learning Scalaz: day 4


Hey there. There's an updated html5 book version, if you want.

Yesterday we reviewed kinds and types, explored Tagged type, and started looking at Semigroup and Monoid as a way of abstracting binary operations over various types.

I've gotten a few feedbacks. First, paulp suggested that I could use companion type like Option.type for our kind calculator. Using the updated version, we get the following:

scala> kind[Functor.type]
res1: String = Functor's kind is (* -> *) -> *. This is a type constructor that takes type constructor(s): a higher-kinded type.

Also a comment from Jason Zaugg:

This might be a good point to pause and discuss the laws by which a well behaved type class instance must abide.

I've been skipping all the sections in Learn You a Haskell for Great Good about the laws and we got pulled over.

Functor Laws


All functors are expected to exhibit certain kinds of functor-like properties and behaviors.
The first functor law states that if we map the id function over a functor, the functor that we get back should be the same as the original functor.

In other words,

scala> List(1, 2, 3) map {identity} assert_=== List(1, 2, 3)

The second law says that composing two functions and then mapping the resulting function over a functor should be the same as first mapping one function over the functor and then mapping the other one.

In other words,

scala> (List(1, 2, 3) map {{(_: Int) * 3} map {(_: Int) + 1}}) assert_=== (List(1, 2, 3) map {(_: Int) * 3} map {(_: Int) + 1})

These are laws the implementer of the functors must abide, and not something the compiler can check for you. Scalaz 7 ships with FunctorLaw traits that describes this in code:

trait FunctorLaw {
  /** The identity function, lifted, is a no-op. */
  def identity[A](fa: F[A])(implicit FA: Equal[F[A]]): Boolean = FA.equal(map(fa)(x => x), fa)
   * A series of maps may be freely rewritten as a single map on a
   * composed function.
  def associative[A, B, C](fa: F[A], f1: A => B, f2: B => C)(implicit FC: Equal[F[C]]): Boolean = FC.equal(map(map(fa)(f1))(f2), map(fa)(f2 compose f1))

Not only that, it ships with ScalaCheck bindings to test these properties using arbiterary values. Here's the build.sbt to check from REPL. Note this is using slightly older version of Scala 2.10 and Scalaz 7 milestones instead of 7.0.0-M3/2.10.0-M7 that we have been using because scalaz-scalacheck-binding is not there for it:

scalaVersion := "2.10.0-M6"
resolvers += "Typesafe Snapshots" at "http://repo.typesafe.com/typesafe/snapshots/"
libraryDependencies ++= Seq(
  "org.scalaz" % "scalaz-core" % "7.0.0-M2" cross CrossVersion.full,
  "org.scalaz" % "scalaz-scalacheck-binding" % "7.0.0-M2" % "test" cross CrossVersion.full
scalacOptions += "-feature"
initialCommands in console := "import scalaz._, Scalaz._"
initialCommands in console in Test := "import scalaz._, Scalaz._, scalacheck.ScalazProperties._, scalacheck.ScalazArbitrary._,scalacheck.ScalaCheckBinding._"

Instead of the usual sbt console, run sbt test:console:

$ sbt test:console
[info] Starting scala interpreter...
import scalaz._
import Scalaz._
import scalacheck.ScalazProperties._
import scalacheck.ScalazArbitrary._
import scalacheck.ScalaCheckBinding._
Welcome to Scala version 2.10.0-M6 (Java HotSpot(TM) 64-Bit Server VM, Java 1.6.0_33).
Type in expressions to have them evaluated.
Type :help for more information.

Here's how you test if List meets the functor laws:

scala> functor.laws[List].check
+ functor.identity: OK, passed 100 tests.
+ functor.associative: OK, passed 100 tests.

