Data.Either.Extra

This module extends Data.Either with extra operations, particularly to quickly extract from inside an Either. Some of these operations are partial, and should be used with care in production-quality code.

If you need more Either functions see the either.

Documentation

module Data.Either

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

fromEither :: Either a a -> a

Pull the value out of an Either where both alternatives have the same type.

\x -> fromEither (Left x ) == x
\x -> fromEither (Right x) == x

fromLeft' :: Partial => Either l r -> l

The fromLeft' function extracts the element out of a Left and throws an error if its argument is Right. Much like fromJust, using this function in polished code is usually a bad idea.

\x -> fromLeft' (Left  x) == x
\x -> fromLeft' (Right x) == undefined

fromRight' :: Partial => Either l r -> r

The fromRight' function extracts the element out of a Right and throws an error if its argument is Left. Much like fromJust, using this function in polished code is usually a bad idea.

\x -> fromRight' (Right x) == x
\x -> fromRight' (Left  x) == undefined

eitherToMaybe :: Either a b -> Maybe b

Given an Either, convert it to a Maybe, where Left becomes Nothing.

\x -> eitherToMaybe (Left x) == Nothing
\x -> eitherToMaybe (Right x) == Just x

maybeToEither :: a -> Maybe b -> Either a b

Given a Maybe, convert it to an Either, providing a suitable value for the Left should the value be Nothing.

\a b -> maybeToEither a (Just b) == Right b
\a -> maybeToEither a Nothing == Left a

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

The mapLeft function takes a function and applies it to an Either value iff the value takes the form Left _.

mapLeft show (Left 1) == Left "1"
mapLeft show (Right True) == Right True

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

The mapRight function takes a function and applies it to an Either value iff the value takes the form Right _.

mapRight show (Left 1) == Left 1
mapRight show (Right True) == Right "True"