Add Semigroup.instance method

kernel/src/main/scala/cats/kernel/Semigroup.scala

@@ -1,7 +1,7 @@
 package cats.kernel
 
 import scala.{ specialized => sp }
-import scala.annotation.{ tailrec }
+import scala.annotation.tailrec
 
 /**
  * A semigroup is any set `A` with an associative operation (`combine`).
@@ -76,4 +76,11 @@ object Semigroup extends SemigroupFunctions[Semigroup] {
    * Access an implicit `Semigroup[A]`.
    */
   @inline final def apply[A](implicit ev: Semigroup[A]): Semigroup[A] = ev
+
+  /**
+   * Create a `Semigroup` instance from the given function.
+   */
+  @inline def instance[A](cmb: (A, A) => A): Semigroup[A] = new Semigroup[A] {
+    override def combine(x: A, y: A): A = cmb(x, y)
+  }
 }

tests/src/test/scala/cats/tests/SemigroupSuite.scala

@@ -2,10 +2,10 @@ package cats
 package tests
 
 import org.scalatest._
+import org.scalatest.prop.GeneratorDrivenPropertyChecks
 
 
-
-class SemigroupSuite extends FunSuite {
+class SemigroupSuite extends FunSuite with Matchers with GeneratorDrivenPropertyChecks {
   {
     import cats.implicits._
     Invariant[Semigroup]
@@ -19,4 +19,14 @@ class SemigroupSuite extends FunSuite {
     Semigroupal[Semigroup]
     InvariantMonoidal[Semigroup]
   }
+
+  test("Semigroup.instance creates a Semigroup from the given function") {
+    val mult: (Int, Int) => Int = (a, b) => a * b
+    val add: (Int, Int) => Int = (a, b) => a + b
+
+    forAll { (a: Int, b: Int) =>
+      Semigroup.instance(mult).combine(a, b) should === (a * b)
+      Semigroup.instance(add).combine(a, b) should === (a + b)
+    }
+  }
 }