Add scan1Semigroup method to Stream

core/shared/src/main/scala/fs2/Stream.scala

@@ -2533,6 +2533,17 @@ final class Stream[+F[_], +O] private[fs2] (private val free: FreeC[F, O, Unit])
       case Some((hd, tl)) => Pull.output1(hd) >> tl.scan_(hd: O2)(f)
     }.stream
 
+  /**
+    * Like `[[scan1]], but uses the implicitly available `Semigroup[O2]` to combine elements.
+    *
+    * @example {{{
+    * scala> Stream(1,2,3,4).scan1Semigroup.toList
+    * res0: List[Int] = List(1, 3, 6, 10)
+    * }}}
+    */
+  def scan1Semigroup[O2 >: O](implicit O2: Semigroup[O2]): Stream[F, O2] =
+    scan1(O2.combine)
+
   /**
     * Like `scan` but `f` is applied to each chunk of the source stream.
     * The resulting chunk is emitted and the result of the chunk is used in the

core/shared/src/test/scala/fs2/StreamCombinatorsSuite.scala

@@ -1159,6 +1159,16 @@ class StreamCombinatorsSuite extends Fs2Suite {
     }
   }
 
+  property("scan1Semigroup") {
+    forAll { (s: Stream[Pure, Int]) =>
+      val v = s.toVector
+      val f = (a: Int, b: Int) => a + b
+      s.scan1Semigroup.toVector == v.headOption.fold(Vector.empty[Int])(h =>
+        v.drop(1).scanLeft(h)(f)
+      )
+    }
+  }
+
   test("sleep") {
     val delay = 200.millis
     // force a sync up in duration, then measure how long sleep takes