Either

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").

Examples

The type Either String Int is the type of values which can be either a String or an Int. The Left constructor can be used only on Strings, and the Right constructor can be used only on Ints:

>>> let s = Left "foo" :: Either String Int
>>> s
Left "foo"
>>> let n = Right 3 :: Either String Int
>>> n
Right 3
>>> :type s
s :: Either String Int
>>> :type n
n :: Either String Int

The fmap from our Functor instance will ignore Left values, but will apply the supplied function to values contained in a Right:

>>> let s = Left "foo" :: Either String Int
>>> let n = Right 3 :: Either String Int
>>> fmap (*2) s
Left "foo"
>>> fmap (*2) n
Right 6

The Monad instance for Either allows us to chain together multiple actions which may fail, and fail overall if any of the individual steps failed. First we'll write a function that can either parse an Int from a Char, or fail.

>>> import Data.Char ( digitToInt, isDigit )
>>> :{
    let parseEither :: Char -> Either String Int
        parseEither c
          | isDigit c = Right (digitToInt c)
          | otherwise = Left "parse error"
>>> :}

The following should work, since both '1' and '2' can be parsed as Ints.

>>> :{
    let parseMultiple :: Either String Int
        parseMultiple = do
          x <- parseEither '1'
          y <- parseEither '2'
          return (x + y)
>>> :}

>>> parseMultiple
Right 3

But the following should fail overall, since the first operation where we attempt to parse 'm' as an Int will fail:

>>> :{
    let parseMultiple :: Either String Int
        parseMultiple = do
          x <- parseEither 'm'
          y <- parseEither '2'
          return (x + y)
>>> :}

>>> parseMultiple
Left "parse error"

Constructors

Left a

Right b

Instances

ToJSON2 Either

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON2 :: (a -> Value) -> ([a] -> Value) -> (b -> Value) -> ([b] -> Value) -> Either a b -> Value

liftToJSONList2 :: (a -> Value) -> ([a] -> Value) -> (b -> Value) -> ([b] -> Value) -> [Either a b] -> Value

liftToEncoding2 :: (a -> Encoding) -> ([a] -> Encoding) -> (b -> Encoding) -> ([b] -> Encoding) -> Either a b -> Encoding

liftToEncodingList2 :: (a -> Encoding) -> ([a] -> Encoding) -> (b -> Encoding) -> ([b] -> Encoding) -> [Either a b] -> Encoding

FromJSON2 Either

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON2 :: (Value -> Parser a) -> (Value -> Parser [a]) -> (Value -> Parser b) -> (Value -> Parser [b]) -> Value -> Parser (Either a b)

liftParseJSONList2 :: (Value -> Parser a) -> (Value -> Parser [a]) -> (Value -> Parser b) -> (Value -> Parser [b]) -> Value -> Parser [Either a b]

Bitraversable Either

Since: base-4.10.0.0

Defined in Data.Bitraversable

Methods

bitraverse :: Applicative f => (a -> f c) -> (b -> f d) -> Either a b -> f (Either c d)

Bifoldable Either

Since: base-4.10.0.0

Defined in Data.Bifoldable

Methods

bifold :: Monoid m => Either m m -> m

bifoldMap :: Monoid m => (a -> m) -> (b -> m) -> Either a b -> m

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

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

Bifunctor Either

Since: base-4.8.0.0

Defined in Data.Bifunctor

Methods

bimap :: (a -> b) -> (c -> d) -> Either a c -> Either b d

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

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

Eq2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftEq2 :: (a -> b -> Bool) -> (c -> d -> Bool) -> Either a c -> Either b d -> Bool

Ord2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftCompare2 :: (a -> b -> Ordering) -> (c -> d -> Ordering) -> Either a c -> Either b d -> Ordering

Read2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftReadsPrec2 :: (Int -> ReadS a) -> ReadS [a] -> (Int -> ReadS b) -> ReadS [b] -> Int -> ReadS (Either a b)

liftReadList2 :: (Int -> ReadS a) -> ReadS [a] -> (Int -> ReadS b) -> ReadS [b] -> ReadS [Either a b]

liftReadPrec2 :: ReadPrec a -> ReadPrec [a] -> ReadPrec b -> ReadPrec [b] -> ReadPrec (Either a b)

liftReadListPrec2 :: ReadPrec a -> ReadPrec [a] -> ReadPrec b -> ReadPrec [b] -> ReadPrec [Either a b]

