Data.Either

The Either type, and associated operations.

Documentation

data Either a b

The Either type represents values with two possibilities: a value of type Either a b is either Left a or Right b.

The Either type is sometimes used to represent a value which is either correct or an error; by convention, the Left constructor is used to hold an error value and the Right constructor is used to hold a correct value (mnemonic: "right" also means "correct").

Constructors

Left a

Right b

Instances

Monad (Either e)

Functor (Either a)

MonadFix (Either e)

Applicative (Either e)

Foldable (Either a)

Traversable (Either a)

Generic1 (Either a)

(Eq a, Eq b) => Eq (Either a b)

(Data a, Data b) => Data (Either a b)

(Ord a, Ord b) => Ord (Either a b)

(Read a, Read b) => Read (Either a b)

(Show a, Show b) => Show (Either a b)

Generic (Either a b)

Typeable (* -> * -> *) Either

type Rep1 (Either a)

type Rep (Either a b)

type (==) (Either k k1) a b

either :: (a -> c) -> (b -> c) -> Either a b -> c

Case analysis for the Either type. If the value is Left a, apply the first function to a; if it is Right b, apply the second function to b.

lefts :: [Either a b] -> [a]

Extracts from a list of Either all the Left elements All the Left elements are extracted in order.

rights :: [Either a b] -> [b]

Extracts from a list of Either all the Right elements All the Right elements are extracted in order.

isLeft :: Either a b -> Bool

Return True if the given value is a Left-value, False otherwise.

Since: 4.7.0.0

isRight :: Either a b -> Bool

Return True if the given value is a Right-value, False otherwise.

Since: 4.7.0.0

partitionEithers :: [Either a b] -> ([a], [b])

Partitions a list of Either into two lists All the Left elements are extracted, in order, to the first component of the output. Similarly the Right elements are extracted to the second component of the output.