I see Code B frequently, and I see Code A seldom.
Is Code A correct?
Is block: (canvas: Canvas) -> Unit equivalent to block: Canvas.() -> Unit in Kotlin?
Code A
fun DrawScope.drawNormal(block: (canvas: Canvas) -> Unit) {
drawIntoCanvas {
block(it)
}
}
Code B
fun DrawScope.drawNormal(block: Canvas.() -> Unit) {
drawIntoCanvas {
block(it)
}
}
Code A declare a function who takes a Canvas as parameters.
You'll be able to use Canvas using it or by naming it.
drawNormal { canvas ->
// do whatever you want with canvas
}
In code B you are using (like #gidds said) a function literal with receiver. You provide a scope where the anonymous function will be running. So in the anonymous function, the this will be the canvas.
drawNormal {
// no parameters, you can simply use all canvas functions in this block
}
Both approach are good but it depends on what you are trying to achieve.
It's like scoped functions with let and apply
The code A will be better if you just want to get canvas attributes like width and length to build another things or do some actions with it.
The code B will be better if you want to modify the canvas reference (like modifying width and length)
They are not equivalent from the point of view of the caller of your function.
In Code A, block is a function that takes the Canvas as an argument. This means that the caller of drawNormal will access it as it or via a named parameter in the lambda:
drawNormal {
it.rotate(90f)
}
drawNormal { canvas ->
canvas.rotate(90f)
}
In Code B, block is a function that takes a Canvas as a receiver. This means that the caller will access it as this or call functions from Canvas directly:
drawNormal {
this.rotate(90f)
rotate(90f)
}
Both are equally correct. It depends on the experience you want to provide to your caller.
Related
I'm writing a Kotlin inline class to make Decimal4J more convenient without instantiating any objects. I'm worried that scope functions might create lambda objects, thereby making the whole thing pointless.
Consider the function compareTo in the following example.
/* imports and whatnot */
#JvmInline
value class Quantity(val basis: Long) {
companion object {
val scale: Int = 12
val metrics: ScaleMetrics = Scales.getScaleMetrics(scale)
val arithmetic: DecimalArithmetic = metrics.defaultArithmetic
}
operator fun compareTo(alt: Number): Int {
with(arithmetic) {
val normal = when (alt) {
is Double -> fromDouble(alt)
is Float -> fromFloat(alt)
is Long -> fromLong(alt)
is BigDecimal -> fromBigDecimal(alt)
is BigInteger -> fromBigInteger(alt)
else -> fromLong(alt.toLong())
}
return compare(basis, normal)
}
}
}
Does the with(arithmetic) scope create a lambda in the heap? The docs on kotlinlang.org consistently refer to the scoped code as a lambda expression. Is there any way to use scope functions without creating objects?
All of the built-in scoping functions, including with, are marked inline, which means the implementation gets planted directly in the code that's calling it. Once that happens, the lambda call can be optimized away.
To be more concrete, here's the implementation of with (with the Kotlin contracts stuff removed, since that's not relevant here)
public inline fun <T, R> with(receiver: T, block: T.() -> R): R {
return receiver.block()
}
Extension methods are, and always have been, syntax sugar resolved at compile time, so this is effectively
public inline fun <T, R> with(receiver: T, block: (T) -> R): R {
return block(receiver) // (with `this` renamed by the compiler)
}
So when we call
operator fun compareTo(alt: Number): Int {
with (arithmetic) {
println("Hi :)")
println(foobar()) // Assuming foobar is a method on arithmetic
}
}
The inline will transform this into
operator fun compareTo(alt: Number): Int {
({
println("Hi :)")
println(it.foobar()) // Assuming foobar is a method on arithmetic
})(arithmetic)
}
And any optimizer worth its salt can see that this is a function that's immediately evaluated, so we should go ahead and do that now. What we end up with is
operator fun compareTo(alt: Number): Int {
println("Hi :)")
println(arithmetic.foobar()) // Assuming foobar is a method on arithmetic
}
which is what you would have written to begin with.
So, tl;dr, the compiler is smart enough to figure it out. You don't have to worry about it. It's one of the perks of working in a high-level language.
By the way, this isn't just abstract. I just compiled the above code on my own machine and then decompiled the JVM bytecode to see what it really did. It was quite a bit noisier (since the JVM, by necessity, has a lot of noise), but there was no lambda object allocated, and the function was just one straight shot that calls println twice.
