Enum Either

Source

pub enum Either<L, R> {
    Left(L),
    Right(R),
}

Expand description

The enum Either with variants Left and Right is a general purpose sum type with two cases.

The Either type is symmetric and treats its variants the same way, without preference. (For representing success or error, use the regular Result enum instead.)

Variants§

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Left(L)

A value of type L.

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Right(R)

A value of type R.

Implementations§

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impl<L, R> Either<L, R>

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pub fn is_left(&self) -> bool

Return true if the value is the Left variant.

use either::*;

let values = [Left(1), Right("the right value")];
assert_eq!(values[0].is_left(), true);
assert_eq!(values[1].is_left(), false);

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pub fn is_right(&self) -> bool

Return true if the value is the Right variant.

use either::*;

let values = [Left(1), Right("the right value")];
assert_eq!(values[0].is_right(), false);
assert_eq!(values[1].is_right(), true);

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pub fn left(self) -> Option<L>

Convert the left side of Either<L, R> to an Option<L>.

use either::*;

let left: Either<_, ()> = Left("some value");
assert_eq!(left.left(),  Some("some value"));

let right: Either<(), _> = Right(321);
assert_eq!(right.left(), None);

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pub fn right(self) -> Option<R>

Convert the right side of Either<L, R> to an Option<R>.

use either::*;

let left: Either<_, ()> = Left("some value");
assert_eq!(left.right(),  None);

let right: Either<(), _> = Right(321);
assert_eq!(right.right(), Some(321));

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pub fn as_ref(&self) -> Either<&L, &R> ⓘ

Convert &Either<L, R> to Either<&L, &R>.

use either::*;

let left: Either<_, ()> = Left("some value");
assert_eq!(left.as_ref(), Left(&"some value"));

let right: Either<(), _> = Right("some value");
assert_eq!(right.as_ref(), Right(&"some value"));

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pub fn as_mut(&mut self) -> Either<&mut L, &mut R> ⓘ

Convert &mut Either<L, R> to Either<&mut L, &mut R>.

use either::*;

fn mutate_left(value: &mut Either<u32, u32>) {
    if let Some(l) = value.as_mut().left() {
        *l = 999;
    }
}

let mut left = Left(123);
let mut right = Right(123);
mutate_left(&mut left);
mutate_left(&mut right);
assert_eq!(left, Left(999));
assert_eq!(right, Right(123));

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pub fn as_pin_ref(self: Pin<&Either<L, R>>) -> Either<Pin<&L>, Pin<&R>> ⓘ

Convert Pin<&Either<L, R>> to Either<Pin<&L>, Pin<&R>>, pinned projections of the inner variants.

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pub fn as_pin_mut( self: Pin<&mut Either<L, R>>, ) -> Either<Pin<&mut L>, Pin<&mut R>> ⓘ

Convert Pin<&mut Either<L, R>> to Either<Pin<&mut L>, Pin<&mut R>>, pinned projections of the inner variants.

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pub fn flip(self) -> Either<R, L> ⓘ

Convert Either<L, R> to Either<R, L>.

use either::*;

let left: Either<_, ()> = Left(123);
assert_eq!(left.flip(), Right(123));

let right: Either<(), _> = Right("some value");
assert_eq!(right.flip(), Left("some value"));

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pub fn map_left<F, M>(self, f: F) -> Either<M, R> ⓘ
where F: FnOnce(L) -> M,

Apply the function f on the value in the Left variant if it is present rewrapping the result in Left.

use either::*;

let left: Either<_, u32> = Left(123);
assert_eq!(left.map_left(|x| x * 2), Left(246));

let right: Either<u32, _> = Right(123);
assert_eq!(right.map_left(|x| x * 2), Right(123));

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pub fn map_right<F, S>(self, f: F) -> Either<L, S> ⓘ
where F: FnOnce(R) -> S,

Apply the function f on the value in the Right variant if it is present rewrapping the result in Right.

use either::*;

let left: Either<_, u32> = Left(123);
assert_eq!(left.map_right(|x| x * 2), Left(123));

let right: Either<u32, _> = Right(123);
assert_eq!(right.map_right(|x| x * 2), Right(246));

