I think an extension lambda requires that you pass in the correct argument, but i seems not to do so in the following example.
open class Base {
open fun f() = 1
}
class Derived : Base() {
override fun f() = 99
}
fun Base.g(): Int { return f()}
fun Base.h(xl: Base.() -> Int): Int { return xl()}
fun main() {
val b: Base = Derived() // Upcast
println(b.g())
println(b.h { f()}) // [1]
}
I understand that Base.h takes a function that takes a Base object as its parameter. But line [1] shows that it accepts f(), which is a function that takes no parameter. I was thinking hard about this and I prefixed it with this.f() and it still worked. Not convinced, I modified the code as follows:
open class Base {
open fun f() = 1
}
class Derived : Base() {
override fun f() = 99
}
fun Base.g(): Int { return f()}
fun Base.h(xl: (Base) -> Int): Int { return xl(Base())}
fun test(i:Int) = 1
fun main() {
val b: Base = Derived() // Upcast
println(b.g())
println(b.h { test(1) })
}
This code works. I've run it to verify. And as you can see, b.h() accepts test(), which takes an Int. And this is contrary to the fact that Base.h() takes a Base.
Could you explain this? Thank you for reading.
Note the curly brackets around the functions that are passed in! They change everything.
In the second code, b.h { test(1) } is not passing the function test to b.h. The syntax to pass test to b.h would be b.h(::test), and that does produce an error as you would expect.
b.h { test(1) } passes a function (a lambda expression) that takes a Base as parameter, ignores that parameter, calls test(1) and returns the result. You are basically passing a function that looks like this to b.h:
fun foo(p: Base) = test(1)
You might be wondering how Kotlin knows about Base when you did not write the word Base in the call at all. Well, it can just look at the declaration of b.h, and see that { test(1) } must take a parameter of Base.
The first code snippet is a bit different, because b.h accepts a Base.() -> Int in that case. Base.() -> Int represents a function whose receiver type is Base, that is, a function func that can be called like someBaseObject.func(). Compare this to a function func that takes a Base object as parameter, which can be called like func(someBaseObject).
Again, { f() } is not passing the function f. It is a lambda expression that does nothing but calls f. In this case though, f itself can be passed to b.h (b.h(Base::f)), because it is a function with a receiver type of Base! You can do someBaseObject.f(), can't you? Passing the lambda is similar to passing an extension function that is declared like this (you're just "wrapping" f in another function):
fun Base.foo() = f()
And since the receiver of the function is Base, you are able to access other functions that has Base as the receiver (such as f) in the lambda. You can also specify the receiver (which is this) explicitly.
There are two functions, say func1 and func2. func2 is an inner function of func1 and based on a condition I wish to return from func1, meaning end execution of func1. How do I do that in kotlin?
fun func1(){
fun func2(){
if(someCondition){
...
return#func1 //How do I do this? since it says return not allowed here
}
}
...
func2()
}
This wouldn't make sense in all cases, as a nested function might outlive the scope of the containing function in some cases. Here's an example of that:
var x: () -> Unit = {}
fun func1() {
fun func2() {}
x = ::func2
}
Here, it would not make sense to allow returns for func1 from func2. By the time x is called, there might not even be an active call to func1. This is basically the topic of non-local returns, which you need inline functions for (see the official documentation). Those, unfortunately, are not available as local functions (at least not yet).
For your specific case, you're probably stuck with signaling that you want a return from func1 by using the return value of func2 and checking for it in func1. (Or an exception, which wouldn't be nice to use for control flow like this.)
I noticed that I get the same effect if I define this trivial function:
fun double ( i: Int ) = i*2
and if I define a variable and assign a lambda (with an identical body) to it:
var double = { i : Int -> i*2 }
I get the same result if I call double(a) with either declaration.
This leaves me confused. When is it needed, recommended, advantageous to define a variable as a lambda rather than define a function to it?
When is it needed, recommended, advantageous to define a variable as a lambda rather than define a function to it?