In case you're interested, Kotlin takes this example function
fun compareTo(alt: Number): Unit {
return with(arithmetic) {
println("Hi :)")
println(foobar())
}
}
to this Java, after being decompiled,
public static final void compareTo-impl(long arg0, #NotNull Number alt) {
Intrinsics.checkNotNullParameter((Object)alt, (String)"alt");
long l = arithmetic;
boolean bl = false;
boolean bl2 = false;
long $this$compareTo_impl_u24lambda_u2d0 = l;
boolean bl3 = false;
String string = "Hi :)";
boolean bl4 = false;
System.out.println((Object)string);
int n = so_quant.foobar-impl($this$compareTo_impl_u24lambda_u2d0);
bl4 = false;
System.out.println(n);
}
Quite a bit noisier, but the idea is exactly the same. And all of those pointless local variables will be taken care of by a good JIT engine.
Just some additional info to help clear up the terminology that led to your confusion.
The word “lambda” is defined as a syntax for writing a function. The word does not describe a function itself, so the word lambda has nothing to do with whether a function object is being allocated or not.
In Kotlin, there are multiple different syntaxes you can choose from to define or refer to a function. Lambda is only one of these.
// lambda assigned to variable
val x: (String) -> Unit = {
println(it)
}
// anonymous function assigned to variable
val y: (String) -> Unit = fun(input: String) {
println(input)
}
// reference to existing named function assigned to variable
val z: (String) -> Unit = ::println
// lambda passed to higher order function
“Hello World”.let { println(it) }
// anonymous function passed to higher order function
“Hello World”.let(fun(input: Any) { println(input) })
// reference to existing named function passed to higher order function
“Hello World”.let(::println)
// existing functional reference passed to higher order function
“Hello World”.let(x)
There is actually no such thing as a lambda object that can be passed around. The object is a function that could have been defined using any of the above syntaxes. Once a functional reference exists, the syntax that was used to create it is irrelevant.
With inline higher order functions, as the standard library scope functions are, the compiler optimizes away the creation of the functional object altogether. Of the four higher order calls in my example above, the first three will compile to the same thing. The last is a bit different because the function x already exists so it will be x itself that is invoked in the inlined code. Its contents don’t get hoisted out and called directly in the inlined code.
The advantage of using lambda syntax for higher order inline function calls is that it enables you to use keywords for the outer scope (non-local returns), such as return, continue, or break.
For example:
private fun TextView.onEndDrawableClicked(onClicked: (view: TextView) -> Unit) {
this.setOnTouchListener { v, event ->
var hasConsumed = false
if (v is TextView) {
if (event.x >= v.width - v.totalPaddingRight) {
if (event.action == MotionEvent.ACTION_UP) {
onClicked(this)
}
hasConsumed = true
}
}
hasConsumed
}
}
In the example above we see extension function. I know what it is and can use use/create similar ones. But there's onClicked: (view: TextView) -> Unit in the example's parameters. What is this? Callback? I see this kind of parameters too often, but don't have any idea how to understand that. Does it reference to some lambda function? Can someone send me detailed manual/example of this kind of parameters/functions/whatever?
onClicked is a function that gets passed to onEndDrawableClicked. It's type is a functional type (view: TextView) -> Unit. That function, when called, expects one parameter view of type TextView and returns Unit.
So yes, it is a callback, passed as a lambda. It can be used like:
val textView: TextView = ...
textView.onEndDrawableClicked {
// code that should be executed when onClicked gets called.
}
Functions are first class citiziens in Kotlin. Meaning you can store function in variables and pass them to other functions. Or let functions return functions.
When to use?
You can pass a function whenever you want to pass behavior (rather than state) to or from your functions. So the caller can decide what to do in those cases. That allows you to write highly flexible code / API's.
Alternatively you could create an interface with one ore many functions to be called and pass an instance of that interface to your function.
I'm trying to understand the following code (source).
class HTML {
fun body() { ... }
}
fun html(init: HTML.() -> Unit): HTML {
val html = HTML() // create the receiver object
html.init() // pass the receiver object to the lambda
return html
}
html { // lambda with receiver begins here
body() // calling a method on the receiver object
}
What I really cannot grasp is the line
html.init() // pass the receiver object to the lambda
What is happening here?
Can somebody please explain in simple words what is going on here?
First, let's make this example a little easier and see what the problem is then.
We could build the html function like this:
fun html(init: (HTML) -> Unit): HTML {
val html = HTML()
init(html)
return html
}
This would be easier to grasp (at first), because we are just passing a usual one-parameter lambda to the html function.
But now the call-site is not builder like:
html { it: HTML -> // just for clarity
it.body() // not very nice
}
Wouldn't it be nice if we could invoke body() inside html without it? That's possible! All we need is a lambda with receiver.
fun html(init: HTML.() -> Unit): HTML { // <-- only this changed
val html = HTML()
init(html)
return html
}
See how html is passed as an argument to init like before?