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pub fn map_either<F, G, M, S>(self, f: F, g: G) -> Either<M, S> ⓘ
where F: FnOnce(L) -> M, G: FnOnce(R) -> S,

Apply the functions f and g to the Left and Right variants respectively. This is equivalent to bimap in functional programming.

use either::*;

let f = |s: String| s.len();
let g = |u: u8| u.to_string();

let left: Either<String, u8> = Left("loopy".into());
assert_eq!(left.map_either(f, g), Left(5));

let right: Either<String, u8> = Right(42);
assert_eq!(right.map_either(f, g), Right("42".into()));

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pub fn map_either_with<Ctx, F, G, M, S>( self, ctx: Ctx, f: F, g: G, ) -> Either<M, S> ⓘ
where F: FnOnce(Ctx, L) -> M, G: FnOnce(Ctx, R) -> S,

Similar to map_either, with an added context ctx accessible to both functions.

use either::*;

let mut sum = 0;

// Both closures want to update the same value, so pass it as context.
let mut f = |sum: &mut usize, s: String| { *sum += s.len(); s.to_uppercase() };
let mut g = |sum: &mut usize, u: usize| { *sum += u; u.to_string() };

let left: Either<String, usize> = Left("loopy".into());
assert_eq!(left.map_either_with(&mut sum, &mut f, &mut g), Left("LOOPY".into()));

let right: Either<String, usize> = Right(42);
assert_eq!(right.map_either_with(&mut sum, &mut f, &mut g), Right("42".into()));

assert_eq!(sum, 47);

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pub fn either<F, G, T>(self, f: F, g: G) -> T
where F: FnOnce(L) -> T, G: FnOnce(R) -> T,

Apply one of two functions depending on contents, unifying their result. If the value is Left(L) then the first function f is applied; if it is Right(R) then the second function g is applied.

use either::*;

fn square(n: u32) -> i32 { (n * n) as i32 }
fn negate(n: i32) -> i32 { -n }

let left: Either<u32, i32> = Left(4);
assert_eq!(left.either(square, negate), 16);

let right: Either<u32, i32> = Right(-4);
assert_eq!(right.either(square, negate), 4);

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pub fn either_with<Ctx, F, G, T>(self, ctx: Ctx, f: F, g: G) -> T
where F: FnOnce(Ctx, L) -> T, G: FnOnce(Ctx, R) -> T,

Like either, but provide some context to whichever of the functions ends up being called.

// In this example, the context is a mutable reference
use either::*;

let mut result = Vec::new();

let values = vec![Left(2), Right(2.7)];

for value in values {
    value.either_with(&mut result,
                      |ctx, integer| ctx.push(integer),
                      |ctx, real| ctx.push(f64::round(real) as i32));
}

assert_eq!(result, vec![2, 3]);

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pub fn left_and_then<F, S>(self, f: F) -> Either<S, R> ⓘ
where F: FnOnce(L) -> Either<S, R>,

Apply the function f on the value in the Left variant if it is present.

use either::*;

let left: Either<_, u32> = Left(123);
assert_eq!(left.left_and_then::<_,()>(|x| Right(x * 2)), Right(246));

let right: Either<u32, _> = Right(123);
assert_eq!(right.left_and_then(|x| Right::<(), _>(x * 2)), Right(123));

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pub fn right_and_then<F, S>(self, f: F) -> Either<L, S> ⓘ
where F: FnOnce(R) -> Either<L, S>,

Apply the function f on the value in the Right variant if it is present.

use either::*;

let left: Either<_, u32> = Left(123);
assert_eq!(left.right_and_then(|x| Right(x * 2)), Left(123));

let right: Either<u32, _> = Right(123);
assert_eq!(right.right_and_then(|x| Right(x * 2)), Right(246));

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pub fn into_iter( self, ) -> Either<<L as IntoIterator>::IntoIter, <R as IntoIterator>::IntoIter> ⓘ
where L: IntoIterator, R: IntoIterator<Item = <L as IntoIterator>::Item>,

Convert the inner value to an iterator.