Whenever you have the choice of either, you should use a fun declaration. Even with a fun you can still get a first-class callable object from it by using a function reference.
On the JVM, a fun is significantly more lightweight, both in terms of RAM and invocation overhead. It compiles into a Java method, whereas a val compiles into an instance field + getter + a synthetic class that implements a functional interface + a singleton instance of that class that you must fetch, dereference, and invoke a method on it.
You should consider a function-typed val or var only when something is forcing you to do it. One example is that you can dynamically replace a var and effectively change the definition of the function. You may also receive function objects from the outside, or you may need to comply with an API that needs them.
In any case, if you ever use a function-typed property of a class, you'll know why you're doing it.
First, if I understand you right, your question is "Why are functions first-class citizens in Kotlin -- And when to use them as such?", right?
Kotlin functions are first-class, which means that they can be stored in variables and data structures, passed as arguments to and returned from other higher-order functions. You can operate with functions in any way that is possible for other non-function values. (see here)
As stated in the docs, one use case are higher-order functions. As a first step, I will leave the wikipedia link here: https://en.wikipedia.org/wiki/Higher-order_function
Basically, a higher-order function is a function that takes functions as parameters, or returns a function.
This means that a higher-order function has at least one parameter of a function type or returns a value of a function type.
Following a short example of a higher-order function that receives a parameter of function type (Int) -> Boolean:
fun foo(pred: (Int) -> Boolean) : String = if(pred(x)) "SUCCESS" else "FAIL"
This higher-order function can now be called with any (Int) -> Boolean function.
The docs also state ... [can be used] in any way that is possible for other non-function values.
This means that you can, for example, assign different functions to a variable, depending on your current context.
For example:
// This example is verbose on purpose ;)
var checker: (Int) -> Boolean
if (POSITIVE_CHECK) {
checker = { x -> x > 0 } // Either store this function ...
} else {
checker = { x -> x < 0 } // ... or this one ...
}
if (checker(someNumber)) { // ... and use whatever function is now stored in variable "checker" here
print("Check was fine")
}
(Code untested)
You can define variable and assign it lambda when you want change behaviour for some reason. For example, you have different formula for several cases.
val formula: (Int) -> Int = when(value) {
CONDITION1 -> { it*2 }
CONDITION2 -> { it*3 }
else -> { it }
}
val x: Int = TODO()
val result = formula(x)
If you simply need helper function, you should define it as fun.
If you pass a lambda as a parameter of a function it will be stored in a variable. The calling application might need to save that (e.g. event listener for later use). Therefore you need to be able to store it as a variable as well. As said in the answer however, you should do this only when needed!
For me, I would write the Lambda variable as followed:
var double: (Int) -> Int = { i -> //no need to specify parameter name in () but in {}
i*2
}
So that you can easily know that its type is (i: Int) -> Int, read as takes an integer and returns an integer.
Then you can pass it to somewhere say a function like:
fun doSomething(double: (Int) -> Int) {
double(i)
}
When to use functioneExpression rather than function declaration in Go?
I searched Function Expression vs Function Declaration (in JS), it's about hoisting. How about Golang?
There is one unique property to each:
A function declaration binds an identifier, the function name, to a function; so the function name will be an identifier which you can refer to.
A function literals represents an anonymous function. Function literals are closures, they capture the surrounding environment: they may refer to variables defined in a surrounding function. Those variables are then shared between the surrounding function and the function literal, and they survive as long as they are accessible.
Don't be deluded: syntactically whenever a function literal is used, a declared function could also be used. For example the following code is a valid and working Go code:
func do() {}
func main() {
go do()
defer do()
}
The answer to when to use one over the other lies in the unique properties listed above.
Use function declaration when you want to refer to the function, when you want to reuse it. This is also a good way to separate code, function declaration must be at the file level: you can't declare a function in another function. See Golang nested class inside function for details.
