typelevel / scalaz-outlaws   0.2


outcasts no longer allowed in the ivory tower

Scala versions: 2.10


"An outlaw can be defined as somebody who lives outside the law, beyond the law and not necessarily against it." ― Hunter S. Thompson


scalaz-outlaws aims to be a repository to collect Typeclasses, Typeclass instances and other curios which are not welcome in scalaz-core because they are either lawless, or don't strictly adhere to all the laws of a given typeclass.

Using scalaz-outlaws

Add the following to your SBT build:

resolvers += "Scalaz Bintray Repo" at "http://dl.bintray.com/stew/snapshots"

And add the library dependency:

libraryDependencies += "org.typelevel" %% "scalaz-outlaws" % "0.2"

What is here and why?


Set isn't considered to be a functor. You should be able to easily find some flamewars about this

in order to import the Set instances from scalaz-outlaws:

import scalaz.outlaws.std.set._


Try isn't a valid functor. read about it here

What should you consider using instead? Instead of:

    Try {
        might throw


    \/.fromTryCatchThrowable {
        might throw

or perhaps:

    Task.delay {
        might throw

in order to import the Try instances from scalaz-outlaws:

import scalaz.outlaws.std.utilTry._


This is a typeclass with a single function "each" which is like "foreach" found on many scala standard library collection classes. There are no meaningful laws for Each. It is only used for side-effects, which are considered harmful.

What should you use instead of side-effecting?

instead of:



    val sideEffectingComputation = xxx.traverse_(x => Task.delay(sideeffect(x))

followed eventually by:

    val didItWork: Throwable \/ Unit = sideEffectingComputation.attemptRun

Monoid[Double] — Monoid[Float]

scalaz doesn't have a monoid instance for Double/Float since you cannot count on + operations on Double/Float to be associative.

import scalaz.outlaws.std.double._
import scalaz.outlaws.std.float._
import scalaz.syntax.monoid._

val x: Double = 1D |+| 2D
val y: Float = 1F |+| 2F