Breaking the law

Following the book, let's try breaking the law.

scala> :paste
// Entering paste mode (ctrl-D to finish)
sealed trait COption[+A] {}
case class CSome[A](counter: Int, a: A) extends COption[A]
case object CNone extends COption[Nothing]
implicit def coptionEqual[A]: Equal[COption[A]] = Equal.equalA
implicit val coptionFunctor = new Functor[COption] {
  def map[A, B](fa: COption[A])(f: A => B): COption[B] = fa match {
    case CNone => CNone
    case CSome(c, a) => CSome(c + 1, f(a))
// Exiting paste mode, now interpreting.
defined trait COption
defined class CSome
defined module CNone
coptionEqual: [A]=> scalaz.Equal[COption[A]]
coptionFunctor: scalaz.Functor[COption] = $anon$1@42538425
scala> (CSome(0, "ho"): COption[String]) map {(_: String) + "ha"}
res4: COption[String] = CSome(1,hoha)
scala> (CSome(0, "ho"): COption[String]) map {identity}
res5: COption[String] = CSome(1,ho)

It's breaking the first law. Let's see if we can catch this.

scala> functor.laws[COption].check
<console>:26: error: could not find implicit value for parameter af: org.scalacheck.Arbitrary[COption[Int]]

So now we have to supply "arbitrary" COption[A] implicitly:

scala> import org.scalacheck.{Gen, Arbitrary}
import org.scalacheck.{Gen, Arbitrary}
scala> implicit def COptionArbiterary[A](implicit a: Arbitrary[A]): Arbitrary[COption[A]] =
         a map { a => (CSome(0, a): COption[A]) }
COptionArbiterary: [A](implicit a: org.scalacheck.Arbitrary[A])org.scalacheck.Arbitrary[COption[A]]

This is pretty cool. ScalaCheck on its own does not ship map method, but Scalaz injected it as a Functor[Arbitrary]! Not much of an arbitrary COption, but I don't know enough ScalaCheck, so this will have to do.

scala> functor.laws[COption].check
! functor.identity: Falsified after 0 passed tests.
> ARG_0: CSome(0,-170856004)
! functor.associative: Falsified after 0 passed tests.
> ARG_0: CSome(0,1)
> ARG_1: <function1>
> ARG_2: <function1>

And the test fails as expected.

Applicative Laws

Here are the laws for Applicative:

  trait ApplicativeLaw extends FunctorLaw {
    def identityAp[A](fa: F[A])(implicit FA: Equal[F[A]]): Boolean =
      FA.equal(ap(fa)(point((a: A) => a)), fa)
    def composition[A, B, C](fbc: F[B => C], fab: F[A => B], fa: F[A])(implicit FC: Equal[F[C]]) =
      FC.equal(ap(ap(fa)(fab))(fbc), ap(fa)(ap(fab)(ap(fbc)(point((bc: B => C) => (ab: A => B) => bc compose ab)))))
    def homomorphism[A, B](ab: A => B, a: A)(implicit FB: Equal[F[B]]): Boolean =
      FB.equal(ap(point(a))(point(ab)), point(ab(a)))
    def interchange[A, B](f: F[A => B], a: A)(implicit FB: Equal[F[B]]): Boolean =
      FB.equal(ap(point(a))(f), ap(f)(point((f: A => B) => f(a))))

LYAHFGG is skipping the details on this, so I am skipping too.

Semigroup Laws

Here are the Semigroup Laws:

   * A semigroup in type F must satisfy two laws:
    *  - '''closure''': `∀ a, b in F, append(a, b)` is also in `F`. This is enforced by the type system.
    *  - '''associativity''': `∀ a, b, c` in `F`, the equation `append(append(a, b), c) = append(a, append(b , c))` holds.
  trait SemigroupLaw {
    def associative(f1: F, f2: F, f3: F)(implicit F: Equal[F]): Boolean =
      F.equal(append(f1, append(f2, f3)), append(append(f1, f2), f3))

Remember, 1 * (2 * 3) and (1 * 2) * 3 must hold, which is called associative.

scala> semigroup.laws[Int @@ Tags.Multiplication].check
+ semigroup.associative: OK, passed 100 tests.