Use function literals when you want the function to have access to the local variables and other identifiers surrounding it. Since you can't declare a function in another function, to capture the local variables and identifiers, the only option here is a function literal (unless you want to pass everything as arguments to a declared function, which could quickly get dirty if the function needs to modify the values in which case they need to be "transformed" into pointers and addresses need to be passed). For an interesting related question, check out Define a recursive function within a function in Go.
When the function does not need to have a name, and the function does not need access to the local variables, you may choose which one to use, relying on your judgement, examples seen in other (quality) projects and prior experiences. If the function body is "small", easier / simpler to use a function literal. If the function body is "big", code will be easier to read and understand if you declare it as a separate function and provide sufficient documentation to it.
An interesting / related topic is a variable of function type, which you may initialize using either a function literal or with a declared function. It has the benefits that it is an identifier so you can refer to it, and that its value can be changed and a new function can be assigned to it.
For example:
func do() { fmt.Println("Doing...") }
var f = do
func main() {
f()
f = func() { fmt.Println("Not doing!") }
f()
}
Output of the above (try it on the Go Playground):
Doing...
Not doing!
Very useful for mocking functions in tests. For an example, see Testing os.Exit scenarios in Go with coverage information (coveralls.io/Goveralls).
Anonymous functions in Go are used in various situations:
- to start a goroutine
go func() {
// body
}()
- with a defer statement
defer func() {
// body
}()
- as an argument to another function
err := filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
// todo
})
- as a closure (examples)
Function declarations are used when you want to refer to the function by name. Maybe to call it from multiple places in your own code, or to expose it to other packages.
I know that an inline function will maybe improve performance & cause the generated code to grow, but I'm not sure when it is correct to use one.
lock(l) { foo() }
Instead of creating a function object for the parameter and generating a call, the compiler could emit the following code. (Source)
l.lock()
try {
foo()
}
finally {
l.unlock()
}
but I found that there is no function object created by kotlin for a non-inline function. why?
/**non-inline function**/
fun lock(lock: Lock, block: () -> Unit) {
lock.lock();
try {
block();
} finally {
lock.unlock();
}
}
Let's say you create a higher order function that takes a lambda of type () -> Unit (no parameters, no return value), and executes it like so:
fun nonInlined(block: () -> Unit) {
println("before")
block()
println("after")
}
In Java parlance, this will translate to something like this (simplified!):
public void nonInlined(Function block) {
System.out.println("before");
block.invoke();
System.out.println("after");
}
And when you call it from Kotlin...
nonInlined {
println("do something here")
}
Under the hood, an instance of Function will be created here, that wraps the code inside the lambda (again, this is simplified):
nonInlined(new Function() {
#Override
public void invoke() {
System.out.println("do something here");
}
});
So basically, calling this function and passing a lambda to it will always create an instance of a Function object.
On the other hand, if you use the inline keyword:
inline fun inlined(block: () -> Unit) {
println("before")
block()
println("after")
}
When you call it like this:
inlined {
println("do something here")
}
No Function instance will be created, instead, the code around the invocation of block inside the inlined function will be copied to the call site, so you'll get something like this in the bytecode:
System.out.println("before");
System.out.println("do something here");
System.out.println("after");
In this case, no new instances are created.
Let me add: When not to use inline:
If you have a simple function that doesn't accept other functions as an argument, it does not make sense to inline them. IntelliJ will warn you:
Expected performance impact of inlining '...' is insignificant.
Inlining works best for functions with parameters of functional types
Even if you have a function "with parameters of functional types", you may encounter the compiler telling you that inlining does not work. Consider this example:
inline fun calculateNoInline(param: Int, operation: IntMapper): Int {
val o = operation //compiler does not like this
return o(param)
}
This code won't compile, yielding the error:
Illegal usage of inline-parameter 'operation' in '...'. Add 'noinline' modifier to the parameter declaration.
The reason is that the compiler is unable to inline this code, particularly the operation parameter. If operation is not wrapped in an object (which would be the result of applying inline), how can it be assigned to a variable at all? In this case, the compiler suggests making the argument noinline. Having an inline function with a single noinline function does not make any sense, don't do that. However, if there are multiple parameters of functional types, consider inlining some of them if required.
