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>;
}try_trait_v2)Expand description
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 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:
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:
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:
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)
}Required Associated Types§
sourcetype Output
type Output
try_trait_v2)The type of the value produced by ? when not short-circuiting.
sourcetype Residual
type Residual
try_trait_v2)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§
sourcefn from_output(output: Self::Output) -> Self
fn from_output(output: Self::Output) -> Self
try_trait_v2)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),
);
assert_eq!(Option::from_output(4)?, 4);
// 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));sourcefn branch(self) -> ControlFlow<Self::Residual, Self::Output>
fn branch(self) -> ControlFlow<Self::Residual, Self::Output>
try_trait_v2)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)),
);