The following trivial Kotlin code snippet
fun main() {}
compiles just fine, but the following
val main : () -> Unit = {}
makes the compiler complain that "No main method found in project.", while I was expecting them to be equivalent (I expect a programming language to be as conceptually uniform as possible).
Why does this happen? Is it related only to main, or does this behaviour concern a larger class of functions? Is there some subtle difference between declaring functions with "fun" and declaring them as lambdas?
Conceptually, they are different things. To see that, let's take a look at roughly what the equivalent Java would be. I'll use JVM for examples in this answer, but the same principles apply to all of the other Kotlin backends.
object Foo {
fun main() { ... }
}
This is roughly
class Foo {
public static void main() { ... }
}
Again, roughly. Technically, you'll get a singleton object and a method on it unless you use #JvmStatic (I assume there's some special handling for main that produces a static function on JVM, but I don't know that for a fact)
On the other hand,
object Foo {
val main: () -> Unit = { ... }
}
Here, we're declaring a property, which in Java is going to get implemented as a getter-setter pair
class Foo {
// Singleton instance
public static Foo instance = new Foo();
public Supplier<Void> main;
Foo() {
main = new Supplier<Void>() {
Void get() {
...
}
}
}
}
That is, there isn't actually a main method. There's a main field which, deep down somewhere, has a function inside of it. In my example above, that function is called get. In Kotlin, it's called invoke.
The way I like to think of it is this. Methods in Kotlin (i.e. the things you define on objects that designate their behavior) are not themselves first-class objects. They're second-class citizens which exist on an object. You can convert them to first-class objects by making them into functions. Functions are ordinary objects, like any other. If you take an ordinary object, which may or may not be a function, and call it with (), then you're actually invoking the method .invoke(...) on it. That is, () is an operator on objects which really ends up calling a method. So in Kotlin, functions are really just objects with a custom invoke and a lot of syntax sugar.
Your val defines a field which is a function. Your fun defines a method. Both of these can be called with (), but only one is a genuine method call; the other is secretly calling .invoke on another object. The fact that they look syntactically the same is irrelevant.
As the old adage goes, functions are a poor man's objects, and objects are a poor man's functions.
There is a subtle (or more than subtle) difference. Declaring it with val means that main is a property containing a reference to an anonymous function (which you defined with the lambda). If you define it with val, then when you call main(), you are actually calling the getter of the main property, and then using the invoke() operator to call invoke() on the return value of the property (the anonymous function).
I'd like to use ES6 public class fields:
class Superclass {
constructor() {
// would like to write modular code that applies to all
// subclasses here, or similarly somewhere in Superclass
this.example++; // does NOT WORK (not intialized)
//e.g. doStuffWith(this.fieldTemplates)
}
}
class Subclass extends Superclass {
example = 0
static fieldTemplates = [
Foo,
function() {this.example++},
etc
]
}
Problem:
ES6 public fields are NOT initialized before the constructors, only before the current constructor. For example, when calling super(), any child field will not yet have been defined, like this.example will not yet exist. Static fields will have already been defined. So for example if one were to execute the code function(){this.example++} with .bind as appropriate, called from the superclass constructor, it would fail.
Workaround:
One workaround would be to put all initialization logic after all ES6 public classes have been properly initialized. For example:
class Subclass extends Superclass {
example = 0
lateConstructor = (function(){
this.example++; // works fine
}).bind(this)()
}
What's the solution?
However, this would involve rewriting every single class. I would like something like this by just defining it in the Superclass.constructor, something magic like Object.defineProperty(this, 'lateConstructor', {some magic}) (Object.defineProperty is allegedly internally how es6 static fields are defined, but I see no such explanation how to achieve this programatically in say the mozilla docs; after using Object.getOwnPropertyDescriptor to inspect my above immediately-.binded-and-evaluated cludge I'm inclined to believe there is no way to define a property descriptor as a thunk; the definition is probably executed after returning from super(), that is probably immediately evaluated and assigned to the class like let exampleValue = eval(...); Object.defineProperty(..{value:exampleValue})). Alternatively I could do something horrible like do setTimeout(this.lateConstructor,0) in the Superclass.constructor but that would break many things and not compose well.
I could perhaps try to just use a hierarchy of Objects everywhere instead, but is there some way to implement some global logic for all subclasses in the parent class? Besides making everything lazy with getters? Thanks for any insight.
References:
Run additional action after constructor -- (problems: this requires wrapping all subclasses)
Can I create a thunk to run after the constructor?
No, that is not possible.
How to run code after class fields are initialized, in a sane way?
Put the code in the constructor of the class that defines those fields.
Is there some way to implement some global logic for all subclasses in the parent class?
Yes: define a method. The subclass can call it from its constructor.
