Add Applicative.unit (#1586)

* Add Applicative#unit * Fix comment
typelevel · Apr 4, 2017 · 43d205f · 43d205f
1 parent e77bb99
commit 43d205f
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diff --git a/core/src/main/scala/cats/Applicative.scala b/core/src/main/scala/cats/Applicative.scala
@@ -22,6 +22,14 @@ import simulacrum.typeclass
    */
   def pure[A](x: A): F[A]
 
+  /**
+   * Returns an `F[Unit]` value, equivalent with `pure(())`.
+   *
+   * A useful shorthand, also allowing implementations to optimize the
+   * returned reference (e.g. it can be a `val`).
+   */
+  def unit: F[Unit] = pure(())
+
   override def map[A, B](fa: F[A])(f: A => B): F[B] =
     ap(pure(f))(fa)
 

diff --git a/laws/src/main/scala/cats/laws/ApplicativeLaws.scala b/laws/src/main/scala/cats/laws/ApplicativeLaws.scala
@@ -35,6 +35,9 @@ trait ApplicativeLaws[F[_]] extends ApplyLaws[F] {
   def apProductConsistent[A, B](fa: F[A], f: F[A => B]): IsEq[F[B]] =
     F.ap(f)(fa) <-> F.map(F.product(f, fa)) { case (f, a) => f(a) }
 
+  def applicativeUnit[A](a: A): IsEq[F[A]] =
+    F.unit.map(_ => a) <-> F.pure(a)
+
   // The following are the lax monoidal functor identity laws - the associativity law is covered by
   // Cartesian's associativity law.
 

diff --git a/laws/src/main/scala/cats/laws/discipline/ApplicativeTests.scala b/laws/src/main/scala/cats/laws/discipline/ApplicativeTests.scala
@@ -31,6 +31,7 @@ trait ApplicativeTests[F[_]] extends ApplyTests[F] {
       "applicative homomorphism" -> forAll(laws.applicativeHomomorphism[A, B] _),
       "applicative interchange" -> forAll(laws.applicativeInterchange[A, B] _),
       "applicative map" -> forAll(laws.applicativeMap[A, B] _),
+      "applicative unit" -> forAll(laws.applicativeUnit[A] _),
       "ap consistent with product + map" -> forAll(laws.apProductConsistent[A, B] _),
       "monoidal left identity" -> forAll((fa: F[A]) => iso.leftIdentity(laws.monoidalLeftIdentity(fa))),
       "monoidal right identity" -> forAll((fa: F[A]) => iso.rightIdentity(laws.monoidalRightIdentity(fa))))