Trait Try

trait Try: ~const FromResidual

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:

# #![feature(try_trait_v2)]
# use std::ops::Try;
fn simple_try_fold_1<A, T, R: Try<Output = A>>(
    iter: impl Iterator<Item = T>,
    mut accum: A,
    mut f: impl FnMut(A, T) -> R,
) -> R {
    todo!()
}

If we get through the entire iterator, we need to wrap up the accumulator into the return type using [Try::from_output]:

# #![feature(try_trait_v2)]
# use std::ops::{ControlFlow, Try};
fn simple_try_fold_2<A, T, R: Try<Output = A>>(
    iter: impl Iterator<Item = T>,
    mut accum: A,
    mut f: impl FnMut(A, T) -> R,
) -> R {
    for x in iter {
        let cf = f(accum, x).branch();
        match cf {
            ControlFlow::Continue(a) => accum = a,
            ControlFlow::Break(_) => todo!(),
        }
    }
    R::from_output(accum)
}

We'll also need FromResidual::from_residual to turn the residual back into the original type. But because it's a supertrait of Try, we don't need to mention it in the bounds. All types which implement Try can be recreated from their corresponding residual, so we'll just call it:

# #![feature(try_trait_v2)]
# use std::ops::{ControlFlow, Try};
pub fn simple_try_fold_3<A, T, R: Try<Output = A>>(
    iter: impl Iterator<Item = T>,
    mut accum: A,
    mut f: impl FnMut(A, T) -> R,
) -> R {
    for x in iter {
        let cf = f(accum, x).branch();
        match cf {
            ControlFlow::Continue(a) => accum = a,
            ControlFlow::Break(r) => return R::from_residual(r),
        }
    }
    R::from_output(accum)
}

But this "call branch, then match on it, and return if it was a Break" is exactly what happens inside the ? operator. So rather than do all this manually, we can just use ? instead:

# #![feature(try_trait_v2)]
# use std::ops::Try;
fn simple_try_fold<A, T, R: Try<Output = A>>(
    iter: impl Iterator<Item = T>,
    mut accum: A,
    mut f: impl FnMut(A, T) -> R,
) -> R {
    for x in iter {
        accum = f(accum, x)?;
    }
    R::from_output(accum)
}

Associated Types

type Output

The type of the value produced by ? when not short-circuiting.

type Residual

The type of the value passed to FromResidual::from_residual as part of ? when short-circuiting.

This represents the possible values of the Self type which are not represented by the Output type.

Note to Implementors

The choice of this type is critical to interconversion. Unlike the Output type, which will often be a raw generic type, this type is typically a newtype of some sort to "color" the type so that it's distinguishable from the residuals of other types.

This is why Result<T, E>::Residual is not E, but Result<Infallible, E>. That way it's distinct from ControlFlow<E>::Residual, for example, and thus ? on ControlFlow cannot be used in a method returning Result.

If you're making a generic type Foo<T> that implements Try<Output = T>, then typically you can use Foo<std::convert::Infallible> as its Residual type: that type will have a "hole" in the correct place, and will maintain the "foo-ness" of the residual so other types need to opt-in to interconversion.

Required Methods

fn from_output(output: <Self as >::Output) -> Self

Constructs the type from its Output type.

This should be implemented consistently with the branch method such that applying the ? operator will get back the original value: Try::from_output(x).branch() --> ControlFlow::Continue(x).

Examples

#![feature(try_trait_v2)]
use std::ops::Try;

assert_eq!(<Result<_, String> as Try>::from_output(3), Ok(3));
assert_eq!(<Option<_> as Try>::from_output(4), Some(4));
assert_eq!(
    <std::ops::ControlFlow<String, _> as Try>::from_output(5),
    std::ops::ControlFlow::Continue(5),
);

# fn make_question_mark_work() -> Option<()> {
assert_eq!(Option::from_output(4)?, 4);
# None }
# make_question_mark_work();

// This is used, for example, on the accumulator in `try_fold`:
let r = std::iter::empty().try_fold(4, |_, ()| -> Option<_> { unreachable!() });
assert_eq!(r, Some(4));

fn branch(self: Self) -> ControlFlow<<Self as >::Residual, <Self as >::Output>

Used in ? to decide whether the operator should produce a value (because this returned ControlFlow::Continue) or propagate a value back to the caller (because this returned ControlFlow::Break).

Examples

#![feature(try_trait_v2)]
use std::ops::{ControlFlow, Try};

assert_eq!(Ok::<_, String>(3).branch(), ControlFlow::Continue(3));
assert_eq!(Err::<String, _>(3).branch(), ControlFlow::Break(Err(3)));

assert_eq!(Some(3).branch(), ControlFlow::Continue(3));
assert_eq!(None::<String>.branch(), ControlFlow::Break(None));

assert_eq!(ControlFlow::<String, _>::Continue(3).branch(), ControlFlow::Continue(3));
assert_eq!(
    ControlFlow::<_, String>::Break(3).branch(),
    ControlFlow::Break(ControlFlow::Break(3)),
);

Implementors