either

问题 I tried to validate the construction of a Record with Applicatives and the Either Monad . It works fine. But I can't see all Error Messages. Only the first is visible because the Right Path of the Either Monad ignores them. Here is my code: import Data.Either (either) import Text.Printf (printf) data Record = Record { fieldA :: String , fieldB :: String , fieldC :: String } deriving (Show, Eq) type Err = String setField :: String -> String -> Either Err String setField field value | length

问题 I understand it would be difficult to change now without breaking existing code, but I'm wondering why it was done that way in the first place. Why not just: sealed trait Either[+A, +B] case class Left[A](x: A) extends Either[A, Nothing] case class Right[B](x: B) extends Either[Nothing, B] Is there some drawback here that I'm failing to see...? 回答1: Not sure how relevant this answer really is to Scala, but it certainly is in Haskell which is evidently where Scala's Either was borrowed from

问题 I understand it would be difficult to change now without breaking existing code, but I'm wondering why it was done that way in the first place. Why not just: sealed trait Either[+A, +B] case class Left[A](x: A) extends Either[A, Nothing] case class Right[B](x: B) extends Either[Nothing, B] Is there some drawback here that I'm failing to see...? 回答1: Not sure how relevant this answer really is to Scala, but it certainly is in Haskell which is evidently where Scala's Either was borrowed from

问题 I understand it would be difficult to change now without breaking existing code, but I'm wondering why it was done that way in the first place. Why not just: sealed trait Either[+A, +B] case class Left[A](x: A) extends Either[A, Nothing] case class Right[B](x: B) extends Either[Nothing, B] Is there some drawback here that I'm failing to see...? 回答1: Not sure how relevant this answer really is to Scala, but it certainly is in Haskell which is evidently where Scala's Either was borrowed from

问题 I have a function that handles errors via Either : funErrViaEither :: a -> Either SomeException b I want to use this function in another function that should be more flexible and return MonadThrow m : funErrViaThrow :: MonadThrow m => a -> m b funErrViaThrow x = if x = someCondition then funErrViaEither else throwM (SomeException MyCustomException) This does not compile; the type checker complains that the return type of funErrViaEither does not match the expected type m b . I don't

问题 I have a function that handles errors via Either : funErrViaEither :: a -> Either SomeException b I want to use this function in another function that should be more flexible and return MonadThrow m : funErrViaThrow :: MonadThrow m => a -> m b funErrViaThrow x = if x = someCondition then funErrViaEither else throwM (SomeException MyCustomException) This does not compile; the type checker complains that the return type of funErrViaEither does not match the expected type m b . I don't

问题 I am looking to extract a value easily from a method that return a type Either<Exception, Object> . I am doing some tests but unable to test easily the return of my methods. For example: final Either<ServerException, TokenModel> result = await repository.getToken(...); To test I am able to do that expect(result, equals(Right(tokenModelExpected))); // => OK Now how can I retrieve the result directly? final TokenModel modelRetrieved = Left(result); ==> Not working.. I found that I have to cast

问题 I have a for comprehension like this: for { (value1: String, value2: String, value3: String) <- getConfigs(args) // more stuff using those values } getConfigs returns an Either[Throwable, (Seq[String], String, String)] and when I try to compile I get this error: value withFilter is not a member of Either[Throwable,(Seq[String], String, String)] How can I use this method (that returns an Either ) in the for comprehension? 回答1: Like this: for { tuple <- getConfigs() } println(tuple) Joking

问题 I have a for comprehension like this: for { (value1: String, value2: String, value3: String) <- getConfigs(args) // more stuff using those values } getConfigs returns an Either[Throwable, (Seq[String], String, String)] and when I try to compile I get this error: value withFilter is not a member of Either[Throwable,(Seq[String], String, String)] How can I use this method (that returns an Either ) in the for comprehension? 回答1: Like this: for { tuple <- getConfigs() } println(tuple) Joking

问题 I came across this dead end while trying to replace exceptions with either monad in c#. Which leads me to think maybe it is not only language specific problem and more technique related missing feature. Let me try to re-explain it more globally: Given: I have a 3rd party function( a function that is imported into my code and I have no access to) which receives a lazy list (c# IEnumerable,f# Seq...) and consume it I Want: To apply a function (LINQ select,map...) on the method's lazy list