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use crate::ops::ControlFlow; /// 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)] /// # #![feature(try_trait_transition)] /// # use std::ops::TryV2 as 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)] /// # #![feature(try_trait_transition)] /// # #![feature(control_flow_enum)] /// # use std::ops::{ControlFlow, TryV2 as 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)] /// # #![feature(try_trait_transition)] /// # #![feature(control_flow_enum)] /// # use std::ops::{ControlFlow, TryV2 as 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: /// ```compile_fail (enable again once ? converts to the new trait) /// # #![feature(try_trait_v2)] /// # #![feature(try_trait_transition)] /// # use std::ops::TryV2 as 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) /// } /// ``` #[unstable(feature = "try_trait_v2", issue = "84277")] pub trait Try: FromResidual { /// The type of the value produced by `?` when *not* short-circuiting. #[unstable(feature = "try_trait_v2", issue = "84277")] type Output; /// 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. #[unstable(feature = "try_trait_v2", issue = "84277")] type Residual; /// 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)] /// #![feature(control_flow_enum)] /// #![feature(try_trait_transition)] /// use std::ops::TryV2 as 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)); /// ``` #[unstable(feature = "try_trait_v2", issue = "84277")] fn from_output(output: Self::Output) -> Self; /// 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)] /// #![feature(control_flow_enum)] /// #![feature(try_trait_transition)] /// use std::ops::{ControlFlow, TryV2 as 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)), /// ); /// ``` #[unstable(feature = "try_trait_v2", issue = "84277")] fn branch(self) -> ControlFlow<Self::Residual, Self::Output>; } /// Used to specify which residuals can be converted into which [`Try`] types. /// /// Every `Try` type needs to be recreatable from its own associated /// `Residual` type, but can also have additional `FromResidual` implementations /// to support interconversion with other `Try` types. #[unstable(feature = "try_trait_v2", issue = "84277")] pub trait FromResidual<R = <Self as Try>::Residual> { /// Constructs the type from a compatible `Residual` type. /// /// This should be implemented consistently with the `branch` method such /// that applying the `?` operator will get back an equivalent residual: /// `FromResidual::from_residual(r).branch() --> ControlFlow::Break(r)`. /// (It may not be an *identical* residual when interconversion is involved.) /// /// # Examples /// /// ``` /// #![feature(try_trait_v2)] /// #![feature(control_flow_enum)] /// use std::ops::{ControlFlow, FromResidual}; /// /// assert_eq!(Result::<String, i64>::from_residual(Err(3_u8)), Err(3)); /// assert_eq!(Option::<String>::from_residual(None), None); /// assert_eq!( /// ControlFlow::<_, String>::from_residual(ControlFlow::Break(5)), /// ControlFlow::Break(5), /// ); /// ``` #[unstable(feature = "try_trait_v2", issue = "84277")] fn from_residual(residual: R) -> Self; }