return this }
extension function also takes the invoking class as a lambda function parameter and returns nothing.
Example:
val phoneNumber = "8899665544"
phoneNumber.also { number ->
println(number.contains("8"))
println(number.length)
}
apply
public inline fun <T> T.apply(block: T.() -> Unit): T { block(); return this }
Same as also but the same invoking object passed as the function so you can use the functions and other properties without calling it or parameter name.
Example:
val phoneNumber = "8899665544"
phoneNumber.apply {
println(contains("8"))
println(length)
}
You can see in the above example the functions of String class directly invoked inside the lambda funtion.
takeIf
public inline fun <T> T.takeIf(predicate: (T) -> Boolean): T? = if (predicate(this)) this else null
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeIf { it.matches("[0-9]+".toRegex()) }
In above example number will have a string of phoneNumber only it matches the regex. Otherwise, it will be null.
takeUnless
public inline fun <T> T.takeUnless(predicate: (T) -> Boolean): T? = if (!predicate(this)) this else null
It is the reverse of takeIf.
Example:
val phoneNumber = "8899665544"
val number = phoneNumber.takeUnless { it.matches("[0-9]+".toRegex()) }
number will have a string of phoneNumber only if not matches the regex. Otherwise, it will be null.
You can view similar answers which is usefull here difference between kotlin also, apply, let, use, takeIf and takeUnless in Kotlin
According to my experience, since such functions are inline syntactic sugar with no performance difference, you should always choose the one that requires writing the least amount of code in the lamda.
To do this, first determine whether you want the lambda to return its result (choose run/let) or the object itself (choose apply/also); then in most cases when the lambda is a single expression, choose the ones with the same block function type as that expression, because when it's a receiver expression, this can be omitted, when it's a parameter expression, it is shorter than this:
val a: Type = ...
fun Type.receiverFunction(...): ReturnType { ... }
a.run/*apply*/ { receiverFunction(...) } // shorter because "this" can be omitted
a.let/*also*/ { it.receiverFunction(...) } // longer
fun parameterFunction(parameter: Type, ...): ReturnType { ... }
a.run/*apply*/ { parameterFunction(this, ...) } // longer
a.let/*also*/ { parameterFunction(it, ...) } // shorter because "it" is shorter than "this"
However, when the lambda consists of a mix of them, it's up to you then to choose the one that fits better into the context or you feel more comfortable with.
Also, use the ones with parameter block function when deconstruction is needed:
val pair: Pair<TypeA, TypeB> = ...
pair.run/*apply*/ {
val (first, second) = this
...
} // longer
pair.let/*also*/ { (first, second) -> ... } // shorter
Here is a brief comparison among all these functions from JetBrains's official Kotlin course on Coursera Kotlin for Java Developers:
I must admit that the difference is not so obvious at first glance, among other things because these 5 functions are often interchangeable. Here is my understanding :
APPLY -> Initialize an object with theses properties and wait for the object
val paint = Paint().apply {
this.style = Paint.Style.FILL
this.color = Color.WHITE
}
LET -> Isolate a piece of code and wait for the result
val result = let {
val b = 3
val c = 2
b + c
}
or
val a = 1
val result = a.let {
val b = 3
val c = 2
it + b + c
}
or
val paint: Paint? = Paint()
paint?.let {
// here, paint is always NOT NULL
// paint is "Paint", not "Paint?"
}
ALSO -> Execute 2 operations at the same time and wait for the result
var a = 1
var b = 3
a = b.also { b = a }
WITH -> Do something with this variable/object and don't wait for a result (chaining NOT allowed )
with(canvas) {
this.draw(x)
this.draw(y)
}
RUN -> Do something with this variable/object and don't wait for a result (chaining allowed)
canvas.run {
this.draw(x)
this.draw(y)
}
or
canvas.run {this.draw(x)}.run {this.draw(x)}