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Data.Either

Copyright (c) The University of Glasgow 2001
License BSD-style (see the file libraries/base/LICENSE)
Maintainer [email protected]
Stability experimental
Portability portable
Safe Haskell Trustworthy
Language Haskell2010

Description

The Either type, and associated operations.

data Either a b Source

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

Show2 Either

Since: 4.9.0.0

Methods

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

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

Read2 Either

Since: 4.9.0.0

Methods

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

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

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

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

Ord2 Either

Since: 4.9.0.0

Methods

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

Eq2 Either

Since: 4.9.0.0

Methods

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

Bifunctor Either

Since: 4.8.0.0

Methods

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

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

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

Bifoldable Either

Since: 4.10.0.0

Methods

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

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

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

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

Bitraversable Either

Since: 4.10.0.0

Methods

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

Monad (Either e)

Since: 4.4.0.0

Methods

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

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

return :: a -> Either e a Source

fail :: String -> Either e a Source

Functor (Either a)

Since: 3.0

Methods

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

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

MonadFix (Either e)

Since: 4.3.0.0

Methods

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

Applicative (Either e)

Since: 3.0

Methods

pure :: a -> Either e a Source

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

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

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

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

Foldable (Either a)

Since: 4.7.0.0

Methods

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

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

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

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

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

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

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

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

toList :: Either a a -> [a] Source

null :: Either a a -> Bool Source

length :: Either a a -> Int Source

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

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

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

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

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

Traversable (Either a)

Since: 4.7.0.0

Methods

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

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

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

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

Show a => Show1 (Either a)

Since: 4.9.0.0

Methods

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

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

Read a => Read1 (Either a)

Since: 4.9.0.0

Methods

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

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

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

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

Ord a => Ord1 (Either a)

Since: 4.9.0.0

Methods

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

Eq a => Eq1 (Either a)

Since: 4.9.0.0

Methods

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

Generic1 * (Either a)

Associated Types

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

Methods

from1 :: f a -> Rep1 (Either a) f a Source

to1 :: Rep1 (Either a) f a -> f a Source

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

Methods

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

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

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

Since: 4.0.0.0

Methods

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

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

toConstr :: Either a b -> Constr Source

dataTypeOf :: Either a b -> DataType Source

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

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

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

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

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

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

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

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

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

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

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

Methods

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

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

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

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

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

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

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

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

Methods

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

show :: Either a b -> String Source

showList :: [Either a b] -> ShowS Source

Generic (Either a b)

Associated Types

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

Methods

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

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

Semigroup (Either a b)

Since: 4.9.0.0

Methods

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

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

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

type Rep1 * (Either a)
type Rep (Either a b)
type (==) (Either k1 k2) a b
type (==) (Either k1 k2) a b

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

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

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

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] Source

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 Source

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: 4.7.0.0

isRight :: Either a b -> Bool Source

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: 4.7.0.0

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

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: 4.10.0.0

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

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: 4.10.0.0

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

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

© The University of Glasgow and others
Licensed under a BSD-style license (see top of the page).
https://downloads.haskell.org/~ghc/8.2.1/docs/html/libraries/base-4.10.0.0/Data-Either.html