src/libcore/ops/try.rs

   1 // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 /// A trait for customizing the behavior of the `?` operator.
  12 ///
  13 /// A type implementing `Try` is one that has a canonical way to view it
  14 /// in terms of a success/failure dichotomy.  This trait allows both
  15 /// extracting those success or failure values from an existing instance and
  16 /// creating a new instance from a success or failure value.
  17 #[unstable(feature = "try_trait", issue = "42327")]
  18 #[rustc_on_unimplemented(
  19    on(all(
  20        any(from_method="from_error", from_method="from_ok"),
  21        from_desugaring="?"),
  22       message="the `?` operator can only be used in a \
  23                function that returns `Result` \
  24                (or another type that implements `{Try}`)",
  25       label="cannot use the `?` operator in a function that returns `{Self}`"),
  26    on(all(from_method="into_result", from_desugaring="?"),
  27       message="the `?` operator can only be applied to values \
  28                that implement `{Try}`",
  29       label="the `?` operator cannot be applied to type `{Self}`")
  30 )]
  31 pub trait Try {
  32     /// The type of this value when viewed as successful.
  33     #[unstable(feature = "try_trait", issue = "42327")]
  34     type Ok;
  35     /// The type of this value when viewed as failed.
  36     #[unstable(feature = "try_trait", issue = "42327")]
  37     type Error;
  38
  39     /// Applies the "?" operator. A return of `Ok(t)` means that the
  40     /// execution should continue normally, and the result of `?` is the
  41     /// value `t`. A return of `Err(e)` means that execution should branch
  42     /// to the innermost enclosing `catch`, or return from the function.
  43     ///
  44     /// If an `Err(e)` result is returned, the value `e` will be "wrapped"
  45     /// in the return type of the enclosing scope (which must itself implement
  46     /// `Try`). Specifically, the value `X::from_error(From::from(e))`
  47     /// is returned, where `X` is the return type of the enclosing function.
  48     #[unstable(feature = "try_trait", issue = "42327")]
  49     fn into_result(self) -> Result<Self::Ok, Self::Error>;
  50
  51     /// Wrap an error value to construct the composite result. For example,
  52     /// `Result::Err(x)` and `Result::from_error(x)` are equivalent.
  53     #[unstable(feature = "try_trait", issue = "42327")]
  54     fn from_error(v: Self::Error) -> Self;
  55
  56     /// Wrap an OK value to construct the composite result. For example,
  57     /// `Result::Ok(x)` and `Result::from_ok(x)` are equivalent.
  58     #[unstable(feature = "try_trait", issue = "42327")]
  59     fn from_ok(v: Self::Ok) -> Self;
  60 }