# Enum either::Either

``````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

### Left(L)

A value of type `L`.

### Right(R)

A value of type `R`.

## Implementations

Return true if the value is the `Left` variant.

``````use either::*;

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

Return true if the value is the `Right` variant.

``````use either::*;

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

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);``````

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));``````

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"));``````

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));``````

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"));``````

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));``````

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));``````

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);``````

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]);``````

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));``````

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));``````

Convert the inner value to an iterator.

``````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]));``````

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");``````

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());``````

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");``````

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");``````

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);``````

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);``````

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();``````

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();``````

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");``````

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");``````

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!));
assert_eq!(left.factor_first().0, 123);

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

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!, 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);``````

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);``````

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));``````

## Trait Implementations

Converts this type into a mutable reference of the (usually inferred) input type.

Requires crate feature `use_std`.

Converts this type into a mutable reference of the (usually inferred) input type.

Requires crate feature `use_std`.

Converts this type into a mutable reference of the (usually inferred) input type.

Requires crate feature `use_std`.

Converts this type into a mutable reference of the (usually inferred) input type.
Converts this type into a mutable reference of the (usually inferred) input type.
Converts this type into a mutable reference of the (usually inferred) input type.
Converts this type into a shared reference of the (usually inferred) input type.

Requires crate feature `use_std`.

Converts this type into a shared reference of the (usually inferred) input type.

Requires crate feature `use_std`.

Converts this type into a shared reference of the (usually inferred) input type.

Requires crate feature `use_std`.

Converts this type into a shared reference of the (usually inferred) input type.
Converts this type into a shared reference of the (usually inferred) input type.
Converts this type into a shared reference of the (usually inferred) input type.

Requires crate feature `"use_std"`

Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty. Read more
Tells this buffer that `amt` bytes have been consumed from the buffer, so they should no longer be returned in calls to `read`. Read more
🔬This is a nightly-only experimental API. (`buf_read_has_data_left`)
Check if the underlying `Read` has any data left to be read. Read more
Read all bytes into `buf` until the delimiter `byte` or EOF is reached. Read more
Read all bytes until a newline (the `0xA` byte) is reached, and append them to the provided buffer. You do not need to clear the buffer before appending. Read more
Returns an iterator over the contents of this reader split on the byte `byte`. Read more
Returns a copy of the value. Read more
Performs copy-assignment from `source`. Read more
Formats the value using the given formatter. Read more
The resulting type after dereferencing.
Dereferences the value.
Mutably dereferences the value.
Formats the value using the given formatter. Read more
Removes and returns an element from the end of the iterator. Read more
🔬This is a nightly-only experimental API. (`iter_advance_by`)
Advances the iterator from the back by `n` elements. Read more
Returns the `n`th element from the end of the iterator. Read more
This is the reverse version of `Iterator::try_fold()`: it takes elements starting from the back of the iterator. Read more
An iterator method that reduces the iterator’s elements to a single, final value, starting from the back. Read more
Searches for an element of an iterator from the back that satisfies a predicate. Read more

`Either` implements `Error` if both `L` and `R` implement it.

👎Deprecated since 1.42.0: use the Display impl or to_string()
👎Deprecated since 1.33.0: replaced by Error::source, which can support downcasting
The lower-level source of this error, if any. Read more
🔬This is a nightly-only experimental API. (`error_generic_member_access`)
Returns the exact remaining length of the iterator. Read more
🔬This is a nightly-only experimental API. (`exact_size_is_empty`)
Returns `true` if the iterator is empty. Read more
Extends a collection with the contents of an iterator. Read more
🔬This is a nightly-only experimental API. (`extend_one`)
Extends a collection with exactly one element.
🔬This is a nightly-only experimental API. (`extend_one`)
Reserves capacity in a collection for the given number of additional elements. Read more

Convert from `Result` to `Either` with `Ok => Right` and `Err => Left`.

Converts to this type from the input type.
Feeds this value into the given `Hasher`. Read more
Feeds a slice of this type into the given `Hasher`. Read more

Convert from `Either` to `Result` with `Right => Ok` and `Left => Err`.

