I'm playing around with Scala(z) to learn functionnal programming. I have a value of type Future[List[Error \/ Double]] and want to transform it to something with the type Future[[List[Error] \/ ...

Given a typeclass where instance selection should be performed based on the return type: case class Monoid[A](m0: A) // We only care about the zero here implicit def s[T] : Monoid[Set[T]] = Monoid...

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...

I'm trying to stack monad transfromers of scalaz in a haskell way: statyReader :: (MonadReader Int m, MonadState Int m) => m Int scala: def statyReader[F[_]](implicit r: MonadReader[F, Int]...

Introduction Scala's Future (new in 2.10 and now 2.9.3) is an applicative functor, which means that if we have a traversable type F, we can take an F[A] and a function A => Future[B] and turn t...

With Scalaz, a function can be mapped over another function. When would I want to use map over andThen? Is there a clear advantage using map? Thanks For example, val f: Int => Int = (a) => ...

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[_]...

For (a cut-down) example, class SemigroupOps[F] (val self: F)(val F: Semigroup[F]) extends Ops[F] { def |+|(other: => F): F = F.append(self, other) } F in this case is a variable so I'd expe...

In OOP it is good practice to talk to interfaces not to implementations. So, e.g., you write something like this (by Seq I mean scala.collection.immutable.Seq :)): // talk to the interface - good ...

How to turn a List of Eithers to a Either of Lists, using MonadPlus.separate? In this answer the author claims this solution, but fails to provide the imports or complete example: If scalaz is ...

Applicative functors are often mentioned as an alternative to monads when your computation steps are independent. One of their often-mentioned advantages is that you don't need transformers when yo...

Having read the book Learn you a Haskell For Great Good, and the very helpful wiki book article Haskell Category Theory which helped me overcome the common category mistake of confusing category ob...

The goal is to implement different type classes (like Semigroup, Monad, Functor, etc.) provided by Scalaz for Spark's RDD (distributed collection). Unfortunately, I cannot make any of the type clas...

I have two (or more) functions defined as: val functionM: String => Option[Int] = s => Some(s.length) val functionM2: Int => Option[String] = i => Some(i.toString) I also have some d...

I have the following situation, given two types MA and MB, I would like to be able to prove that they both not only have an Applicative but also that they both have the same underlying shape. I tri...

We can describe monad, as the computational context, and monad implementation exactly preserves the meaning of that context. For example Option - the context meaning is that the value might exist. ...

I've found this pattern quite a few times in my code: if (doIt) object.callAMethod else object I'm wondering if there could be a syntactically more pleasing way to write the code above, espe...

I would like to create an RDD to collect the results of an iterative calculation . How can I use a loop (or any alternative) to replace the following code: import org.apache.spark.mllib.random.R...

Suppose I want to aggregate data from 2 remote services, and serve response as fast as I can: def loadUser: Future[User] def loadData: Future[Data] case class Payload(user: User, data: Data) ...

Scalaz State monad's modify has the following signature: def modify[S](f: S => S): State[S, Unit] This allows the state to be replaced by state of the same type, which does not work well whe...

I'm trying to understand the idea and purpose behind scalaz concurrent package, primarily Future and Task classes, but when using them in some application, it's now far from simple sequential analo...

We used the WriterT monad transformer on Futures hoping to get more organized logs from an asynchronous application, but we ran into some trouble. If I compile the application below, I get the fol...

I haven't seen many examples of the scalaz state monad. There is this example but it is hard to understand and there is only one other question on stack overflow it seems. I'm going to post a few ...

As I known, shapeless provides the HList (Heterogenous list) type which can include multiple types. Is it possible to fold HList? for example, // ref - Composable application architecture with re...

I would like to implement my asynchronous processing with scalaz.concurrent.Task. I need a function (Task[A], Task[B]) => Task[(A, B)] to return a new task that works as follows: run Task[A] ...