Trait Try

pub trait Try: FromResidual {
    type Output;
    type Residual;

    // Required methods
    fn from_output(output: Self::Output) -> Self;
    fn branch(self) -> ControlFlow<Self::Residual, Self::Output>;
}

🔬This is a nightly-only experimental API. (try_trait_v2)

The ? operator and try {} blocks.

try_* methods typically involve a type implementing this trait. For example, the closures passed to Iterator::try_fold and Iterator::try_for_each must return such a type.

Try types are typically those containing two or more categories of values, some subset of which are so commonly handled via early returns that it’s worth providing a terse (but still visible) syntax to make that easy.

This is most often seen for error handling with Result and Option. The quintessential implementation of this trait is on ControlFlow.

Using Try in Generic Code

Iterator::try_fold was stabilized to call back in Rust 1.27, but this trait is much newer. To illustrate the various associated types and methods, let’s implement our own version.

As a reminder, an infallible version of a fold looks something like this:

fn simple_fold<A, T>(
    iter: impl Iterator<Item = T>,
    mut accum: A,
    mut f: impl FnMut(A, T) -> A,
) -> A {
    for x in iter {
        accum = f(accum, x);
    }
    accum
}

So instead of f returning just an A, we’ll need it to return some other type that produces an A in the “don’t short circuit” path. Conveniently, that’s also the type we need to return from the function.

Let’s add a new generic parameter R for that type, and bound it to the output type that we want:

fn simple_try_fold_1<A, T, R: Try<Output = A>>(
    iter: impl Ite