scalaz | 易学教程

Map on Scalaz Validation failure

阅读更多关于 Map on Scalaz Validation failure

问题 import scalaz._ import Scalaz._ "abc".parseInt This will return a Validation[NumberFormatException, Int] . Is there a way I can apply a function on the failure side (such as toString ) to get a Validation[String, Int] ? 回答1: There is a pair of methods <-: and :-> defined on MAB[M[_,_], A, B] that map on the left and right side of any M[A, B] as long as there is a Bifunctor[M] . Validation happens to be a bifunctor, so you can do this: ((_:NumberFormatException).toString) <-: "123".parseInt

Provide implicits for all subtypes of sealed type

阅读更多关于 Provide implicits for all subtypes of sealed type

In my application I have multiple case classes and objects which are part of sealed trait hierarchy. I use them as messages in Akka. Those classes need to be converted to user friendly form before sending through websocket. Previously I used big pattern match to convert them in single place, but as number of types grows I would like to use implicit conversion: object Types { sealed trait Type case object SubType1 extends Type case object SubType2 extends Type case object SubType3 extends Type trait Converter[T] { def convert(t: T): Int } } object Implicits { import Types._ implicit object

Idiomatic Scala way of deserializing delimited strings into case classes

阅读更多关于 Idiomatic Scala way of deserializing delimited strings into case classes

问题 Suppose I was dealing with a simple colon-delimited text protocol that looked something like: Event:005003:information:2013 12 06 12 37 55:n3.swmml20861:1:Full client swmml20861 registered [entry=280 PID=20864 queue=0x4ca9001b] RSET:m3node:AUTRS:1-1-24:A:0:LOADSHARE:INHIBITED:0 M3UA_IP_LINK:m3node:AUT001LKSET1:AUT001LK1:r OPC:m3node:1-10-2(P):A7:NAT0 .... I'd like to deserialize each line as an instance of a case class, but in a type-safe way. My first attempt uses type classes to define

How to transform disjunction of Future to Future of disjunction

阅读更多关于 How to transform disjunction of Future to Future of disjunction

问题 I have the result of a method: val res: Future[Int] Xor Future[String] = getResult(x) and would like to transform it and use it as Future[Int Xor String] I could not extrapolate my use case from the herding cats blog and am not sure whether a monad transformer would be the right tool here, perhaps rather some form of traverse ? Xor from cats stands in for any disjunction. Scalaz \/ or stdlib Either would be fine as well (though I would prefer a biased disjunction). Thank you 回答1: Just as

Validating a sequence of XML elements with Writer monad

阅读更多关于 Validating a sequence of XML elements with Writer monad

This is a follow-up to my old question . Suppose I need to validate an XML: <a><a1/><a2/><a3/></a> I need to make sure that the root a has children a1 , a2 , and a3 (in this order). I'd like to use a List (instead of scalaz.Validation ) and Writer monad to collect validation errors. So I define a function to validate a sequence of XML elements like this: type Validate = List[Elem] => Writer[List[String], List[Elem]] Now I can add a function to validate the label of an XML element: val label: String => Validate = ... If I redefine Validate as Kleisli I'll be able to compose the Validate

How to combine sequence of Kleisli

阅读更多关于 How to combine sequence of Kleisli

Given a method that return a Kleisli given a parameter def k[A, B, C](a: A) : Kleisli[Future, B, Option[C]] = ??? I want to write a combinator that deals with a sequence of this parameter def ks[A, B, C](as: Seq[A]) : Kleisli[Future, B, Seq[C]] = Kleisli[Future, B, Seq[C]] { ctx => Future.sequence(as.map(a => k(a).run(ctx))).map(_.flatten) } Is there a better way ? Travis Brown There is a better way: import scala.concurrent.ExecutionContext.Implicits.global import scala.concurrent.Future import scalaz._, Scalaz._ def k[A, B, C](a: A): Kleisli[Future, B, Option[C]] = ??? def ks[A, B, C](as:

Help me understand this Scala code: scalaz IO Monad and implicits

阅读更多关于 Help me understand this Scala code: scalaz IO Monad and implicits

问题 This is a followup to this question. Here's the code I'm trying to understand (it's from http://apocalisp.wordpress.com/2010/10/17/scalaz-tutorial-enumeration-based-io-with-iteratees/): object io { sealed trait IO[A] { def unsafePerformIO: A } object IO { def apply[A](a: => A): IO[A] = new IO[A] { def unsafePerformIO = a } } implicit val IOMonad = new Monad[IO] { def pure[A](a: => A): IO[A] = IO(a) def bind[A,B](a: IO[A], f: A => IO[B]): IO[B] = IO { implicitly[Monad[Function0]].bind(() => a

Scalaz: how to compose a map lens with a value lens?

阅读更多关于 Scalaz: how to compose a map lens with a value lens?

问题 There's an example of a Scalaz map lens here: Dan Burton calls it containsKey , and it's inspired by the Edward Kmett talk. There is also something called mapVPLens in Scalaz 7 which is useful for modifying values in a map. My question is: if I have a lens for modifying type V , and a lens for a Map[K,V] , how can I compose them? I've been searching for a while for a good simple example, but there's still a dearth of examples in Scalaz. I'm interested in both Scalaz 6 and Scalaz 7 solutions.

Summing a List of Options with Applicative Functors

阅读更多关于 Summing a List of Options with Applicative Functors

问题 I have a List[Option[Int]] and want to sum over it using applicative functors. From [1] I understand that it should be something like the following import scalaz._ import Scalaz._ List(1,2,3).map(some(_)).foldLeft(some(0))({ case (acc,value) => (acc <|*|> value){_+_} }) however I am simply not able to figure out the correct way to write this. I would be glad if somebody could help me with this. Thank you very much [1] How to combine Option values in Scala? Edit Thanks for all the great

Handling fail-fast failures when the return type is Option[Error]

阅读更多关于 Handling fail-fast failures when the return type is Option[Error]

I have posted quite several questions about failure handling in Scala and I really thank you all for your answers. I understand my options when dealing with Either and Scalaz or a for comprehension, and I have another (last?) question: How to do a fail-fast sequence of operations when the operations are dealing with the outside non-functional world, like a DB? I mean I have a method like that: def insertItem(item: Item): Either[Error,Item] Thanks to Either and these answers, I know how to do it with Either: Chaining method calls with Either and Method parameters validation in Scala, with for