| Safe Haskell | Safe |
|---|---|
| Language | Haskell2010 |
Data.Either.Extra
Description
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.
Synopsis
- module Data.Either
- isLeft :: Either a b -> Bool
- isRight :: Either a b -> Bool
- fromLeft :: a -> Either a b -> a
- fromRight :: b -> Either a b -> b
- fromEither :: Either a a -> a
- fromLeft' :: Partial => Either l r -> l
- fromRight' :: Partial => Either l r -> r
- eitherToMaybe :: Either a b -> Maybe b
- maybeToEither :: a -> Maybe b -> Either a b
Documentation
module Data.Either
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
fromEither :: Either a a -> a Source #
Pull the value out of an Either where both alternatives
have the same type.
\x -> fromEither (Left x ) == x \x -> fromEither (Right x) == x
fromRight' :: Partial => Either l r -> r Source #
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 Source #