Feature request: Convert variables from mutable to immutable at runtime

[LiamSwayne]

It would be helpful to be able to make a variable immutable based on control flow or keywords.

If fix was the keyword used to convert mutable data types to an immutable state, it could look like this:

var x:Int = 1
if runtime_expr:
    fix x

x += 1 # this line will only cause an error if runtime_expr evaluates to True

This behavior can be produced with structs, but to my knowledge requires running control flow and checking for mutability after every function call to an instance. I assume there is a much more efficient way to implement a feature like this, even in a struct. Perhaps a boolean tacked on to the instance could hold it's value until it is freed by the fix call, and subsequent checks for mutability would throw an error when trying to mutate the variable due to being unable to access it's correlating boolean. I am sure there is a better approach, but even a decorator on a struct to provide this functionality with some overhead would be very useful.

[Brian-M-J]

I assume there is a much more efficient way to implement a feature like this

I am sure there is a better approach

let proceeds to quietly cry in the corner...

Also, I'm not really sure what you're trying to accomplish here, that can't already be done with Mojo:

var x: Int = 1
if not runtime_expr:
    x += 1 # No runtime mutability checking needed

Without a compelling real world example, I don't see a need for something like this.