either

Suppose I have a few functions that raise exceptions. I am wrapping them to return Either[Throwable, <function return type>] . (Let's assume I need Either rather than Try ). def fooWrapper(arg1: FooArg1, arg2: FooArg2) = try Right(foo(arg1, arg2)) catch { case NonFatal(e) => Left(e) } def barWrapper(arg1: BarArg1, arg2: BarArg2, a3: BarArg3) = try Right(bar(arg1, arg2, artg3)) catch { case NonFatal(e) => Left(e) } ... Now I would like to write a generic wrapper to get rid of the bolierpllate code. What would you suggest ? I would write something of the form like this: def wrap[Value](f: =>

I'd like to know if it is possible to create some kind of "method call chain", with all methods returning the same Either[Error,Result]. What i'd like to do is: call all the methods successively, and when method returns a Left(Error), then stop the method calls and return the first Left found in the call chain. I've tryied some stuff, with fold, map, projections... but i'm new to Scala and don't find any elegant solution. I've tryed some thing like that: def createUserAndMandatoryCategories(user: User) : Either[Error,User] = { User.create(user).right.map { Logger.info("User created") Category

if a scala function is def A(): Either[Exception, ArrayBuffer[Int]] = { ... } what should be the right way to process the returned result? val a = A() and ? I generally prefer using fold . You can use it like map: scala> def a: Either[Exception,String] = Right("On") a.fold(l => Left(l), r => Right(r.length)) res0: Product with Either[Exception,Int] = Right(2) Or you can use it like a pattern match: scala> a.fold( l => { | println("This was bad") | }, r => { | println("Hurray! " + r) | }) Hurray! On Or you can use it like getOrElse in Option : scala> a.fold( l => "Default" , r => r ) res2:

Suppose I need to convert Option[Int] to Either[String, Int] in Scala. I'd like to do it like this: def foo(ox: Option[Int]): Either[String, Int] = ox.fold(Left("No number")) {x => Right(x)} Unfortunately the code above doesn't compile and I need to add type Either[String, Int] explicitly: ox.fold(Left("No number"): Either[String, Int]) { x => Right(x) } Is it possible to convert Option to Either this way without adding the type ? How would you suggest convert Option to Either ? No, if you do it this way, you can't leave out the type. The type of Left("No number") is inferred to be Either

In my free time I'm learning Haskell, so this is a beginner question. In my readings I came across an example illustrating how Either a is made an instance of Functor : instance Functor (Either a) where fmap f (Right x) = Right (f x) fmap f (Left x) = Left x Now, I'm trying to understand why the implementation maps in the case of a Right value constructor, but doesn't in the case of a Left ? Here is my understanding: First let me rewrite the above instance as instance Functor (Either a) where fmap g (Right x) = Right (g x) fmap g (Left x) = Left x Now: I know that fmap :: (c -> d) -> f c -> f