So here are some suggested rules:
You can inline when all functional type parameters are called directly or passed to other inline function
You should inline when ↑ is the case.
You cannot inline when function parameter is being assigned to a variable inside the function
You should consider inlining if at least one of your functional type parameters can be inlined, use noinline for the others.
You should not inline huge functions, think about generated byte code. It will be copied to all places the function is called from.
Another use case is reified type parameters, which require you to use inline. Read here.
Use inline for preventing object creation
Lambdas are converted to classes
In Kotlin/JVM, function types (lambdas) are converted to anonymous/regular classes that extend the interface Function. Consider the following function:
fun doSomethingElse(lambda: () -> Unit) {
println("Doing something else")
lambda()
}
The function above, after compilation will look like following:
public static final void doSomethingElse(Function0 lambda) {
System.out.println("Doing something else");
lambda.invoke();
}
The function type () -> Unit is converted to the interface Function0.
Now let's see what happens when we call this function from some other function:
fun doSomething() {
println("Before lambda")
doSomethingElse {
println("Inside lambda")
}
println("After lambda")
}
Problem: objects
The compiler replaces the lambda with an anonymous object of Function type:
public static final void doSomething() {
System.out.println("Before lambda");
doSomethingElse(new Function() {
public final void invoke() {
System.out.println("Inside lambda");
}
});
System.out.println("After lambda");
}
The problem here is that, if you call this function in a loop thousands of times, thousands of objects will be created and garbage collected. This affects performance.
Solution: inline
By adding the inline keyword before the function, we can tell the compiler to copy that function's code at call-site, without creating the objects:
inline fun doSomethingElse(lambda: () -> Unit) {
println("Doing something else")
lambda()
}
This results in the copying of the code of the inline function as well as the code of the lambda() at the call-site:
public static final void doSomething() {
System.out.println("Before lambda");
System.out.println("Doing something else");
System.out.println("Inside lambda");
System.out.println("After lambda");
}
This doubles the speed of the execution, if you compare with/without inline keyword with a million repetitions in a for loop. So, the functions that take other functions as arguments are faster when they are inlined.
Use inline for preventing variable capturing
When you use the local variables inside the lambda, it is called variable capturing(closure):
fun doSomething() {
val greetings = "Hello" // Local variable
doSomethingElse {
println("$greetings from lambda") // Variable capture
}
}
If our doSomethingElse() function here is not inline, the captured variables are passed to the lambda via the constructor while creating the anonymous object that we saw earlier:
public static final void doSomething() {
String greetings = "Hello";
doSomethingElse(new Function(greetings) {
public final void invoke() {
System.out.println(this.$greetings + " from lambda");
}
});
}
If you have many local variables used inside the lambda or calling the lambda in a loop, passing every local variable through the constructor causes the extra memory overhead. Using the inline function in this case helps a lot, since the variable is directly used at the call-site.
So, as you can see from the two examples above, the big chunk of performance benefit of inline functions is achieved when the functions take other functions as arguments. This is when the inline functions are most beneficial and worth using. There is no need to inline other general functions because the JIT compiler already makes them inline under the hood, whenever it feels necessary.
Use inline for better control flow
Since non-inline function type is converted to a class, we can't write the return statement inside the lambda:
fun doSomething() {
doSomethingElse {
return // Error: return is not allowed here
}
}
This is known as non-local return because it's not local to the calling function doSomething(). The reason for not allowing the non-local return is that the return statement exists in another class (in the anonymous class shown previously). Making the doSomethingElse() function inline solves this problem and we are allowed to use non-local returns because then the return statement is copied inside the calling function.
Use inline for reified type parameters
While using generics in Kotlin, we can work with the value of type T. But we can't work with the type directly, we get the error Cannot use 'T' as reified type parameter. Use a class instead:
fun <T> doSomething(someValue: T) {
println("Doing something with value: $someValue") // OK
println("Doing something with type: ${T::class.simpleName}") // Error
}
This is because the type argument that we pass to the function is erased at runtime. So, we cannot possibly know exactly which type we are dealing with.
