Is it possible to map a function over a Vec without allocating a new Vec?

Question

I have the following:

enum SomeType {
    VariantA(String),
    VariantB(String, i32),
}

fn transform(x: SomeType) -> SomeType {
    // very complicated          


        

Answer 1

Your first problem is not map, it's transform.

transform takes ownership of its argument, while Vec has ownership of its arguments. Either one has to give, and poking a hole in the Vec would be a bad idea: what if transform panics?

---

The best fix, thus, is to change the signature of transform to:

fn transform(x: &mut SomeType) { ... }

then you can just do:

for x in &mut data { transform(x) }

---

Other solutions will be clunky, as they will need to deal with the fact that transform might panic.