This requires the Left and Right iterators to have the same item type. See factor_into_iter to iterate different types.

use either::*;

let left: Either<_, Vec<u32>> = Left(vec![1, 2, 3, 4, 5]);
let mut right: Either<Vec<u32>, _> = Right(vec![]);
right.extend(left.into_iter());
assert_eq!(right, Right(vec![1, 2, 3, 4, 5]));

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pub fn iter( &self, ) -> Either<<&L as IntoIterator>::IntoIter, <&R as IntoIterator>::IntoIter> ⓘ
where &'a L: for<'a> IntoIterator, &'a R: for<'a> IntoIterator<Item = <&'a L as IntoIterator>::Item>,

Borrow the inner value as an iterator.

This requires the Left and Right iterators to have the same item type. See factor_iter to iterate different types.

use either::*;

let left: Either<_, &[u32]> = Left(vec![2, 3]);
let mut right: Either<Vec<u32>, _> = Right(&[4, 5][..]);
let mut all = vec![1];
all.extend(left.iter());
all.extend(right.iter());
assert_eq!(all, vec![1, 2, 3, 4, 5]);

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pub fn iter_mut( &mut self, ) -> Either<<&mut L as IntoIterator>::IntoIter, <&mut R as IntoIterator>::IntoIter> ⓘ
where &'a mut L: for<'a> IntoIterator, &'a mut R: for<'a> IntoIterator<Item = <&'a mut L as IntoIterator>::Item>,

Mutably borrow the inner value as an iterator.

This requires the Left and Right iterators to have the same item type. See factor_iter_mut to iterate different types.

use either::*;

let mut left: Either<_, &mut [u32]> = Left(vec![2, 3]);
for l in left.iter_mut() {
    *l *= *l
}
assert_eq!(left, Left(vec![4, 9]));

let mut inner = [4, 5];
let mut right: Either<Vec<u32>, _> = Right(&mut inner[..]);
for r in right.iter_mut() {
    *r *= *r
}
assert_eq!(inner, [16, 25]);

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pub fn factor_into_iter( self, ) -> IterEither<<L as IntoIterator>::IntoIter, <R as IntoIterator>::IntoIter>
where L: IntoIterator, R: IntoIterator,

Converts an Either of Iterators to be an Iterator of Eithers

Unlike into_iter, this does not require the Left and Right iterators to have the same item type.

use either::*;
let left: Either<_, Vec<u8>> = Left(&["hello"]);
assert_eq!(left.factor_into_iter().next(), Some(Left(&"hello")));

let right: Either<&[&str], _> = Right(vec![0, 1]);
assert_eq!(right.factor_into_iter().collect::<Vec<_>>(), vec![Right(0), Right(1)]);

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pub fn factor_iter( &self, ) -> IterEither<<&L as IntoIterator>::IntoIter, <&R as IntoIterator>::IntoIter>
where &'a L: for<'a> IntoIterator, &'a R: for<'a> IntoIterator,

Borrows an Either of Iterators to be an Iterator of Eithers

Unlike iter, this does not require the Left and Right iterators to have the same item type.

use either::*;
let left: Either<_, Vec<u8>> = Left(["hello"]);
assert_eq!(left.factor_iter().next(), Some(Left(&"hello")));

let right: Either<[&str; 2], _> = Right(vec![0, 1]);
assert_eq!(right.factor_iter().collect::<Vec<_>>(), vec![Right(&0), Right(&1)]);

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pub fn factor_iter_mut( &mut self, ) -> IterEither<<&mut L as IntoIterator>::IntoIter, <&mut R as IntoIterator>::IntoIter>
where &'a mut L: for<'a> IntoIterator, &'a mut R: for<'a> IntoIterator,

Mutably borrows an Either of Iterators to be an Iterator of Eithers

Unlike iter_mut, this does not require the Left and Right iterators to have the same item type.

use either::*;
let mut left: Either<_, Vec<u8>> = Left(["hello"]);
left.factor_iter_mut().for_each(|x| *x.unwrap_left() = "goodbye");
assert_eq!(left, Left(["goodbye"]));

let mut right: Either<[&str; 2], _> = Right(vec![0, 1, 2]);
right.factor_iter_mut().for_each(|x| if let Right(r) = x { *r = -*r; });
assert_eq!(right, Right(vec![0, -1, -2]));

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pub fn left_or(self, other: L) -> L

Return left value or given value

Arguments passed to left_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use left_or_else, which is lazily evaluated.