Using an inline function along with the reified type parameter solves this problem:
inline fun <reified T> doSomething(someValue: T) {
println("Doing something with value: $someValue") // OK
println("Doing something with type: ${T::class.simpleName}") // OK
}
Inlining causes the actual type argument to be copied in place of T. So, for example, the T::class.simpleName becomes String::class.simpleName, when you call the function like doSomething("Some String"). The reified keyword can only be used with inline functions.
Avoid inline when calls are repetitive
Let's say we have the following function that is called repetitively at different abstraction levels:
inline fun doSomething() {
println("Doing something")
}
First abstraction level
inline fun doSomethingAgain() {
doSomething()
doSomething()
}
Results in:
public static final void doSomethingAgain() {
System.out.println("Doing something");
System.out.println("Doing something");
}
At first abstraction level, the code grows at: 21 = 2 lines.
Second abstraction level
inline fun doSomethingAgainAndAgain() {
doSomethingAgain()
doSomethingAgain()
}
Results in:
public static final void doSomethingAgainAndAgain() {
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
}
At second abstraction level, the code grows at: 22 = 4 lines.
Third abstraction level
inline fun doSomethingAgainAndAgainAndAgain() {
doSomethingAgainAndAgain()
doSomethingAgainAndAgain()
}
Results in:
public static final void doSomethingAgainAndAgainAndAgain() {
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
System.out.println("Doing something");
}
At third abstraction level, the code grows at: 23 = 8 lines.
Similarly, at the fourth abstraction level, the code grows at 24 = 16 lines and so on.
The number 2 is the number of times the function is called at each abstraction level. As you can see the code grows exponentially not only at the last level but also at every level, so that's 16 + 8 + 4 + 2 lines. I have shown only 2 calls and 3 abstraction levels here to keep it concise but imagine how much code will be generated for more calls and more abstraction levels. This increases the size of your app. This is another reason why you shouldn't inline each and every function in your app.
Avoid inline in recursive cycles
Avoid using the inline function for recursive cycles of function calls as shown in the following code:
// Don't use inline for such recursive cycles
inline fun doFirstThing() { doSecondThing() }
inline fun doSecondThing() { doThirdThing() }
inline fun doThirdThing() { doFirstThing() }
This will result in a never ending cycle of the functions copying the code. The compiler gives you an error: The 'yourFunction()' invocation is a part of inline cycle.
Can't use inline when hiding implementation
The public inline functions cannot access private functions, so they cannot be used for implementation hiding:
inline fun doSomething() {
doItPrivately() // Error
}
private fun doItPrivately() { }
In the inline function shown above, accessing the private function doItPrivately() gives an error: Public-API inline function cannot access non-public API fun.
Checking the generated code
Now, about the second part of your question:
but I found that there is no function object created by kotlin for a
non-inline function. why?
The Function object is indeed created. To see the created Function object, you need to actually call your lock() function inside the main() function as follows:
fun main() {
lock { println("Inside the block()") }
}
Generated class
The generated Function class doesn't reflect in the decompiled Java code. You need to directly look into the bytecode. Look for the line starting with:
final class your/package/YourFilenameKt$main$1 extends Lambda implements Function0 { }
This is the class that is generated by the compiler for the function type that is passed to the lock() function. The main$1 is the name of the class that is created for your block() function. Sometimes the class is anonymous as shown in the example in the first section.
Generated object
In the bytecode, look for the line starting with:
GETSTATIC your/package/YourFilenameKt$main$1.INSTANCE
INSTANCE is the object that is created for the class mentioned above. The created object is a singleton, hence the name INSTANCE.
That's it! Hope that provides useful insight into inline functions.
