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Rollup merge of rust-lang#47193 - cramertj:result-opts, r=TimNN
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Add transpose conversions for nested Option and Result

These impls are useful when working with combinator
methods that expect an option or a result, but you
have a `Result<Option<T>, E>` instead of an `Option<Result<T, E>>`
or vice versa.
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GuillaumeGomez authored Jan 20, 2018
2 parents fe811eb + c9ae249 commit 0e270fc
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29 changes: 29 additions & 0 deletions src/libcore/option.rs
Original file line number Diff line number Diff line change
Expand Up @@ -881,6 +881,35 @@ impl<T: Default> Option<T> {
}
}

impl<T, E> Option<Result<T, E>> {
/// Transposes an `Option` of a `Result` into a `Result` of an `Option`.
///
/// `None` will be mapped to `Ok(None)`.
/// `Some(Ok(_))` and `Some(Err(_))` will be mapped to `Ok(Some(_))` and `Err(_)`.
///
/// # Examples
///
/// ```
/// #![feature(transpose_result)]
///
/// #[derive(Debug, Eq, PartialEq)]
/// struct SomeErr;
///
/// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
/// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
/// assert_eq!(x, y.transpose());
/// ```
#[inline]
#[unstable(feature = "transpose_result", issue = "47338")]
pub fn transpose(self) -> Result<Option<T>, E> {
match self {
Some(Ok(x)) => Ok(Some(x)),
Some(Err(e)) => Err(e),
None => Ok(None),
}
}
}

// This is a separate function to reduce the code size of .expect() itself.
#[inline(never)]
#[cold]
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29 changes: 29 additions & 0 deletions src/libcore/result.rs
Original file line number Diff line number Diff line change
Expand Up @@ -909,6 +909,35 @@ impl<T: Default, E> Result<T, E> {
}
}

impl<T, E> Result<Option<T>, E> {
/// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
///
/// `Ok(None)` will be mapped to `None`.
/// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
///
/// # Examples
///
/// ```
/// #![feature(transpose_result)]
///
/// #[derive(Debug, Eq, PartialEq)]
/// struct SomeErr;
///
/// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
/// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
/// assert_eq!(x.transpose(), y);
/// ```
#[inline]
#[unstable(feature = "transpose_result", issue = "47338")]
pub fn transpose(self) -> Option<Result<T, E>> {
match self {
Ok(Some(x)) => Some(Ok(x)),
Ok(None) => None,
Err(e) => Some(Err(e)),
}
}
}

// This is a separate function to reduce the code size of the methods
#[inline(never)]
#[cold]
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57 changes: 57 additions & 0 deletions src/test/run-pass/result-opt-conversions.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,57 @@
// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

#![feature(transpose_result)]

#[derive(Copy, Clone, Debug, PartialEq)]
struct BadNumErr;

fn try_num(x: i32) -> Result<i32, BadNumErr> {
if x <= 5 {
Ok(x + 1)
} else {
Err(BadNumErr)
}
}

type ResOpt = Result<Option<i32>, BadNumErr>;
type OptRes = Option<Result<i32, BadNumErr>>;

fn main() {
let mut x: ResOpt = Ok(Some(5));
let mut y: OptRes = Some(Ok(5));
assert_eq!(x, y.transpose());
assert_eq!(x.transpose(), y);

x = Ok(None);
y = None;
assert_eq!(x, y.transpose());
assert_eq!(x.transpose(), y);

x = Err(BadNumErr);
y = Some(Err(BadNumErr));
assert_eq!(x, y.transpose());
assert_eq!(x.transpose(), y);

let res: Result<Vec<i32>, BadNumErr> =
(0..10)
.map(|x| {
let y = try_num(x)?;
Ok(if y % 2 == 0 {
Some(y - 1)
} else {
None
})
})
.filter_map(Result::transpose)
.collect();

assert_eq!(res, Err(BadNumErr))
}

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