Generic function that returns different types based on the value of an argument

dependent-type, generics, rust

Solution

You can't.

Sorry, but there's just no way of doing this in Rust. You'd need dependent types, which Rust doesn't have. You could perhaps return an instance of an `enum` that just contains two variants (one for each type). Or you could accept one callback for each "path" and let the caller decide how to resolve the problem.

fn read_register<FnU8, FnU16, R>(
    &self,
    register: Register,
    with_u8: FnU8,
    with_u16: FnU16,
) -> R
where
    FnU8: FnOnce(u8) -> R,
    FnU16: FnOnce(u16) -> R,
{
    match register {
        Register::V0 => with_u8(self.v0),
        Register::V1 => with_u8(self.v1),
        Register::V2 => with_u16(self.v2),
        Register::V3 => with_u16(self.v3),
    }
}

Problem

I have a struct which holds registers. I want my `read_register` function to return a `u8` for `Register::V0` and `Register::V1` but a `u16` for `Register::V2` and `Register::V3`. I'm not sure how to make the function generic over the return type. I'm getting the error `match arms have incompatible types` which does make sense because the types are different. ``` struct Registers { v0: u8, v1: u8, v2: u16, v3: u16, } enum Register { V0, V1, V2, V3, } impl Registers { fn read_register<T>(&self, register: Register) -> T { match register { Register::V0 => self.v0, Register::V1 => self.v1, Register::V2 => self.v2, Register::V3 => self.v3, } } } ```

Original source