scalaz

问题 I'm trying to understand Lens es in scalaz (surprisingly didn't find something similar in cats-core ) and I came across the so called Store which is a type alias: type StoreT[F[_], A, B] = IndexedStoreT[F, A, A, B] type IndexedStore[I, A, B] = IndexedStoreT[Id, I, A, B] type Store[A, B] = StoreT[Id, A, B] Where final case class IndexedStoreT[F[_], +I, A, B](run: (F[A => B], I)) The question is how to treat this type? The documentation just referes to Lens es. Can someone give an explanation

问题 The Free implementation in Haskell is: data Free f a = Pure a | Free (f (Free f a)) whereas, the implementation in Scalaz is: sealed abstract class Free[S[_], A] private case class Return[S[_], A](a: A) extends Free[S, A] private case class Suspend[S[_], A](a: S[A]) extends Free[S, A] private case class Gosub[S[_], B, C](a: Free[S, C], f: C => Free[S, B]) extends Free[S, B] why isn't the scalaz implementation similar to Haskell, like: sealed trait Free[F[_],A] case class Return[F[_],A](a: A)

问题 I'm trying to understand the following piece of code (from the Scalaz library): def kleisliIdApplicative[R]: Applicative[Kleisli[Id, R, ?]] = ... I'm assuming that a type of the form T[P0, ?] is a type-constructor that takes a parameter. However I'm no able to find documentation that explains the usage of question marks in type parameters. A related question is what is the difference between the question mark and an underscore? Is there a place where all this is well-documented? 回答1: The

问题 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:

问题 I am looking at disjunction type of scalaz and I noticed method ap /** Apply a function in the environment of the right of this disjunction. */ def ap[AA >: A, C](f: => AA \/ (B => C)): (AA \/ C) = f flatMap (ff => map(ff(_))) I guess I understand what it does. Now I wonder when and why one should actually use it ? Are there any examples of using this ap function ? 回答1: The disjunction you are looking for: import scalaz.{ \/, -\/ \/-, EitherT } import scalaz.syntax.ToIdOps object Testing

问题 I'm trying to use the scalaz iteratee package to process a large zip file in constant space. I have a long-running process I need to perform on each file in the zip file. Those processes can (and should) be run in parallel. I created an EnumeratorT that inflates each ZipEntry into a File object. The signature looks like: def enumZipFile(f:File):EnumeratorT[IoExceptionOr[IO[File]], IO] I want to attach an IterateeT that will perform the long-running process on each file. I basically end up

问题 Let's say I have the following tuples: scala> val t1 = Tuple2("abcd", "efg") t1: (java.lang.String, java.lang.String) = (abcd,efg) scala> val t2 = Tuple2(1234, "lmnop") t2: (Int, java.lang.String) = (1234,lmnop) scala> val t3 = Tuple3("qrs", "tuv", "wxyz") t3: (java.lang.String, java.lang.String, java.lang.String) = (qrs,tuv,wxyz) Is there a friendly way to combine them (in two steps if necessary) into a Tuple7? I'm really looking for a general answer for combining tuples of arbitrary size,

问题 In Scala, for reading a text file and uploading it into an array, a common approach is scala.io.Source.fromFile("file.txt").getLines.toArray Especially for very large files, is there a faster approach perhaps by reading blocks of bytes into memory first and then splitting them by new line characters ? (See Read entire file in Scala for commonly used approaches.) Many Thanks. 回答1: The performance problem has nothing to do with the way the data is read. It is already buffered. Nothing happens

问题 following is a toy example to demonstrate real life legacy methods' shape weirdness and point of the question. As you can see anotherFunc , after mapping over personList expands type to \/[Throwable,List[\/[Throwable,String]]] which isn't intended return type but effect of map ing personList . again what's illustrated below inside anotherFunc is for demo purpose(in reality there is more meaningful things happening instead of Option("fakeString") or any of that. Requirement is to map

问题 I know in Scala, you can do type ===>[A, B] = Map[A, B] and then you can use infix notation to define def foo: String ===> Int which is same as saying def foo: Map[String, Int] . Is there any way to exploit this infix notation to create types with >2 arguments? For example, I want something like this: type A ~> B ~~~> C to be an alias of say Map[A, Pair[B, C]] ? Is there anyway I can write something line this: type A to B -> C as alias for (A, B, C) type? 回答1: Interestingly operator