Show2 Either

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftShowsPrec2 :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> (Int -> b -> ShowS) -> ([b] -> ShowS) -> Int -> Either a b -> ShowS

liftShowList2 :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> (Int -> b -> ShowS) -> ([b] -> ShowS) -> [Either a b] -> ShowS

NFData2 Either

Since: deepseq-1.4.3.0

Defined in Control.DeepSeq

Methods

liftRnf2 :: (a -> ()) -> (b -> ()) -> Either a b -> ()

Hashable2 Either

Defined in Data.Hashable.Class

Methods

liftHashWithSalt2 :: (Int -> a -> Int) -> (Int -> b -> Int) -> Int -> Either a b -> Int

Bitraversable1 Either

Defined in Data.Semigroup.Traversable.Class

Methods

bitraverse1 :: Apply f => (a -> f b) -> (c -> f d) -> Either a c -> f (Either b d)

bisequence1 :: Apply f => Either (f a) (f b) -> f (Either a b)

Swapped Either

Defined in Control.Lens.Iso

Methods

swapped :: (Profunctor p, Functor f) => p (Either b a) (f (Either d c)) -> p (Either a b) (f (Either c d))

Bifoldable1 Either

Defined in Data.Semigroup.Foldable.Class

Methods

bifold1 :: Semigroup m => Either m m -> m

bifoldMap1 :: Semigroup m => (a -> m) -> (b -> m) -> Either a b -> m

() :=> (Monad (Either a))

Defined in Data.Constraint

Methods

ins :: () :- Monad (Either a)

() :=> (Functor (Either a))

Defined in Data.Constraint

Methods

ins :: () :- Functor (Either a)

() :=> (Applicative (Either a))

Defined in Data.Constraint

Methods

ins :: () :- Applicative (Either a)

MonadError e (Either e)

Defined in Control.Monad.Error.Class

Methods

throwError :: e -> Either e a

catchError :: Either e a -> (e -> Either e a) -> Either e a

Monad (Either e)

Since: base-4.4.0.0

Defined in Data.Either

Methods

(>>=) :: Either e a -> (a -> Either e b) -> Either e b

(>>) :: Either e a -> Either e b -> Either e b

return :: a -> Either e a

fail :: String -> Either e a

Functor (Either a)

Since: base-3.0

Defined in Data.Either

Methods

fmap :: (a0 -> b) -> Either a a0 -> Either a b

(<$) :: a0 -> Either a b -> Either a a0

MonadFix (Either e)

Since: base-4.3.0.0

Defined in Control.Monad.Fix

Methods

mfix :: (a -> Either e a) -> Either e a

Applicative (Either e)

Since: base-3.0

Defined in Data.Either

Methods

pure :: a -> Either e a

(<*>) :: Either e (a -> b) -> Either e a -> Either e b

liftA2 :: (a -> b -> c) -> Either e a -> Either e b -> Either e c

(*>) :: Either e a -> Either e b -> Either e b

(<*) :: Either e a -> Either e b -> Either e a

Foldable (Either a)

Since: base-4.7.0.0

Defined in Data.Foldable

Methods

fold :: Monoid m => Either a m -> m

foldMap :: Monoid m => (a0 -> m) -> Either a a0 -> m

foldr :: (a0 -> b -> b) -> b -> Either a a0 -> b

foldr' :: (a0 -> b -> b) -> b -> Either a a0 -> b

foldl :: (b -> a0 -> b) -> b -> Either a a0 -> b

foldl' :: (b -> a0 -> b) -> b -> Either a a0 -> b

foldr1 :: (a0 -> a0 -> a0) -> Either a a0 -> a0

foldl1 :: (a0 -> a0 -> a0) -> Either a a0 -> a0

toList :: Either a a0 -> [a0]

null :: Either a a0 -> Bool

length :: Either a a0 -> Int

elem :: Eq a0 => a0 -> Either a a0 -> Bool

maximum :: Ord a0 => Either a a0 -> a0

minimum :: Ord a0 => Either a a0 -> a0

sum :: Num a0 => Either a a0 -> a0

product :: Num a0 => Either a a0 -> a0

Traversable (Either a)

Since: base-4.7.0.0

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a0 -> f b) -> Either a a0 -> f (Either a b)

sequenceA :: Applicative f => Either a (f a0) -> f (Either a a0)

mapM :: Monad m => (a0 -> m b) -> Either a a0 -> m (Either a b)

sequence :: Monad m => Either a (m a0) -> m (Either a a0)