Higher-order functions are very helpful and they can really improve the reusability of code. However, one of the biggest concerns about using them is efficiency. Lambda expressions are compiled to classes (often anonymous classes), and object creation in Java is a heavy operation. We can still use higher-order functions in an effective way, while keeping all the benefits, by making functions inline.
here comes the inline function into picture
When a function is marked as inline, during code compilation the compiler will replace all the function calls with the actual body of the function. Also, lambda expressions provided as arguments are replaced with their actual body. They will not be treated as functions, but as actual code.
In short:- Inline-->rather than being called ,they are replaced by the function's body code at compile time...
In Kotlin, using a function as a parameter of another function (so called higher-order functions) feels more natural than in Java.
Using lambdas has some disadvantages, though. Since they’re anonymous classes (and therefore, objects), they need memory (and might even add to the overall method count of your app).
To avoid this, we can inline our methods.
fun notInlined(getString: () -> String?) = println(getString())
inline fun inlined(getString: () -> String?) = println(getString())
From the above example:- These two functions do exactly the same thing - printing the result of the getString function. One is inlined and one is not.
If you’d check the decompiled java code, you would see that the methods are completely identical. That’s because the inline keyword is an instruction to the compiler to copy the code into the call-site.
However, if we are passing any function type to another function like below:
//Compile time error… Illegal usage of inline function type ftOne...
inline fun Int.doSomething(y: Int, ftOne: Int.(Int) -> Int, ftTwo: (Int) -> Int) {
//passing a function type to another function
val funOne = someFunction(ftOne)
/*...*/
}
To solve that, we can rewrite our function as below:
inline fun Int.doSomething(y: Int, noinline ftOne: Int.(Int) -> Int, ftTwo: (Int) -> Int) {
//passing a function type to another function
val funOne = someFunction(ftOne)
/*...*/}
Suppose we have a higher order function like below:
inline fun Int.doSomething(y: Int, noinline ftOne: Int.(Int) -> Int) {
//passing a function type to another function
val funOne = someFunction(ftOne)
/*...*/}
Here, the compiler will tell us to not use the inline keyword when there is only one lambda parameter and we are passing it to another function. So, we can rewrite above function as below:
fun Int.doSomething(y: Int, ftOne: Int.(Int) -> Int) {
//passing a function type to another function
val funOne = someFunction(ftOne)
/*...*/
}
Note:-we had to remove the keyword noinline as well because it can be used only for inline functions!
Suppose we have function like this -->
fun intercept() {
// ...
val start = SystemClock.elapsedRealtime()
val result = doSomethingWeWantToMeasure()
val duration = SystemClock.elapsedRealtime() - start
log(duration)
// ...}
This works fine but the meat of the function’s logic is polluted with measurement code making it harder for your colleagues to work what’s going on. :)
Here’s how an inline function can help this code:
fun intercept() {
// ...
val result = measure { doSomethingWeWantToMeasure() }
// ...
}
}
inline fun <T> measure(action: () -> T) {
val start = SystemClock.elapsedRealtime()
val result = action()
val duration = SystemClock.elapsedRealtime() - start
log(duration)
return result
}
Now I can concentrate on reading what the intercept() function’s main intention is without skipping over lines of measurement code. We also benefit from the option of reusing that code in other places where we want to
inline allows you to call a function with a lambda argument within a closure ({ ... }) rather than passing the lambda like measure(myLamda)
When is this useful?
The inline keyword is useful for functions that accept other functions, or lambdas, as arguments.
Without the inline keyword on a function, that function's lambda argument gets converted at compile time to an instance of a Function interface with a single method called invoke(), and the code in the lambda is executed by calling invoke() on that Function instance inside the function body.
With the inline keyword on a function, that compile time conversion never happens. Instead, the body of the inline function gets inserted at its call site and its code is executed without the overhead of creating a function instance.
Hmmm? Example in android -->
Let's say we have a function in an activity router class to start an activity and apply some extras
fun startActivity(context: Context,
activity: Class<*>,
applyExtras: (intent: Intent) -> Unit) {
val intent = Intent(context, activity)
applyExtras(intent)
context.startActivity(intent)
}
This function creates an intent, applies some extras by calling the applyExtras function argument, and starts the activity.