Just thought of a workaround (that is hierarchically composable). To answer my own question, in a somewhat unfulfilling way (people should feel free to post better solutions):
// The following illustrates a way to ensure all public class fields have been defined and initialized
// prior to running 'constructor' code. This is achieved by never calling new directly, but instead just
// running Someclass.make(...). All constructor code is instead written in an init(...) function.
class Superclass {
init(opts) { // 'constructor'
this.toRun(); // custom constructor logic example
}
static make() { // the magic that makes everything work
var R = new this();
R.init(...arguments);
return R;
}
}
class Subclass extends Superclass {
subclassValue = 0 // custom public class field example
init(toAdd, opts) { // 'constructor'
// custom constructor logic example
this.subclassValue += toAdd; // may use THIS before super.init
super.init(opts);
// may do stuff afterwards
}
toRun() { // custom public class method example
console.log('.subclassValue = ', this.subclassValue);
}
}
Demo:
> var obj = Subclass.make(1, {});
.subclassValue = 1
> console.log(obj);
Subclass {
subclassValue: 1
__proto__: Superclass
}
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.
the code will output
1
2
2
the 1 only print once. I have no idea, how to write it in genie.
below is vala code:
public static void main() {
var a = new One();
var b = new One();
}
class One : Object {
class construct {
stdout.puts("1\n");
}
public One () {
stdout.puts("2\n");
}
}
What is the equivalent code class construct method for genie?
If Genie's init? (NOW) it is diffrent from vala's class construct?
Genie's init
below can't works!
init
var a = new One
var b = new One
class One
construct ()
stdout.puts("2\n")
init
stdout.puts("1\n")
construct block require GLib.object
construct block => init block
but Vala's class construct dosen't.
so vala will works, but Genie dosen't,
vala code:
class One {
class construct {
stdout.puts("1\n");
}
public One () {
stdout.puts("2\n");
}
}
Why this functionality is useful?
Actually I never use class before.
but I think it is useful, very useful.
example: init some static fileds ( or class fields: that another question).
I don't know why? Why Vala implement this feature?
It must be useful if Vala implement it.
Fom what I can tell from the documentation I thought it was init. Although I'm not sure why you would want to use it. It might be useful for some lazy loading of static data used in all instances of the class, but I've never done that myself.
In object oriented programming terminology a class is "instantiated" when it is created as an object. The process of instantiation can be customized with a "constructor". In Vala and Genie it is also possible to customize the process when an object is no longer needed with a "destructor".
If a method does not act upon the data in the object it is called a static method. By definition a constructor does not act upon the data in the object, but returns a new object. So constructors are always static and static construct is the wrong name. In Vala there is class construct and if you change your code to use that you get the same result. See Vala different type of constructors for a thorough treatment on this subject in Vala. The part about class construct is at the end.
My understanding was Genie's init was the equivalent of Vala's class construct. I think your question has uncovered a problem in Genie. My understanding was this code would answer your question:
[indent = 4]
init
var a = new One()
var b = new One()
var c = new One.alternative_constructor()
class One:Object
init
print "Class 'One' is registered with GType"
construct()
print "Object of type 'One' created"
construct alternative_constructor()
print """Obect of type 'One' created with named constructor
'alternative_constructor', but the design of your
class is probably too complex if it has multiple
constructors."""
def One()
print "This is not a constructor"
final
print "Object of type 'One' destroyed"
This, however, outputs:
Class 'One' is registered with GType
Object of type 'One' created
Class 'One' is registered with GType
Object of type 'One' created
Class 'One' is registered with GType
Obect of type 'One' created with named constructor
'alternative_constructor', but the design of your
class is probably too complex if it has multiple
constructors.
Object of type 'One' destroyed
Object of type 'One' destroyed
Object of type 'One' destroyed
Whereas the registration with GType only happens once so the init code is being put in the wrong place and getting called with each instantiation not with the type registration. So at present I don't think there is an equivalent of Vala's class construct in Genie.
Changing the behaviour of init may be one solution, but I may have misunderstood its original purpose and there is probably plenty of code out there now that relies on its current behaviour. The other solution is to write a patch for the Genie parser that implements this. You would need to make a good case why this functionality is useful.
I'm trying to learn Scala and the Play Framework at the same time. Scala looks to me like it has a lot of really cool ideas, but one of my frustrations is trying to understand all of the different syntaxes for methods/functions/lambdas/anonymous functions/etc.
So I have my main application controller like so:
object Application extends Controller {
def index = Action {
Ok(views.html.index("Your new application is ready."))
}
}
This tells me I have a singleton Application that has one method, index, that returns what type?? I was expecting index to have a definition more like:
def index(req: Request) : Result = { ... }
Looking at Play Framework's documentation, it looks as though Action is a trait, that transforms a request to a result, by I'm having a hard time understanding what this line is saying:
def index = Action { ... }
I come from a Java background, so I don't know what this is saying? (this statement feels like it's saying "method index = [some interface Action]", which doesn't make sense to me; it seems something beautiful is happening, but it is magic to me, and I feel uneasy with magic in my code ;))
When you invoke an object as if it were a function, that's translated into a call to apply. I.e.:
foo(bar)
is translated into
foo.apply(bar)
So, inside index you are calling the Action object as if it were a function, which means you are actually calling Action.apply.
The return type of index is omitted because the compiler can infer it to be the return type of Action.apply (which I guess from the name is Unit).
So the short answer to this question is that there's some stuff going on behind the scenes that makes the above work: namely that the compiler is inferring types, and in Scala, objects with an apply method can get called as if they were functions.
So what's going on here is that this code:
def index = Action {
Ok("Hello World!")
}
...is equivalent to (or rather shorthand for) this code:
def index : Action[AnyContent] = Action.apply(
(req: Request[AnyContent]) => {
Ok(views.html.index("Hello World!"))
} : Result
)
The magic is happening here: ... = Action {...}. Action {...} says "call Action with this anonymous function {...}".
Because Action.apply is defined as apply(block: => Result): Action[AnyContent], all of the argument-/return- types can be inferred.