Of course, we can invoke it like this too: html.init() as shown in the example. The instance of HTML becomes this inside the block of the lambda.
Now, we can do this:
html {
this.body()
}
Since this can be omitted, we arrive here:
html {
body()
}
So, in the end lambdas with receivers make the code more concise and allow us to use a nice builder syntax.
Here is step by step explaination:
1. Creation of function, receiver type lambda.
fun html(init: HTML.() -> Unit): HTML {
here function html accept a parameter init of type HTML.() -> Unit i.e. it indicated that it is a receiver of HTML and can only be called with help of a real HTML object. And : HTML indicates that the function obviously returns HTML object.
2. call of init at html
html.init()
Here init() function is called as a receiver of HTML by a real HTML object.
Alright enough of formal talking, Here is what a receiver is:
So if you remember extension function defined as fun A.myFun(...): ReturnType {}, in that case you get a variable this which act as an instance of type A it was called on.
Similarly receiver lambda gives you a this variable inside that,
In a particular example:
class A {
fun thisCanBeCalledByAInstance() {
println("I've got called")
}
}
fun main() {
val receiver: A.() -> Unit = { // this: A
thisCanBeCalledByAInstance() // prints: I've got called
// or traditional way this.thisCanBeCalledByAInstance()
}
val a: A = A()
a.receiver()
}
Here you were able to call the method(function) from the instance of A even if it was lambda because it was a receiver.
PS: For simple langauge you can think html.init() as init(html) but html is not a parameter but instead works as this vaiable inside the lambda
This is why you were able to call body() on that lambda, because implicitly you were calling this.body() and this has came from html.init()'s html object.
When I look at sample code for the "use" function in Kotlin, I usually see something like this:
private fun readFirstLine(): String {
BufferedReader(FileReader("test.file")).use { return it.readLine() }
}
However, in the following example, I don't understand where "input" comes from, since input -> appears to be a lambda. From my understanding, everything inside of use { } must be an expression:
val streamIn = resources.openRawResource(rawResId)
val streamOut = FileOutputStream(destFilename)
streamIn.use { input ->
streamOut.use { output ->
input.copyTo(output)
}
}
"input" clearly refers to the same object that "streamIn" refers to, but I don't understand how Kotlin knows that.
everything inside of use { } must be an expression
If you looked at the signature, you'll see that use takes a (T) -> R function, so really, any function/lambda that accepts the closable thing as a parameter can be passed to it.
With that misconception cleared up, let's see what the code in question is doing.
streamIn.use { input ->
streamOut.use { output ->
input.copyTo(output)
}
}
First we see streamIn.use {, which means we are going to do something with streamIn and then close it. And from now on streamIn will be called input. Then there is streamOut.use {, which indicates that we are also going to use streamOut to do stuff, and then close it, and we are going to call it output from now on.
I don't understand where "input" comes from
It's basically giving another name to the it as in your first code snippet. Since we have nested lambdas here, we can't use it to refer to the parameters of both lambdas.
"input" clearly refers to the same object that "streamIn" refers to, but I don't understand how Kotlin knows that.
This is because in the implementation of use, there's probably a line like this:
return block(this)
block is the lambda parameter you pass to use, and this is the object on which use is called. Since input is the parameter of the lambda, it refers to this.
Now we have declared that we are going to use two resources, what are going to do with them? input.copyTo(output)! Whatever copyTo returns is going to be returned by streamOut.use, which in turn is going to be returned by streamIn.use. streamOut and streamIn will also be closed one after another.
So overall what have we done? We have basically used 2 resources at the same time and closed them afterwards. This is how you'd compose use to use multiple resources at the same time.
in the lambda, you can define a name for your object so in the following code the input is equivalent to it which is streamIn and output is equivalent to streamOut:
streamIn.use { input ->
streamOut.use { output ->
input.copyTo(output)
}
}
The reason that they define input and output is you cannot use it when you use a lambda block inside another lambda block.
use is an extension function which takes whatever calls it as a parameter.
Assume this example:
file.bufferedReader().use{
println(it.readText()) // it is actually that object that calls `use`
}
According to the API docs of Kotlin, this is the schema of use:
inline fun <T : AutoCloseable?, R> T.use(block: (T) -> R): R
The bufferedReader in my example is a closable class.
When you write somethingClosable.use { }, you are in fact passing a lambda function to it, like:
fun <T, R> function(t: T): R {
// use T and return an R
}
somethingClosable.use(function)
And inside use your function will be called.
More info on extension functions in Kotlin.
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)}