We know the lambda body is lazily well, because if we don't call the lambda the code in the lambda body is never be called.
We also know in any function language that a variable can be used in a function/lambda even if it is not initialized, such as javascript, ruby, groovy and .etc, for example, the groovy code below can works fine:
def foo
def lambda = { foo }
foo = "bar"
println(lambda())
// ^--- return "bar"
We also know we can access an uninitialized variable if the catch-block has initialized the variable when an Exception is raised in try-block in Java, for example:
// v--- m is not initialized yet
int m;
try{ throw new RuntimeException(); } catch(Exception ex){ m = 2;}
System.out.println(m);// println 2
If the lambda is lazily, why does Kotlin can't use an uninitialized variable in lambda? I know Kotlin is a null-safety language, so the compiler will analyzing the code from top to bottom include the lambda body to make sure the variable is initialized. so the lambda body is not "lazily" at compile-time. for example:
var a:Int
val lambda = { a }// lambda is never be invoked
// ^--- a compile error thrown: variable is not initialized yet
a = 2
Q: But why the code below also can't be working? I don't understand it, since the variable is effectively-final in Java, if you want to change the variable value you must using an ObjectRef instead, and this test contradicts my previous conclusions:"lambda body is not lazily at compile-time" .for example:
var a:Int
run{ a = 2 }// a is initialized & inlined to callsite function
// v--- a compile error thrown: variable is not initialized yet
println(a)
So I only can think is that the compiler can't sure the element field in ObjectRef is whether initialized or not, but @hotkey has denied my thoughts. Why?
Q: why does Kotlin inline functions can't works fine even if I initializing the variable in catch-block like as in java? for example:
var a: Int
try {
run { a = 2 }
} catch(ex: Throwable) {
a = 3
}
// v--- Error: `a` is not initialized
println(a)
But, @hotkey has already mentioned that you should using try-catch expression in Kotlin to initializing a variable in his answer, for example:
var a: Int = try {
run { 2 }
} catch(ex: Throwable) {
3
}
// v--- println 2
println(a);
Q: If the actual thing is that, why I don't call the run directly? for example:
val a = run{2};
println(a);//println 2
However the code above can works fine in java, for example:
int a;
try {
a = 2;
} catch (Throwable ex) {
a = 3;
}
System.out.println(a); // println 2
Q: But why the code below also can't be working?
Because code can change. At the point where the lambda is defined the variable is not initialized so if the code is changed and the lambda is invoked directly afterwards it would be invalid. The kotlin compiler wants to make sure there is absolutely no way the uninitialized variable can be accessed before it is initialized, even by proxy.
Q: why does Kotlin inline functions can't works fine even if I initializing the variable in catch-block like as in java?
Because run is not special and the compiler can't know when the body is executed. If you consider the possibility of run not being executed then the compiler cannot guarentee that the variable will be initialized.
In the changed example it uses the try-catch expression to essentially execute a = run { 2 }, which is different from run { a = 2 } because a result is guaranteed by the return type.
Q: If the actual thing is that, why I doesn't call the run directly?
That is essentially what happens. Regarding the final Java code the fact is that Java does not follow the exact same rules of Kotlin and the same happens in reverse. Just because something is possible in Java does not mean it will be valid Kotlin.
You could make the variable lazy with the following...
val a: Int by lazy { 3 }
Obviously, you could use a function in place of the 3. But this allows the compiler to continue and guarantees that a is initialized before use.
Edit
Though the question seems to be "why can't it be done". I am in the same mind frame, that I don't see why not (within reason). I think the compiler has enough information to figure out that a lambda declaration is not a reference to any of the closure variables. So, I think it could show a different error when the lambda is used and the variables it references have not been initialized.
That said, here is what I would do if the compiler writers were to disagree with my assessment (or take too long to get around to the feature).
The following example shows a way to do a lazy local variable initialization (for version 1.1 and later)
import kotlin.reflect.*
//...
var a:Int by object {
private var backing : Int? = null
operator fun getValue(thisRef: Any?, property: KProperty<*>): Int =
backing ?: throw Exception("variable has not been initialized")
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: Int) {
backing = value
}
}
var lambda = { a }
// ...
a = 3
println("a = ${lambda()}")
I used an anonymous object to show the guts of what's going on (and because lazy caused a compiler error). The object could be turned into function like lazy.
Now we are potentially back to a runtime exception if the programmer forgets to initialize the variable before it is referenced. But Kotlin did try at least to help us avoid that.
