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A simple way to use Scala's standard Either as an applicative functor, without Scalaz, via the #apply method
branch: master

Merge pull request #4 from EECOLOR/patch-1

Update -- missing parentheses in code example
latest commit 0175bd9244
@ms-tg ms-tg authored


Build Status

This is a single file approach to treating Scala's standard library Either class like a Scalaz Validation. It provides a mechanism for treating Either as an applicative functor by enriching it with an #apply method when the Right type is a function.

The motivation is to be able to use the lessons from Tony Morris's paper Applicative Programming, Disjoint Unions, Semigroups and Non-breaking Error Handling, but using only the standard library's Either class, and using the #apply method for the applicative functor invocation.

As an example, consider the Person case class presented in the above paper, and the three validations applied to the parameters to Person: validAge, validName, and validPostcode. In this library, these function would return the standard library Either class instead of the Scalaz Validation, and main would look something like this:

def main(args: Array[String]) {
  if(args.length < 3)
    println("Need at least three arguments")
  else {
    // Notice the natural syntax to lift function into Either, and then apply it to Eithers
    Right(Person)(validAge(args(0)), validName(args(1)), validPostcode(args(2))) match {
      case Right(p) => println("We have a person: " + p)
      case Left(e) => e foreach println

Compare to the original version which uses Haskell-like syntax, reverses order of parameters, and requires Scalaz's Validation class instead of the standard library's Either class:

def main(args: Array[String]) {
  if(args.length < 3)
    println("Need at least three arguments")
  else {
    val f = (Person(_, _, _)).curried
    val age = validAge(args(0))
    val name = validName(args(1))
    val postcode = validPostcode(args(2))
    postcode <<*>> (name <<*>> (age map f)) match {
      case Success(p) => println("We have a person: " + p)
      case Failure(e) => e foreach println

How to add as a dependency to your Play 2.x Project

To add this library to your Play 2.x project, you can follow these instructions to add a dependency directly on a tagged version in this git repository.

For example, to add a dependency on EitherValidations v1.0.1 to a Play project:

// project/Build.scala
import sbt._
import Keys._
import PlayProject._

object ApplicationBuild extends Build {
  val appName = "YourAppName"
  val appVersion = "1.0-SNAPSHOT"

  val appDependencies = Seq(
    "org.scalaquery" %% "scalaquery" % "0.10.0-M1",
    // artifact dependencies...

  val gitDependencies: Seq[ClasspathDep[ProjectReference]] = Seq(

  // ...

  val main = PlayProject(appName, appVersion, appDependencies, mainLang = SCALA).dependsOn(gitDependencies : _*)

(still looking for instructions on how to do this from a regular sbt project)

How to Test w/ sbt and Edit w/ IntelliJ

  1. Install sbt, -OR- create a symlink to the sbt installed as part of play

    # I have this as ~/bin/sbt, and the ~/bin directory is in my PATH
    java -Xms512M -Xmx1536M -Xss1M -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=384M -jar $SBT_DIR/sbt-launch.jar "$@"
  2. Run tests

    sbt test
  3. Generate IntelliJ project

    sbt gen-idea


All tests are passing on Scala versions back to 2.9.0-1. See our Travis-CI config for all the versions that are currently tested.


Please feel free to fork this project and make pull requests. Any change to code must come with tests, of course.

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