Converts this type into the (usually inferred) input type.

`Either<L, R>` is an iterator if both `L` and `R` are iterators.

The type of the elements being iterated over.
Returns the bounds on the remaining length of the iterator. Read more
Folds every element into an accumulator by applying an operation, returning the final result. Read more
Consumes the iterator, counting the number of iterations and returning it. Read more
Consumes the iterator, returning the last element. Read more
Returns the `n`th element of the iterator. Read more
Transforms an iterator into a collection. Read more
Tests if every element of the iterator matches a predicate. Read more
🔬This is a nightly-only experimental API. (`iter_next_chunk`)
Advances the iterator and returns an array containing the next `N` values. Read more
🔬This is a nightly-only experimental API. (`iter_advance_by`)
Advances the iterator by `n` elements. Read more
Creates an iterator starting at the same point, but stepping by the given amount at each iteration. Read more
Takes two iterators and creates a new iterator over both in sequence. Read more
‘Zips up’ two iterators into a single iterator of pairs. Read more
🔬This is a nightly-only experimental API. (`iter_intersperse`)
Creates a new iterator which places an item generated by `separator` between adjacent items of the original iterator. Read more
Takes a closure and creates an iterator which calls that closure on each element. Read more
Calls a closure on each element of an iterator. Read more
Creates an iterator which uses a closure to determine if an element should be yielded. Read more
Creates an iterator that both filters and maps. Read more
Creates an iterator which gives the current iteration count as well as the next value. Read more
Creates an iterator which can use the `peek` and `peek_mut` methods to look at the next element of the iterator without consuming it. See their documentation for more information. Read more
Creates an iterator that `skip`s elements based on a predicate. Read more
Creates an iterator that yields elements based on a predicate. Read more
Creates an iterator that both yields elements based on a predicate and maps. Read more
Creates an iterator that skips the first `n` elements. Read more
Creates an iterator that yields the first `n` elements, or fewer if the underlying iterator ends sooner. Read more
An iterator adapter similar to `fold` that holds internal state and produces a new iterator. Read more
Creates an iterator that works like map, but flattens nested structure. Read more
Creates an iterator which ends after the first `None`. Read more
Does something with each element of an iterator, passing the value on. Read more
Borrows an iterator, rather than consuming it. Read more
🔬This is a nightly-only experimental API. (`iter_collect_into`)
Collects all the items from an iterator into a collection. Read more
Consumes an iterator, creating two collections from it. Read more
🔬This is a nightly-only experimental API. (`iter_is_partitioned`)
Checks if the elements of this iterator are partitioned according to the given predicate, such that all those that return `true` precede all those that return `false`. Read more
An iterator method that applies a function as long as it returns successfully, producing a single, final value. Read more
An iterator method that applies a fallible function to each item in the iterator, stopping at the first error and returning that error. Read more
Reduces the elements to a single one, by repeatedly applying a reducing operation. Read more
🔬This is a nightly-only experimental API. (`iterator_try_reduce`)
Reduces the elements to a single one by repeatedly applying a reducing operation. If the closure returns a failure, the failure is propagated back to the caller immediately. Read more
Tests if any element of the iterator matches a predicate. Read more
Searches for an element of an iterator that satisfies a predicate. Read more
Applies function to the elements of iterator and returns the first non-none result. Read more
🔬This is a nightly-only experimental API. (`try_find`)
Applies function to the elements of iterator and returns the first true result or the first error. Read more
Searches for an element in an iterator, returning its index. Read more
Returns the element that gives the maximum value from the specified function. Read more
Returns the element that gives the maximum value with respect to the specified comparison function. Read more
Returns the element that gives the minimum value from the specified function. Read more
Returns the element that gives the minimum value with respect to the specified comparison function. Read more
Converts an iterator of pairs into a pair of containers. Read more
Creates an iterator which copies all of its elements. Read more
Creates an iterator which `clone`s all of its elements. Read more
🔬This is a nightly-only experimental API. (`iter_array_chunks`)
Returns an iterator over `N` elements of the iterator at a time. Read more
Sums the elements of an iterator. Read more
Iterates over the entire iterator, multiplying all the elements Read more
🔬This is a nightly-only experimental API. (`iter_order_by`)
Lexicographically compares the elements of this `Iterator` with those of another with respect to the specified comparison function. Read more
Lexicographically compares the elements of this `Iterator` with those of another. Read more
🔬This is a nightly-only experimental API. (`iter_order_by`)
Lexicographically compares the elements of this `Iterator` with those of another with respect to the specified comparison function. Read more
Determines if the elements of this `Iterator` are equal to those of another. Read more
🔬This is a nightly-only experimental API. (`iter_order_by`)
Determines if the elements of this `Iterator` are equal to those of another with respect to the specified equality function. Read more
Determines if the elements of this `Iterator` are unequal to those of another. Read more
Determines if the elements of this `Iterator` are lexicographically less than those of another. Read more
Determines if the elements of this `Iterator` are lexicographically less or equal to those of another. Read more
Determines if the elements of this `Iterator` are lexicographically greater than those of another. Read more
Determines if the elements of this `Iterator` are lexicographically greater than or equal to those of another. Read more
🔬This is a nightly-only experimental API. (`is_sorted`)
Checks if the elements of this iterator are sorted using the given comparator function. Read more
🔬This is a nightly-only experimental API. (`is_sorted`)
Checks if the elements of this iterator are sorted using the given key extraction function. Read more
This method returns an `Ordering` between `self` and `other`. Read more
Compares and returns the maximum of two values. Read more
Compares and returns the minimum of two values. Read more
Restrict a value to a certain interval. Read more
This method tests for `self` and `other` values to be equal, and is used by `==`. Read more
This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason. Read more
This method returns an ordering between `self` and `other` values if one exists. Read more
This method tests less than (for `self` and `other`) and is used by the `<` operator. Read more
This method tests less than or equal to (for `self` and `other`) and is used by the `<=` operator. Read more
This method tests greater than (for `self` and `other`) and is used by the `>` operator. Read more
This method tests greater than or equal to (for `self` and `other`) and is used by the `>=` operator. Read more

