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Interoperability libraries for Shapeless
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README.md

shapeless-contrib

Interoperability libraries for Shapeless

Build Status

Usage

This library is currently available for Scala 2.11.

To use the latest version, include the following in your build.sbt:

libraryDependencies ++= Seq(
  "org.typelevel" %% "shapeless-scalacheck" % "0.3",
  "org.typelevel" %% "shapeless-spire" % "0.3",
  "org.typelevel" %% "shapeless-scalaz" % "0.3"
)

What does this library do?

shapeless-contrib aims to provide smooth interoperability between Shapeless, Scalaz and Spire. The most prominent feature is automatic derivation of type class instances for case classes, but there is more. If you've got a cool idea for more stuff, please open an issue.

Examples

Scalaz + Shapeless = Profit

Note: The current 0.3.x series depends on shapeless 2.0.0.

The combination of these two libraries allows for some nifty utility functions related to scalaz.Applicative:

  • lifting arbitrary functions (i.e. a generalized liftA1, liftA2, ...)
  • sequencing and traversing an HList (just like sequencing and traversing a List)
import shapeless._
import shapeless.contrib.scalaz._

import scalaz.Applicative
import scalaz.std.option._

// define a function with arbitrarily many parameters
def foo(x: Int, y: String, z: Float) = s"$x - $y - $z"

// lift it into `Option`
val lifted = Applicative[Option].liftA(foo _)

// resulting type: `(Option[Int], Option[String], Option[Float]) => Option[String]`


// define an `HList` consisting of `Option`s
val in = Option(1) :: Option("foo") :: HNil

val sequenced = sequence(in)

// resulting type: `Option[Int :: String :: HNil]`

// works for `Validation`, too:
import scalaz._
import scalaz.std.string._

val v1: Validation[String, Int] = Success(3)
val v2: Validation[String, Float] = Failure("foo")
sequence(v1 :: v2 :: HNil)

// resulting type: `Validation[String, Int :: Float :: HNil]`

Traversing works the same way, but you will also have to specify a shapeless.Poly which maps over the HList first.

In addition to that, it also provides a conversion between their lens types:

import shapeless._
import shapeless.contrib.scalaz._

case class TwoElem(n: Int, x: String)

// Generate a `shapeless.Lens`
val sLens = lens[TwoElem] >> 'n

// Convert it to a `scalaz.Lens`
val zsLens = sLens.asScalaz

// The other way round:
import scalaz.Lens

val zLens = Lens.lensId[Int]
val szLens = zLens.asShapeless

Derive type classes

Consider a simple case class with an addition operation:

case class Vector3(x: Int, y: Int, z: Int) {
  def +(other: Vector3) = Vector3(this.x + other.x, this.y + other.y, this.z + other.z)
}

If we want to use that in a generic setting, e.g. in an algorithm which requires a Monoid, we can define an instance for spire.algebra.Monoid like so:

implicit object Vector3Monoid extends Monoid[Vector3] {
  def id = Vector3(0, 0, 0)
  def op(x: Vector3, y: Vector3) = x + y
}

This will work nicely for that particular case. However, it requires repetition: addition on Vector3 is just pointwise addition of its elements, and the null vector consists of three zeroes. We do not want to repeat that sort of code for all our case classes, and want to derive that automatically. Luckily, this library provides exactly that:

import spire.implicits._
import shapeless.contrib.spire._

// Define the `Vector3` case class without any operations
case class Vector3(x: Int, y: Int, z: Int)

// That's it! `Vector3` is an `AdditiveMonoid` now.
Vector3(1, 2, 3) + Vector3(-1, 3, 0)
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