Enum either::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

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[0].is_left(), true);
assert_eq!(values[1].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[0].is_right(), false);
assert_eq!(values[1].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 Pin<&Either<L, R>> to Either<Pin<&L>, Pin<&R>>, pinned projections of the inner variants.

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

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");

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);

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())));

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())));

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())));

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);

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);

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
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
🔬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
Returns an iterator over the contents of this reader split on the byte byte. Read more
Returns an iterator over the lines of this reader. 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
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
🔬This is a nightly-only experimental API. (iter_advance_by)
Advances the iterator from the back by n elements. Read more
Returns the nth 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

Either implements Error if both L and R implement it.

The lower-level source of this error, if any. Read more
👎Deprecated since 1.42.0:

use the Display impl or to_string()

Read more
👎Deprecated since 1.33.0:

replaced by Error::source, which can support downcasting

🔬This is a nightly-only experimental API. (error_generic_member_access)
Provides type based access to context intended for error reports. Read more
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.

Either<L, R> is a future if both L and R are futures.

The type of value produced on completion.
Attempt to resolve the future to a final value, registering the current task for wakeup if the value is not yet available. Read more
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.
Advances the iterator and returns the next value. Read more
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
Calls a closure on each element of an iterator. 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 nth element of the iterator. Read more
Transforms an iterator into a collection. Read more
Consumes an iterator, creating two collections from it. Read more
Tests if every element of the iterator matches a predicate. 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
Searches for an element in an iterator, returning its index. 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
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 skips 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
🔬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
🔬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
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 clones 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 the exact number of bytes required to fill buf. Read more
Read all bytes until EOF in this source, placing them into buf. Read more
Read all bytes until EOF in this source, appending them to 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 Reader has an efficient read_vectored implementation. 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 Seek if both L and R do.

Requires crate feature "use_std"

Seek to an offset, in bytes, in a stream. Read more
Rewind to the beginning of a stream. Read more
🔬This is a nightly-only experimental API. (seek_stream_len)
Returns the length of this stream (in bytes). Read more
Returns the current seek position from the start of the stream. 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
Attempts to write an entire buffer into this writer. Read more
Writes a formatted string into this writer, returning any error encountered. 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 Writer has an efficient write_vectored implementation. Read more
🔬This is a nightly-only experimental API. (write_all_vectored)
Attempts to write multiple buffers into this writer. Read more
Creates a “by reference” adapter for this instance of Write. Read more

Auto Trait Implementations

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 output that the future will produce on completion.
Which kind of future are we turning this into?
Creates a future from a value. Read more
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.