ToJSON a => ToJSON1 (Either a)

Defined in Data.Aeson.Types.ToJSON

Methods

liftToJSON :: (a0 -> Value) -> ([a0] -> Value) -> Either a a0 -> Value

liftToJSONList :: (a0 -> Value) -> ([a0] -> Value) -> [Either a a0] -> Value

liftToEncoding :: (a0 -> Encoding) -> ([a0] -> Encoding) -> Either a a0 -> Encoding

liftToEncodingList :: (a0 -> Encoding) -> ([a0] -> Encoding) -> [Either a a0] -> Encoding

FromJSON a => FromJSON1 (Either a)

Defined in Data.Aeson.Types.FromJSON

Methods

liftParseJSON :: (Value -> Parser a0) -> (Value -> Parser [a0]) -> Value -> Parser (Either a a0)

liftParseJSONList :: (Value -> Parser a0) -> (Value -> Parser [a0]) -> Value -> Parser [Either a a0]

Eq a => Eq1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftEq :: (a0 -> b -> Bool) -> Either a a0 -> Either a b -> Bool

Ord a => Ord1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftCompare :: (a0 -> b -> Ordering) -> Either a a0 -> Either a b -> Ordering

Read a => Read1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (Int -> ReadS a0) -> ReadS [a0] -> Int -> ReadS (Either a a0)

liftReadList :: (Int -> ReadS a0) -> ReadS [a0] -> ReadS [Either a a0]

liftReadPrec :: ReadPrec a0 -> ReadPrec [a0] -> ReadPrec (Either a a0)

liftReadListPrec :: ReadPrec a0 -> ReadPrec [a0] -> ReadPrec [Either a a0]

Show a => Show1 (Either a)

Since: base-4.9.0.0

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a0 -> ShowS) -> ([a0] -> ShowS) -> Int -> Either a a0 -> ShowS

liftShowList :: (Int -> a0 -> ShowS) -> ([a0] -> ShowS) -> [Either a a0] -> ShowS

e ~ SomeException => MonadMask (Either e)

Since: exceptions-0.8.3

Defined in Control.Monad.Catch

Methods

mask :: ((forall a. Either e a -> Either e a) -> Either e b) -> Either e b

uninterruptibleMask :: ((forall a. Either e a -> Either e a) -> Either e b) -> Either e b

generalBracket :: Either e a -> (a -> ExitCase b -> Either e c) -> (a -> Either e b) -> Either e (b, c)

e ~ SomeException => MonadCatch (Either e)

Since: exceptions-0.8.3

Defined in Control.Monad.Catch

Methods

catch :: Exception e0 => Either e a -> (e0 -> Either e a) -> Either e a

e ~ SomeException => MonadThrow (Either e)

Defined in Control.Monad.Catch

Methods

throwM :: Exception e0 => e0 -> Either e a

NFData a => NFData1 (Either a)

Since: deepseq-1.4.3.0

Defined in Control.DeepSeq

Methods

liftRnf :: (a0 -> ()) -> Either a a0 -> ()

Hashable a => Hashable1 (Either a)

Defined in Data.Hashable.Class

Methods

liftHashWithSalt :: (Int -> a0 -> Int) -> Int -> Either a a0 -> Int

Apply (Either a)

Defined in Data.Functor.Bind.Class

Methods

(<.>) :: Either a (a0 -> b) -> Either a a0 -> Either a b

(.>) :: Either a a0 -> Either a b -> Either a b

(<.) :: Either a a0 -> Either a b -> Either a a0

liftF2 :: (a0 -> b -> c) -> Either a a0 -> Either a b -> Either a c

Pointed (Either a)

Defined in Data.Pointed

Methods

point :: a0 -> Either a a0

Alt (Either a)

Defined in Data.Functor.Alt

Methods

(<!>) :: Either a a0 -> Either a a0 -> Either a a0

some :: Applicative (Either a) => Either a a0 -> Either a [a0]

many :: Applicative (Either a) => Either a a0 -> Either a [a0]

Bind (Either a)

Defined in Data.Functor.Bind.Class

Methods

(>>-) :: Either a a0 -> (a0 -> Either a b) -> Either a b

join :: Either a (Either a a0) -> Either a a0

Extend (Either a)

Defined in Data.Functor.Extend

Methods

duplicated :: Either a a0 -> Either a (Either a a0)

extended :: (Either a a0 -> b) -> Either a a0 -> Either a b

Generic1 (Either a :: * -> *)

