Result with lifetime parameter - enum with lifetime parameter applied to default T<'a> and E<'a> parameter (Rust)

Question

I'm attempting to make a Result with a lifetime parameter, as an additional restrain on the T (::Ok(T)) and E (::Err(E)) generic parameters.

// enums A<'a> and B<'a> elided

pub enum Result<'a, T=A<'a>, E=B<'a>> {
    Ok(T),
    Err(E),
}

unused parameter, suggestion: add PhantomData

So it didn't work. I read that I can hack around the unused parameter error with PhantomData, but that seems dirty to me.

Neither did this work:

pub enum Result<T=A<'a>, E=B<'a>> {
    Ok(T),
    Err(E),
}

undefined parameter, suggestion: add parameter before T
(twice)

This suggestion throws me for a loop!

How do I put a generic type with a lifetime parameter, in my enum generic parameter default value?

Answer 1

The error is correct, this type would make no sense:

Since the lifetime is only used in default parameters, what would the lifetime be in cases like this:

let foo = Result::Ok(42); // What's the lifetime in this instance?

?

The lifetime isn't used, this is where the error message comes from.

---

The question doesn't provide much context, but it does seem like you intend to only use A<'_> and B<'_> in your type. If that is the case, then you must use

pub enum Result<'a> {
    Ok(A<'a>),
    Err(B<'a>),
}

which is valid. However that would raise the question of why you're not using the built-in Result type, maybe combined with a type alias to avoid repetition: type Result<'a> = std::result::Result<A<'a>, B<'a>>;. This is pretty common to do, even the standard library does it a few times:

• std::io::Result: type Result<T> = Result<T, Error>;
• std::fmt::Result: type Result = Result<(), Error>;