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.
Documentation
module Data.Either
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
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.
fromLeft 1 (Left 3) == 3 fromLeft 1 (Right "foo") == 1
fromRight :: b -> Either a b -> b Source
Return the contents of a Right
-value or a default value otherwise.
fromRight 1 (Right 3) == 3 fromRight 1 (Left "foo") == 1
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