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Questions tagged [scalaz]

Scalaz provides type classes and purely functional data structures for Scala

Scalaz is a Scala library for functional programming.

It provides purely functional data structures to complement those from the Scala standard library. It defines a set of foundational type classes (e.g. Functor, Applicative, Monad) and corresponding instances for a large number of data structures.

More information is available on the Scalaz Project page

It is released under the BSD (open source) license.

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Scala functional programming gymnastics

I am trying to do the following in as little code as possible and as functionally as possible: def restrict(floor : Option[Double], cap : Option[Double], amt : Double) : Double Obviously the following works: = (floor -> cap) match { case…
oxbow_lakes
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Scala: question marks in type parameters

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…
Damian Nadales
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The object-functional impedance mismatch

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 OOP practice doSomething[A](xs: Seq[A]) = ??? not…
lambdista
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What's the difference between Task and IO in Scalaz?

These two Scalaz types scalaz.concurrent.Task[+A] scalaz.effect.IO[A] seem very conceptually similar. They both: Represent a potentially side-effecting computation Produce a success (A) or failure (Exception) result Have Monad instances Can be…
Chris Martin
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Already existing functional way for a retry-until in Scala?

Is there a functional/Scala way to call a function repeatedly until it succeeds, while reacting to failed attempts? Let me illustrate with an example. Suppose I want to read an integer from standard-in, and retry if the user did not in fact enter an…
Sebastian N.
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How do I get the scalaz IDEA live templates working for the symbolic methods?

Many of the methods in scalaz have symbolic unicode equivalents, such as forever and ∞ (of course, I have this the wrong way round, the symbolic methods really have ASCII equivalents). The project contains a live templates XML file for IDEA so these…
oxbow_lakes
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How to use Scalaz 7's EitherT with liftM

If I have a monad transformer type taking two type arguments, I can use liftM to lift values into the transformed monad: scala> val o = 1.point[List].liftM[OptionT] o: scalaz.OptionT[List,Int] = OptionT(List(Some(1))) However if I try the same…
Hugh
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Scalaz Lens Composition

Really simple question here. After watching an excellent introduction to lenses: http://www.youtube.com/watch?v=efv0SQNde5Q I thought I might attempt one of the simple examples covered in the talk: import…
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Flattening Nested Scalaz Validations

I'm fairly new to scalaz and I've started out with validations. I have some validation functions of the form: def validateXyz(...): ValidationNEL[String, String] = ... I'm then using the applicative style to combine multiple validations and then…
Chris Turner
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Scalaz : validating in a for-comprehension and logging

I admit that the title is not very explicit : sorry for that. Assume I have a for-comprehension : for {v1<-Validation1(input) v2<-Validation2(v1) v3<-Validation3(v2) } yield result Validation1, Validation2 and Validation3 do some checking…
bhericher
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Summing a List of Options with Applicative Functors

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) =>…
Manuel Schmidt
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Writing type class instances for nested classes in Scala

In this recent Stack Overflow question, the author wanted to change a list of parsers of some type into a parser that returns lists of that type. We can imagine doing this with Scalaz's sequence for applicative functors: import…
Travis Brown
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Use Unapply to extract identical type classes

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 tried doing the following: type UnapplyM[TC[_[_]], MA,…
wheaties
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Can I automatically implement classes?

In Scalaz every Monad instance is automatically an instance of Applicative. implicit val listInstance = new Monad[List] { def point[A](a: => A) = List(a) def bind[A, B](fa: List[A])(f: A => List[B]) = fa flatMap f } List(2) <*> List((x: Int)…
ZhekaKozlov
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What are Tower[A] and IvoryTower in Scalaz?

When I looked at scalaz.effect.IO source code, I noticed that it has a method apply with the following signature: sealed trait IO[A] { def apply(rw: Tower[IvoryTower]): Trampoline[(Tower[IvoryTower], A)] } Tower[A] and IvoryTower are defined…
ZhekaKozlov
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