Defined in GHC.Generics

Associated Types

type Rep1 (Either a) :: k -> *

Methods

from1 :: Either a a0 -> Rep1 (Either a) a0

to1 :: Rep1 (Either a) a0 -> Either a a0

MonadBase (Either e) (Either e)

Defined in Control.Monad.Base

Methods

liftBase :: Either e α -> Either e α

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

Defined in Data.Either

Methods

(==) :: Either a b -> Either a b -> Bool

(/=) :: Either a b -> Either a b -> Bool

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

Since: base-4.0.0.0

Defined in Data.Data

Methods

gfoldl :: (forall d b0. Data d => c (d -> b0) -> d -> c b0) -> (forall g. g -> c g) -> Either a b -> c (Either a b)

gunfold :: (forall b0 r. Data b0 => c (b0 -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (Either a b)

toConstr :: Either a b -> Constr

dataTypeOf :: Either a b -> DataType

dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (Either a b))

dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (Either a b))

gmapT :: (forall b0. Data b0 => b0 -> b0) -> Either a b -> Either a b

gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> Either a b -> r

gmapQr :: (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> Either a b -> r

gmapQ :: (forall d. Data d => d -> u) -> Either a b -> [u]

gmapQi :: Int -> (forall d. Data d => d -> u) -> Either a b -> u

gmapM :: Monad m => (forall d. Data d => d -> m d) -> Either a b -> m (Either a b)

gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> Either a b -> m (Either a b)

gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> Either a b -> m (Either a b)

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

Defined in Data.Either

Methods

compare :: Either a b -> Either a b -> Ordering

(<) :: Either a b -> Either a b -> Bool

(<=) :: Either a b -> Either a b -> Bool

(>) :: Either a b -> Either a b -> Bool

(>=) :: Either a b -> Either a b -> Bool

max :: Either a b -> Either a b -> Either a b

min :: Either a b -> Either a b -> Either a b

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

Defined in Data.Either

Methods

readsPrec :: Int -> ReadS (Either a b)

readList :: ReadS [Either a b]

readPrec :: ReadPrec (Either a b)

readListPrec :: ReadPrec [Either a b]

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

Defined in Data.Either

Methods

showsPrec :: Int -> Either a b -> ShowS

show :: Either a b -> String

showList :: [Either a b] -> ShowS

Generic (Either a b)

Defined in GHC.Generics

Associated Types

type Rep (Either a b) :: * -> *

Methods

from :: Either a b -> Rep (Either a b) x

to :: Rep (Either a b) x -> Either a b

Semigroup (Either a b)

Since: base-4.9.0.0

Defined in Data.Either

Methods

(<>) :: Either a b -> Either a b -> Either a b

sconcat :: NonEmpty (Either a b) -> Either a b

stimes :: Integral b0 => b0 -> Either a b -> Either a b

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

Defined in Language.Haskell.TH.Syntax

Methods

lift :: Either a b -> Q Exp

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

Defined in Data.Hashable.Class

Methods

hashWithSalt :: Int -> Either a b -> Int

hash :: Either a b -> Int

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

Defined in Data.Aeson.Types.ToJSON

Methods

toJSON :: Either a b -> Value

toEncoding :: Either a b -> Encoding

toJSONList :: [Either a b] -> Value

toEncodingList :: [Either a b] -> Encoding

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

Defined in Data.Aeson.Types.FromJSON

Methods

parseJSON :: Value -> Parser (Either a b)

parseJSONList :: Value -> Parser [Either a b]

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

Defined in Control.DeepSeq

Methods

rnf :: Either a b -> ()

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

Since: serialise-0.2.0.0

Defined in Codec.Serialise.Class

Methods

encode :: Either a b -> Encoding

decode :: Decoder s (Either a b)

encodeList :: [Either a b] -> Encoding

decodeList :: Decoder s [Either a b]

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

Defined in Data.Constraint

Methods

ins :: (Eq a, Eq b) :- Eq (Either a b)

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

Defined in Data.Constraint

Methods

ins :: (Ord a, Ord b) :- Ord (Either a b)

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

Defined in Data.Constraint

Methods

ins :: (Read a, Read b) :- Read (Either a b)

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

Defined in Data.Constraint

Methods

ins :: (Show a, Show b) :- Show (Either a b)

type Rep1 (Either a :: * -> *)

Defined in GHC.Generics

type Rep1 (Either a :: * -> *) = D1 (MetaData "Either" "Data.Either" "base" False) (C1 (MetaCons "Left" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 a)) :+: C1 (MetaCons "Right" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) Par1))

type Rep (Either a b)

Defined in GHC.Generics

type Rep (Either a b) = D1 (MetaData "Either" "Data.Either" "base" False) (C1 (MetaCons "Left" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 a)) :+: C1 (MetaCons "Right" PrefixI False) (S1 (MetaSel (Nothing :: Maybe Symbol) NoSourceUnpackedness NoSourceStrictness DecidedLazy) (Rec0 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.

