Mutate type of generic variable in Rust

Question

I know that you can't mutate the type of a variable. So coming from other programming languages where following would work:

struct Point<T> {
    x: T,
    y: T,
}

fn main() {
    let mut p = Point { x: 1, y: 1 }; // p is type of Point<u8> now
    p = Point { x: 1.0, y: 1.0 }; // p should be type of Point<float> now
}

My use case for this would, that you assign a default value to variable and i specific case you change the value. But both are e.g. of the generic type Point<T> and the rest of the code just works with this generic type and not a specific one.

What is the best practice in Rust to achieve this kind of workflow?

EDIT:

A better example to understand what i want to accomplish is maybe following: DateTime<Tz> is given by the crate chrono.


fn main() {
   let is_utc = ....;

   let mut datetime = Local::now(); // datetime is now of type DateType<Local>

   if is_utc {
       datetime = Utc::now(); // datetime is now of type DateType<Utc>
   }

   format(datetime);
}


fn format<Tz: TimeZone>(datetime: DateTime<Tz>) where Tz::Offset : fmt::Display {
    // here is someother code to justify this extra function :)
    datetime.format("%a %b %e %k:%M:%S %Z %Y");
}

So the function format does not are what Tz is except it should be of type TimeZone.

As you said: You can't change the type. Also I don't see why you would need that from your comment. You need beforehand to know which type you expect (you can't change the type of values inside a Vec at runtime as well, can you?). I see two possibilities. Use Default::default and add the requirement T: Default and then you can write: let mut p: Point<f32> = Point::Default — hellow
– hellow, Commented Oct 10, 2019 at 7:52
You could do this in a weakly typed language, but in strongly typed languages which support generics, you cannot change the generic parameter. E.g. if you write Point<int> p = new Point<int>(1, 2); in C#, you cannot write p = new Point<float>(1f, 2f); afterwards. If the class was mutable, you couldn't even write p.X = 2.1f; because p is a struct containing integers. Think of generic classes as templates for concrete classes that are set in stone when you specify a parameter. — vgru
– vgru, Commented Oct 10, 2019 at 8:06
@hellow this not exactly what i want to do. See my edited question. — kegesch
– kegesch, Commented Oct 10, 2019 at 8:07
@Groo yes i understand that. I just want to know a best practice code how to approach my problem. — kegesch
– kegesch, Commented Oct 10, 2019 at 8:08

user9244181 · Accepted Answer · 2020-03-07 21:38:16Z

1

You can set your multi variable type in enum,

enum Num {
    int(i32),
    float(f64)
}

fn main() {
    let mut p = Point { x: Num::int(1), y: Num::int(1) };
    p = Point { x: Num::float(1.0), y: Num::float(1.0) };
}

answered Mar 7, 2020 at 21:38

user9244181

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