Monoid Laws

Here are the Monoid Laws:

   * Monoid instances must satisfy [[scalaz.Semigroup.SemigroupLaw]] and 2 additional laws:
   *  - '''left identity''': `forall a. append(zero, a) == a`
   *  - '''right identity''' : `forall a. append(a, zero) == a`
  trait MonoidLaw extends SemigroupLaw {
    def leftIdentity(a: F)(implicit F: Equal[F]) = F.equal(a, append(zero, a))
    def rightIdentity(a: F)(implicit F: Equal[F]) = F.equal(a, append(a, zero))

This law is simple. I can |+| (mappend) identity value to either left hand side or right hand side. For multiplication:

scala> 1 * 2 assert_=== 2
scala> 2 * 1 assert_=== 2

Using Scalaz:

scala> (Monoid[Int @@ Tags.Multiplication].zero |+| Tags.Multiplication(2): Int) assert_=== 2
scala> (Tags.Multiplication(2) |+| Monoid[Int @@ Tags.Multiplication].zero: Int) assert_=== 2
scala> monoid.laws[Int @@ Tags.Multiplication].check
+ monoid.semigroup.associative: OK, passed 100 tests.
+ monoid.left identity: OK, passed 100 tests.
+ monoid.right identity: OK, passed 100 tests.

Option as Monoid


One way is to treat Maybe a as a monoid only if its type parameter a is a monoid as well and then implement mappend in such a way that it uses the mappend operation of the values that are wrapped with Just.

Let's see if this is how Scalaz does it. See std/Option.scala:

  implicit def optionMonoid[A: Semigroup]: Monoid[Option[A]] = new Monoid[Option[A]] {
    def append(f1: Option[A], f2: => Option[A]) = (f1, f2) match {
      case (Some(a1), Some(a2)) => Some(Semigroup[A].append(a1, a2))
      case (Some(a1), None)     => f1
      case (None, Some(a2))     => f2
      case (None, None)         => None
    def zero: Option[A] = None

The implementation is nice and simple. Context bound A: Semigroup says that A must support |+|. The rest is pattern matching. Doing exactly what the book says.

scala> (none: Option[String]) |+| "andy".some
res23: Option[String] = Some(andy)
scala> (Ordering.LT: Ordering).some |+| none
res25: Option[scalaz.Ordering] = Some(LT)

It works.


But if we don't know if the contents are monoids, we can't use mappend between them, so what are we to do? Well, one thing we can do is to just discard the second value and keep the first one. For this, the First a type exists.

Haskell is using newtype to implement First type constructor. Scalaz 7 does it using mightly Tagged type:

scala> Tags.First('a'.some) |+| Tags.First('b'.some)
res26: scalaz.@@[Option[Char],scalaz.Tags.First] = Some(a)
scala> Tags.First(none: Option[Char]) |+| Tags.First('b'.some)
res27: scalaz.@@[Option[Char],scalaz.Tags.First] = Some(b)
scala> Tags.First('a'.some) |+| Tags.First(none: Option[Char])
res28: scalaz.@@[Option[Char],scalaz.Tags.First] = Some(a)


If we want a monoid on Maybe a such that the second parameter is kept if both parameters of mappend are Just values, Data.Monoid provides a the Last a type.

This is Tags.Last:

scala> Tags.Last('a'.some) |+| Tags.Last('b'.some)
res29: scalaz.@@[Option[Char],scalaz.Tags.Last] = Some(b)
scala> Tags.Last(none: Option[Char]) |+| Tags.Last('b'.some)
res30: scalaz.@@[Option[Char],scalaz.Tags.Last] = Some(b)
scala> Tags.Last('a'.some) |+| Tags.Last(none: Option[Char])
res31: scalaz.@@[Option[Char],scalaz.Tags.Last] = Some(a)



Because there are so many data structures that work nicely with folds, the Foldable type class was introduced. Much like Functor is for things that can be mapped over, Foldable is for things that can be folded up!

The equivalent in Scalaz is also called Foldable. Let's see the typeclass contract:

trait Foldable[F[_]] { self =>
  /** Map each element of the structure to a [[scalaz.Monoid]], and combine the results. */
  def foldMap[A,B](fa: F[A])(f: A => B)(implicit F: Monoid[B]): B
  /**Right-associative fold of a structure. */
  def foldRight[A, B](fa: F[A], z: => B)(f: (A, => B) => B): B

Here are the operators:

/** Wraps a value `self` and provides methods related to `Foldable` */
trait FoldableOps[F[_],A] extends Ops[F[A]] {
  implicit def F: Foldable[F]
  final def foldMap[B: Monoid](f: A => B = (a: A) => a): B = F.foldMap(self)(f)
  final def foldRight[B](z: => B)(f: (A, => B) => B): B = F.foldRight(self, z)(f)
  final def foldLeft[B](z: B)(f: (B, A) => B): B = F.foldLeft(self, z)(f)
  final def foldRightM[G[_], B](z: => B)(f: (A, => B) => G[B])(implicit M: Monad[G]): G[B] = F.foldRightM(self, z)(f)
  final def foldLeftM[G[_], B](z: B)(f: (B, A) => G[B])(implicit M: Monad[G]): G[B] = F.foldLeftM(self, z)(f)
  final def foldr[B](z: => B)(f: A => (=> B) => B): B = F.foldr(self, z)(f)
  final def foldl[B](z: B)(f: B => A => B): B = F.foldl(self, z)(f)
  final def foldrM[G[_], B](z: => B)(f: A => ( => B) => G[B])(implicit M: Monad[G]): G[B] = F.foldrM(self, z)(f)
  final def foldlM[G[_], B](z: B)(f: B => A => G[B])(implicit M: Monad[G]): G[B] = F.foldlM(self, z)(f)
  final def foldr1(f: (A, => A) => A): Option[A] = F.foldr1(self)(f)
  final def foldl1(f: (A, A) => A): Option[A] = F.foldl1(self)(f)
  final def sumr(implicit A: Monoid[A]): A = F.foldRight(self, A.zero)(A.append)
  final def suml(implicit A: Monoid[A]): A = F.foldLeft(self, A.zero)(A.append(_, _))
  final def toList: List[A] = F.toList(self)
  final def toIndexedSeq: IndexedSeq[A] = F.toIndexedSeq(self)
  final def toSet: Set[A] = F.toSet(self)
  final def toStream: Stream[A] = F.toStream(self)
  final def all(p: A => Boolean): Boolean = F.all(self)(p)
  final def(p: A => Boolean): Boolean = F.all(self)(p)
  final def allM[G[_]: Monad](p: A => G[Boolean]): G[Boolean] = F.allM(self)(p)
  final def anyM[G[_]: Monad](p: A => G[Boolean]): G[Boolean] = F.anyM(self)(p)
  final def any(p: A => Boolean): Boolean = F.any(self)(p)
  final def(p: A => Boolean): Boolean = F.any(self)(p)
  final def count: Int = F.count(self)
  final def maximum(implicit A: Order[A]): Option[A] = F.maximum(self)
  final def minimum(implicit A: Order[A]): Option[A] = F.minimum(self)
  final def longDigits(implicit d: A <:< Digit): Long = F.longDigits(self)
  final def empty: Boolean = F.empty(self)
  final def element(a: A)(implicit A: Equal[A]): Boolean = F.element(self, a)
  final def splitWith(p: A => Boolean): List[List[A]] = F.splitWith(self)(p)
  final def selectSplit(p: A => Boolean): List[List[A]] = F.selectSplit(self)(p)
  final def collapse[X[_]](implicit A: ApplicativePlus[X]): X[A] = F.collapse(self)
  final def concatenate(implicit A: Monoid[A]): A = F.fold(self)
  final def traverse_[M[_]:Applicative](f: A => M[Unit]): M[Unit] = F.traverse_(self)(f)

That was impressive. Looks almost like the collection libraries, except it's taking advantage of typeclasses like Order. Let's try folding:

scala> List(1, 2, 3).foldRight (1) {_ * _}
res49: Int = 6
scala> 9.some.foldLeft(2) {_ + _}
res50: Int = 11

These are already in the standard library. Let's try the foldMap operator. Monoid[A] gives us zero and |+|, so that's enough information to fold things over. Since we can't assume that Foldable contains a monoid we need a function to change from A => B where [B: Monoid]:

scala> List(1, 2, 3) foldMap {identity}
res53: Int = 6
scala> List(true, false, true, true) foldMap {Tags.Disjunction}
res56: scalaz.@@[Boolean,scalaz.Tags.Disjunction] = true

This surely beats writing Tags.Disjunction(true) for each of them and connecting them with |+|.

We will pick it up from here later. I'll be out on a business trip, it might slow down.