`Either<L, R>` implements `Read` if both `L` and `R` do.

Requires crate feature `"use_std"`

Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more
Read all bytes until EOF in this source, placing them into `buf`. Read more
Like `read`, except that it reads into a slice of buffers. Read more
🔬This is a nightly-only experimental API. (`can_vector`)
Determines if this `Read`er has an efficient `read_vectored` implementation. Read more
Read all bytes until EOF in this source, appending them to `buf`. Read more
Read the exact number of bytes required to fill `buf`. Read more
🔬This is a nightly-only experimental API. (`read_buf`)
Pull some bytes from this source into the specified buffer. Read more
🔬This is a nightly-only experimental API. (`read_buf`)
Read the exact number of bytes required to fill `cursor`. Read more
Creates a “by reference” adaptor for this instance of `Read`. Read more
Transforms this `Read` instance to an `Iterator` over its bytes. Read more
Creates an adapter which will chain this stream with another. Read more
Creates an adapter which will read at most `limit` bytes from it. Read more

`Either<L, R>` implements `Write` if both `L` and `R` do.

Requires crate feature `"use_std"`

Write a buffer into this writer, returning how many bytes were written. Read more
Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more
Like `write`, except that it writes from a slice of buffers. Read more
🔬This is a nightly-only experimental API. (`can_vector`)
Determines if this `Write`r has an efficient `write_vectored` implementation. Read more
Attempts to write an entire buffer into this writer. Read more
🔬This is a nightly-only experimental API. (`write_all_vectored`)
Attempts to write multiple buffers into this writer. Read more
Writes a formatted string into this writer, returning any error encountered. Read more
Creates a “by reference” adapter for this instance of `Write`. Read more

## Blanket Implementations

Gets the `TypeId` of `self`. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

The type of the elements being iterated over.
Which kind of iterator are we turning this into?
Creates an iterator from a value. Read more
🔬This is a nightly-only experimental API. (`provide_any`)
Data providers should implement this method to provide all values they are able to provide by using `demand`. Read more
The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
Converts the given value to a `String`. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.