If we look at the compiled bytecode and decompile it to Java, this looks something like:
void startActivity(Context context,
Class activity,
Function1 applyExtras) {
Intent intent = new Intent(context, activity);
applyExtras.invoke(intent);
context.startActivity(intent);
}
Let's say we call this from a click listener in an activity:
override fun onClick(v: View) {
router.startActivity(this, SomeActivity::class.java) { intent ->
intent.putExtra("key1", "value1")
intent.putExtra("key2", 5)
}
}
The decompiled bytecode for this click listener would then look like something like this:
#Override void onClick(View v) {
router.startActivity(this, SomeActivity.class, new Function1() {
#Override void invoke(Intent intent) {
intent.putExtra("key1", "value1");
intent.putExtra("key2", 5);
}
}
}
A new instance of Function1 gets created every time the click listener is triggered. This works fine, but it's not ideal!
Now let's just add inline to our activity router method:
inline fun startActivity(context: Context,
activity: Class<*>,
applyExtras: (intent: Intent) -> Unit) {
val intent = Intent(context, activity)
applyExtras(intent)
context.startActivity(intent)
}
Without changing our click listener code at all, we're now able to avoid the creation of that Function1 instance. The Java equivalent of the click listener code would now look something like:
#Override void onClick(View v) {
Intent intent = new Intent(context, SomeActivity.class);
intent.putExtra("key1", "value1");
intent.putExtra("key2", 5);
context.startActivity(intent);
}
Thats it.. :)
To "inline" a function basically means to copy a function's body and paste it at the function's call site. This happens at compile time.
The most important case when we use the inline modifier is when we define util-like functions with parameter functions. Collection or string processing (like filter, map or joinToString) or just standalone functions are a perfect example.
This is why the inline modifier is mostly an important optimization for library developers. They should know how it works and what are its improvements and costs. We should use the inline modifier in our projects when we define our own util functions with function type parameters.
If we don’t have function type parameter, reified type parameter, and we don’t need non-local return, then we most likely shouldn’t use the inline modifier. This is why we will have a warning on Android Studio or IDEA IntelliJ.
Also, there is a code size problem. Inlining a large function could dramatically increase the size of the bytecode because it's copied to every call site. In such cases, you can refactor the function and extract code to regular functions.
One simple case where you might want one is when you create a util function that takes in a suspend block. Consider this.
fun timer(block: () -> Unit) {
// stuff
block()
//stuff
}
fun logic() { }
suspend fun asyncLogic() { }
fun main() {
timer { logic() }
// This is an error
timer { asyncLogic() }
}
In this case, our timer won't accept suspend functions. To solve it, you might be tempted to make it suspend as well
suspend fun timer(block: suspend () -> Unit) {
// stuff
block()
// stuff
}
But then it can only be used from coroutines/suspend functions itself. Then you'll end up making an async version and a non-async version of these utils. The problem goes away if you make it inline.
inline fun timer(block: () -> Unit) {
// stuff
block()
// stuff
}
fun main() {
// timer can be used from anywhere now
timer { logic() }
launch {
timer { asyncLogic() }
}
}
Here is a kotlin playground with the error state. Make the timer inline to solve it.
fun higherOrder(lambda:():Unit){
//invoking lambda
lambda()
}
//Normal function calling higher-order without inline
fun callingHigerOrder() {
higherOrder()
//Here an object will be created for the lambda inside the higher-order function
}
//Normal function calling higher-order with inline
fun callingHigerOrder() {
higherOrder()
//Here there will be no object created and the contents of the lambda will be called directly into this calling function.
}
use inline if you want to avoid object creation at the calling side.
So when using inline, as we understood lambda will be the part of calling function incase if there is a return call inside the lambda block then whole calling function will get returned this is called non-local return.
To avoid non-local return use cross-inline before lambda block in the higher-order function.