Ballerina 中的 final 和 const 有什么区别？

Question

Reading the examples on Ballerina I stumbled upon the example here https://ballerina.io/learn/by-example/variables.html which has the following code:

public const int COUNT = 1;

final int status = 1;

where the first line is only described with

Declare a public compile-time constant

and the second is:

Declare a final variable. The value of the final variable is read-only. Once a value is assigned to a final variable, it becomes immutable. All parameters of a function call are implicitly final.

But this leads to the question: what is the difference between final and const?

Answer 1

The answer is hidden in another example, a lot later in the list: Const and final

The difference between the final variables and constants is that the value of the final variables can be initialized at runtime. However, constants must be initialized at compile time.

Which means

function findFoo() returns int {
    return 42;
}

public function main() {
    // This works
    final int foo = findFoo();
}

however:

function findFoo() returns int {
    // this is not allowed
    return 42;
}

public function main() {
    const int foo = findFoo();
}

Previously there was a bug ( https://github.com/ballerina-platform/ballerina-lang/issues/15044 ) in the language implementation that has now been fixed:

int foo;
// this previously didn't work, but now does
foo = 32;

That is using final allows one to set the value from a function (ie, at runtime) where const doesn't. Currently in both cases one need to set the value where one declares the variable, but in a future version (when the bug is fixed) the definition can be later in the code.

On the other hand (thanks to @dhananjaya for pointing that out) const can be used in other compile time constructs.