§Examples

let left: Either<&str, &str> = Left("left");
assert_eq!(left.left_or("foo"), "left");

let right: Either<&str, &str> = Right("right");
assert_eq!(right.left_or("left"), "left");

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pub fn left_or_default(self) -> L
where L: Default,

Return left or a default

§Examples

let left: Either<String, u32> = Left("left".to_string());
assert_eq!(left.left_or_default(), "left");

let right: Either<String, u32> = Right(42);
assert_eq!(right.left_or_default(), String::default());

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pub fn left_or_else<F>(self, f: F) -> L
where F: FnOnce(R) -> L,

Returns left value or computes it from a closure

§Examples

let left: Either<String, u32> = Left("3".to_string());
assert_eq!(left.left_or_else(|_| unreachable!()), "3");

let right: Either<String, u32> = Right(3);
assert_eq!(right.left_or_else(|x| x.to_string()), "3");

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pub fn right_or(self, other: R) -> R

Return right value or given value

Arguments passed to right_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use right_or_else, which is lazily evaluated.

§Examples

let right: Either<&str, &str> = Right("right");
assert_eq!(right.right_or("foo"), "right");

let left: Either<&str, &str> = Left("left");
assert_eq!(left.right_or("right"), "right");

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pub fn right_or_default(self) -> R
where R: Default,

Return right or a default

§Examples

let left: Either<String, u32> = Left("left".to_string());
assert_eq!(left.right_or_default(), u32::default());

let right: Either<String, u32> = Right(42);
assert_eq!(right.right_or_default(), 42);

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pub fn right_or_else<F>(self, f: F) -> R
where F: FnOnce(L) -> R,

Returns right value or computes it from a closure

§Examples

let left: Either<String, u32> = Left("3".to_string());
assert_eq!(left.right_or_else(|x| x.parse().unwrap()), 3);

let right: Either<String, u32> = Right(3);
assert_eq!(right.right_or_else(|_| unreachable!()), 3);

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pub fn unwrap_left(self) -> L
where R: Debug,

Returns the left value

§Examples

let left: Either<_, ()> = Left(3);
assert_eq!(left.unwrap_left(), 3);

§Panics

When Either is a Right value

let right: Either<(), _> = Right(3);
right.unwrap_left();

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pub fn unwrap_right(self) -> R
where L: Debug,

Returns the right value

§Examples

let right: Either<(), _> = Right(3);
assert_eq!(right.unwrap_right(), 3);

§Panics

When Either is a Left value

let left: Either<_, ()> = Left(3);
left.unwrap_right();

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pub fn expect_left(self, msg: &str) -> L
where R: Debug,

Returns the left value

§Examples

let left: Either<_, ()> = Left(3);
assert_eq!(left.expect_left("value was Right"), 3);

§Panics

When Either is a Right value

let right: Either<(), _> = Right(3);
right.expect_left("value was Right");

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pub fn expect_right(self, msg: &str) -> R
where L: Debug,

Returns the right value

§Examples

let right: Either<(), _> = Right(3);
assert_eq!(right.expect_right("value was Left"), 3);

§Panics

When Either is a Left value

let left: Either<_, ()> = Left(3);
left.expect_right("value was Right");

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pub fn either_into<T>(self) -> T
where L: Into<T>, R: Into<T>,

Convert the contained value into T

§Examples

// Both u16 and u32 can be converted to u64.
let left: Either<u16, u32> = Left(3u16);
assert_eq!(left.either_into::<u64>(), 3u64);
let right: Either<u16, u32> = Right(7u32);
assert_eq!(right.either_into::<u64>(), 7u64);

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impl<L, R> Either<Option<L>, Option<R>>

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pub fn factor_none(self) -> Option<Either<L, R>>

Factors out None from an Either of Option.

use either::*;
let left: Either<_, Option<String>> = Left(Some(vec![0]));
assert_eq!(left.factor_none(), Some(Left(vec![0])));

let right: Either<Option<Vec<u8>>, _> = Right(Some(String::new()));
assert_eq!(right.factor_none(), Some(Right(String::new())));

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impl<L, R, E> Either<Result<L, E>, Result<R, E>>

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pub fn factor_err(self) -> Result<Either<L, R>, E>

Factors out a homogenous type from an Either of Result.

