Add Foldable.intercalate

core/src/main/scala/cats/Foldable.scala

@@ -386,6 +386,22 @@ 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[Vector].intercalate(Vector(1,2,3), 1)
+   * res1: 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

tests/src/test/scala/cats/tests/FoldableTests.scala

@@ -67,6 +67,14 @@ 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[Int], a: Int) =>
+      val list = fa.toList
+      val sum = list.sum + (math.max(list.length - 1, 0) * a)
+      fa.intercalate(a) should === (sum)
+    }
+  }
 }
 
 class FoldableTestsAdditional extends CatsSuite {