Proposal - Better control for initialization

[NN---]

Idea of keywords for better controlling variables initialization.

Samples:

mustinit - Variable that must be initialized before used.

let mut mustinit a = 1; // OK
a = 2; // OK

let mut mustinit x;
f(x); // Error: 'x' is not initialized

let mut mustinit x;
if something { x = 1; } else { x = 2; }
f(x); // OK

let mut mustinit x;
if something { x = 1; }
f(x); // Error: 'x' may not be initialized

initonce - Variable that can be initialized only once and then cannot be changed. It doesn't require the variable to be initialized.

let mut initonce a = 1; // OK
a = 2; // Error 'a' has been already initialized

let mut initonce a;
a = 1; // OK
a = 2; // Error 'a' has been already initialized

[jdm]

How is initonce any different than leaving off the mut qualifier? Similarly, I don't see how the purpose of mustinit - that's exactly how the language behaves today. What's the motivation for making it optional?

[NN---]

The purpose is that you can leave variable uninitialized and intiialize it only when you are ready.
But compiler can tell you that you using uninitialized variable.

'initonce' means that you initialize it only once and cannot change its value later.
'let mut' allows you to change the variable more than once.

I am not good in Rust language, perhaps the Rust language already works this way ? :)

[jdm]

let a;
...
a = 5;
//a = 6; <- error, immutable value.

already works. Any attempts to use a before it's initialized will be an error, and that works with constructs like if/else too.

[thestinger]

Yeah, these features already exist.

[NN---]

And what about mutable variables ?

[thestinger]

Rust never allows use before initialization.

[thestinger]

If you want to leave a variable uninitialized but can't prove safety to the compiler, then you use an unsafe block and initialize with the uninitialized function.

[NN---]

I see.
Great news.
Thanks.

[NN---]

The only thing impossible to do is to seal the mutable variable from other changes.
You put the value and you want to prohibit changing it.
let mut a;
a = 1; // ok
a =2; // I want to prohibit this and further ones.

[jdm]

That's identical to an immutable variable. They don't have to be initialized at declaration; they just need to be initialized before first use.