TIL: Convert Rust enums to inner values

I’ve developed what I think is a pretty neat way to transform a Rust enum instance into one of its variants’ internal values. I don’t want to say I invented it because I think this may be a bit too basic to to be novel.

Here’s the problem: You have an enum with two variants, each of which hold different kinds of data:

enum IpAddr {
  V4([u8; 4])
  V6([&'static str; 8])
}

and you want to have a method on this enum that returns either [u8; 4] if the V4 variant is active, or [&'static str; 8] if the V6 variant is active. Of course you could make everything pub and be done with it, but that’s leaky. A straight-forward way to do this would be to introduce two methods on the enum (Playground):

impl IpAddr {
  pub fn v4(self) -> Result<[u8; 4], _> {
    match self {
      // ...
    }
  }

  pub fn v6(self) -> Result<[&'static str'; 8], _> {
    match self {
      // ...
    }
  }
}

This works perfectly fine, but I don’t like the look of it. I can totally imagine wanting a method that returns a String for the variants instead of fixed-length arrays like we have. We’ll then have to come up with new names for these methods.

The other way is to implement the TryInto/TryFrom traits on the IpAddr enum. This is also a great (idiomatic even) solution, but I wanted something more generic (literally).

Enough teasing, here’s what I want to be able to do:

let ip = IpAddr::V4([0u8; 4]);
let inner = ip.decay::<[u8; 4]>();
assert_eq!(inner, [0u8; 4]);

let ip = IpAddr::V6(["0000"; 8]);
let inner = ip.decay::<[&'static str; 8]>();
assert_eq!(inner, ["0000"; 8]);

We can alias some types if they get unwieldy, but the basic idea is to have something like std::str::parse. A single method you call that lets you control what it returns using the turbofish.

The way you do this is using a custom trait. I call it Decay. It is super simple:

pub trait Decay: Sized {
  fn decay(value: IpAddr) -> Result<Self, EnumError>;
}

Now you can implement this trait for as many conversions as you’d like! Here are a few examples:

// Simple value decay
impl Decay for [u8; 4] {
  fn decay(value: IpAddr) -> Result<Self, EnumError> {
    match value {
      IpAddr::V4(v) => Ok(v),
      _ => Err(EnumError),
    }
  }
}

// More complex transformation.
// Notice how `String` isn't even a possible enum variant data type
impl Decay for String {
  fn decay(value: IpAddr) -> Result<Self, EnumError> {
    match value {
      IpAddr::V4(v) => Ok(format!("{}.{}.{}.{}", v[0], v[1], v[2], v[3])),
      IpAddr::V6(v) => Ok(format!(
        "{}::{}::{}::{}::{}::{}::{}::{}",
        v[0], v[1], v[2], v[3], v[4], v[5], v[6], v[7]
      )),
    }
  }
}

You can then do a conversion this way:

Decay::decay::<String>(ip);

For convenience, let’s create a decay method on our enum directly. We’re not implementing Decay on IpAddr, just creating a method that happens to be called decay as well, because it best describes the action. We could totally call this something else.

impl IpAddr {
  pub fn decay<T: Decay>(self) -> Result<T, EnumError> {
    Decay::decay(self)
  }
}

With this in place, we can finally do some clean-looking transformations:

let ip = IpAddr::V4([0u8; 4]);
let inner = ip.decay::<String>();
println!("{}", inner.unwrap());

Here’s a playground that you can play around with: Playground