Here, the homogeneous type is the Err type of the Result.

use either::*;
let left: Either<_, Result<String, u32>> = Left(Ok(vec![0]));
assert_eq!(left.factor_err(), Ok(Left(vec![0])));

let right: Either<Result<Vec<u8>, u32>, _> = Right(Ok(String::new()));
assert_eq!(right.factor_err(), Ok(Right(String::new())));

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impl<T, L, R> Either<Result<T, L>, Result<T, R>>

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pub fn factor_ok(self) -> Result<T, Either<L, R>>

Factors out a homogenous type from an Either of Result.

Here, the homogeneous type is the Ok type of the Result.

use either::*;
let left: Either<_, Result<u32, String>> = Left(Err(vec![0]));
assert_eq!(left.factor_ok(), Err(Left(vec![0])));

let right: Either<Result<u32, Vec<u8>>, _> = Right(Err(String::new()));
assert_eq!(right.factor_ok(), Err(Right(String::new())));

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impl<T, L, R> Either<(T, L), (T, R)>

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pub fn factor_first(self) -> (T, Either<L, R>)

Factor out a homogeneous type from an either of pairs.

Here, the homogeneous type is the first element of the pairs.

use either::*;
let left: Either<_, (u32, String)> = Left((123, vec![0]));
assert_eq!(left.factor_first().0, 123);

let right: Either<(u32, Vec<u8>), _> = Right((123, String::new()));
assert_eq!(right.factor_first().0, 123);

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impl<T, L, R> Either<(L, T), (R, T)>

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pub fn factor_second(self) -> (Either<L, R>, T)

Factor out a homogeneous type from an either of pairs.

Here, the homogeneous type is the second element of the pairs.

use either::*;
let left: Either<_, (String, u32)> = Left((vec![0], 123));
assert_eq!(left.factor_second().1, 123);

let right: Either<(Vec<u8>, u32), _> = Right((String::new(), 123));
assert_eq!(right.factor_second().1, 123);

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impl<T> Either<T, T>

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pub fn into_inner(self) -> T

Extract the value of an either over two equivalent types.

use either::*;

let left: Either<_, u32> = Left(123);
assert_eq!(left.into_inner(), 123);

let right: Either<u32, _> = Right(123);
assert_eq!(right.into_inner(), 123);

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pub fn map<F, M>(self, f: F) -> Either<M, M> ⓘ
where F: FnOnce(T) -> M,

Map f over the contained value and return the result in the corresponding variant.

use either::*;

let value: Either<_, i32> = Right(42);

let other = value.map(|x| x * 2);
assert_eq!(other, Right(84));

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impl<L, R> Either<&L, &R>

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pub fn cloned(self) -> Either<L, R> ⓘ
where L: Clone, R: Clone,

Maps an Either<&L, &R> to an Either<L, R> by cloning the contents of either branch.

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pub fn copied(self) -> Either<L, R> ⓘ
where L: Copy, R: Copy,

Maps an Either<&L, &R> to an Either<L, R> by copying the contents of either branch.

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impl<L, R> Either<&mut L, &mut R>

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pub fn cloned(self) -> Either<L, R> ⓘ
where L: Clone, R: Clone,

Maps an Either<&mut L, &mut R> to an Either<L, R> by cloning the contents of either branch.

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pub fn copied(self) -> Either<L, R> ⓘ
where L: Copy, R: Copy,

Maps an Either<&mut L, &mut R> to an Either<L, R> by copying the contents of either branch.

Trait Implementations§

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