Merge pull request #1520 from peterneyens/foldable-intercalate

Add Foldable.intercalate
typelevel · Jan 7, 2017 · fd15e07 · fd15e07
2 parents ec0af6e + d02699e
commit fd15e07
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Showing 4 changed files with 53 additions and 2 deletions.
diff --git a/core/src/main/scala/cats/Foldable.scala b/core/src/main/scala/cats/Foldable.scala
@@ -386,6 +386,26 @@ import simulacrum.typeclass
   def nonEmpty[A](fa: F[A]): Boolean =
     !isEmpty(fa)
 
+  /**
+   * Intercalate/insert an element between the existing elements while folding.
+   *
+   * {{{
+   * scala> import cats.implicits._
+   * scala> Foldable[List].intercalate(List("a","b","c"), "-")
+   * res0: String = a-b-c
+   * scala> Foldable[List].intercalate(List("a"), "-")
+   * res1: String = a
+   * scala> Foldable[List].intercalate(List.empty[String], "-")
+   * res2: String = ""
+   * scala> Foldable[Vector].intercalate(Vector(1,2,3), 1)
+   * res3: Int = 8
+   * }}}
+   */
+  def intercalate[A](fa: F[A], a: A)(implicit A: Monoid[A]): A =
+    reduceLeftOption(fa){ (acc, aa) =>
+      A.combine(acc, A.combine(a, aa))
+    }.getOrElse(A.empty)
+
   def compose[G[_]: Foldable]: Foldable[λ[α => F[G[α]]]] =
     new ComposedFoldable[F, G] {
       val F = self

diff --git a/core/src/main/scala/cats/Reducible.scala b/core/src/main/scala/cats/Reducible.scala
@@ -156,6 +156,25 @@ import simulacrum.typeclass
 
   def maximum[A](fa: F[A])(implicit A: Order[A]): A =
     reduceLeft(fa)(A.max)
+
+  /**
+   * Intercalate/insert an element between the existing elements while reducing.
+   *
+   * {{{
+   * scala> import cats.implicits._
+   * scala> import cats.data.NonEmptyList
+   * scala> val nel = NonEmptyList.of("a", "b", "c")
+   * scala> Reducible[NonEmptyList].intercalate1(nel, "-")
+   * res0: String = a-b-c
+   * scala> Reducible[NonEmptyList].intercalate1(NonEmptyList.of("a"), "-")
+   * res1: String = a
+   * }}}
+   */
+  def intercalate1[A](fa: F[A], a: A)(implicit A: Semigroup[A]): A =
+    reduceLeft(fa)((acc, aa) => A.combine(acc, A.combine(a, aa)))
+
+  override def intercalate[A](fa: F[A], a: A)(implicit A: Monoid[A]): A =
+    intercalate1(fa, a)
 }
 
 /**

diff --git a/tests/src/test/scala/cats/tests/FoldableTests.scala b/tests/src/test/scala/cats/tests/FoldableTests.scala
@@ -6,7 +6,7 @@ import org.scalacheck.Arbitrary
 
 import cats.instances.all._
 
-abstract class FoldableCheck[F[_]: Foldable](name: String)(implicit ArbFInt: Arbitrary[F[Int]]) extends CatsSuite with PropertyChecks {
+abstract class FoldableCheck[F[_]: Foldable](name: String)(implicit ArbFInt: Arbitrary[F[Int]], ArbFString: Arbitrary[F[String]]) extends CatsSuite with PropertyChecks {
 
   def iterator[T](fa: F[T]): Iterator[T]
 
@@ -67,6 +67,12 @@ abstract class FoldableCheck[F[_]: Foldable](name: String)(implicit ArbFInt: Arb
       fa.reduceRightOption((x, ly) => ly.map(x - _)).value should === (list.reduceRightOption(_ - _))
     }
   }
+
+  test("intercalate") {
+    forAll { (fa: F[String], a: String) =>
+      fa.intercalate(a) should === (fa.toList.mkString(a))
+    }
+  }
 }
 
 class FoldableTestsAdditional extends CatsSuite {

diff --git a/tests/src/test/scala/cats/tests/ReducibleTests.scala b/tests/src/test/scala/cats/tests/ReducibleTests.scala
@@ -62,7 +62,7 @@ class ReducibleTestsAdditional extends CatsSuite {
 
 }
 
-abstract class ReducibleCheck[F[_]: Reducible](name: String)(implicit ArbFInt: Arbitrary[F[Int]]) extends FoldableCheck[F](name) {
+abstract class ReducibleCheck[F[_]: Reducible](name: String)(implicit ArbFInt: Arbitrary[F[Int]], ArbFString: Arbitrary[F[String]]) extends FoldableCheck[F](name) {
   def range(start: Long, endInclusive: Long): F[Long]
 
   test(s"Reducible[$name].reduceLeftM stack safety") {
@@ -80,4 +80,10 @@ abstract class ReducibleCheck[F[_]: Reducible](name: String)(implicit ArbFInt: A
       fa.toList.toNel should === (Some(fa.toNonEmptyList))
     }
   }
+
+  test(s"Reducible[$name].intercalate1") {
+    forAll { (fa: F[String], a: String) =>
+      fa.intercalate1(a) === (fa.toList.mkString(a))
+    }
+  }
 }