Examples

We create two values of type Either String Int, one using the Left constructor and another using the Right constructor. Then we apply "either" the length function (if we have a String) or the "times-two" function (if we have an Int):

>>> let s = Left "foo" :: Either String Int
>>> let n = Right 3 :: Either String Int
>>> either length (*2) s
3
>>> either length (*2) n
6

eitherM :: Monad m => (a -> m c) -> (b -> m c) -> m (Either a b) -> m c

Monadic generalisation of either.

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

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

Examples

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> lefts list
["foo","bar","baz"]

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

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

Examples

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> rights list
[3,7]

isLeft :: Either a b -> Bool

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

Examples

Basic usage:

>>> isLeft (Left "foo")
True
>>> isLeft (Right 3)
False

Assuming a Left value signifies some sort of error, we can use isLeft to write a very simple error-reporting function that does absolutely nothing in the case of success, and outputs "ERROR" if any error occurred.

This example shows how isLeft might be used to avoid pattern matching when one does not care about the value contained in the constructor:

>>> import Control.Monad ( when )
>>> let report e = when (isLeft e) $ putStrLn "ERROR"
>>> report (Right 1)
>>> report (Left "parse error")
ERROR

Since: base-4.7.0.0

isRight :: Either a b -> Bool

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

Examples

Basic usage:

>>> isRight (Left "foo")
False
>>> isRight (Right 3)
True

Assuming a Left value signifies some sort of error, we can use isRight to write a very simple reporting function that only outputs "SUCCESS" when a computation has succeeded.

This example shows how isRight might be used to avoid pattern matching when one does not care about the value contained in the constructor:

>>> import Control.Monad ( when )
>>> let report e = when (isRight e) $ putStrLn "SUCCESS"
>>> report (Left "parse error")
>>> report (Right 1)
SUCCESS

Since: base-4.7.0.0

fromLeft :: a -> Either a b -> a

Return the contents of a Left-value or a default value otherwise.

Examples

Basic usage:

>>> fromLeft 1 (Left 3)
3
>>> fromLeft 1 (Right "foo")
1

Since: base-4.10.0.0

fromRight :: b -> Either a b -> b

Return the contents of a Right-value or a default value otherwise.

Examples

Basic usage:

>>> fromRight 1 (Right 3)
3
>>> fromRight 1 (Left "foo")
1

Since: base-4.10.0.0

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.

Examples

Basic usage:

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> partitionEithers list
(["foo","bar","baz"],[3,7])

The pair returned by partitionEithers x should be the same pair as (lefts x, rights x):

>>> let list = [ Left "foo", Right 3, Left "bar", Right 7, Left "baz" ]
>>> partitionEithers list == (lefts list, rights list)
True

Optics

_Left :: (Choice p, Applicative f) => p a (f b) -> p (Either a c) (f (Either b c))

This Prism provides a Traversal for tweaking the Left half of an Either:

>>> over _Left (+1) (Left 2)
Left 3

>>> over _Left (+1) (Right 2)
Right 2

>>> Right 42 ^._Left :: String
""

>>> Left "hello" ^._Left
"hello"

It also can be turned around to obtain the embedding into the Left half of an Either:

>>> _Left # 5
Left 5

>>> 5^.re _Left
Left 5

_Right :: (Choice p, Applicative f) => p a (f b) -> p (Either c a) (f (Either c b))

This Prism provides a Traversal for tweaking the Right half of an Either:

>>> over _Right (+1) (Left 2)
Left 2

>>> over _Right (+1) (Right 2)
Right 3

>>> Right "hello" ^._Right
"hello"

>>> Left "hello" ^._Right :: [Double]
[]

It also can be turned around to obtain the embedding into the Right half of an Either:

>>> _Right # 5
Right 5

>>> 5^.re _Right
Right 5