diff --git a/main-settings/src/main/scala/sbt/Append.scala b/main-settings/src/main/scala/sbt/Append.scala
index 9f14995d2..ce77805dc 100644
--- a/main-settings/src/main/scala/sbt/Append.scala
+++ b/main-settings/src/main/scala/sbt/Append.scala
@@ -8,27 +8,25 @@
 package sbt
 
 import java.io.File
-import Def.Classpath
-import scala.annotation.implicitNotFound
-import sbt.internal.util.Attributed
-import Def.Initialize
-import reflect.internal.annotations.compileTimeOnly
+
+import scala.annotation.{ compileTimeOnly, implicitNotFound }
+
+import sbt.Def.{ Classpath, Initialize }
 import sbt.internal.io.Source
+import sbt.internal.util.Attributed
 import sbt.io.{ AllPassFilter, NothingFilter }
 
 object Append {
-  @implicitNotFound(
-    msg = "No implicit for Append.Value[${A}, ${B}] found,\n  so ${B} cannot be appended to ${A}"
-  )
+  @implicitNotFound("No Append.Value[${A}, ${B}] found, so ${B} cannot be appended to ${A}")
   trait Value[A, B] {
     def appendValue(a: A, b: B): A
   }
-  @implicitNotFound(
-    msg = "No implicit for Append.Values[${A}, ${B}] found,\n  so ${B} cannot be appended to ${A}"
-  )
+
+  @implicitNotFound("No Append.Values[${A}, ${B}] found, so ${B} cannot be appended to ${A}")
   trait Values[A, -B] {
     def appendValues(a: A, b: B): A
   }
+
   trait Sequence[A, -B, T] extends Value[A, T] with Values[A, B]
 
   implicit def appendSeq[T, V <: T]: Sequence[Seq[T], Seq[V], V] =
@@ -36,82 +34,82 @@ object Append {
       def appendValues(a: Seq[T], b: Seq[V]): Seq[T] = a ++ b
       def appendValue(a: Seq[T], b: V): Seq[T] = a :+ b
     }
+
   implicit def appendSeqImplicit[T, V](implicit ev: V => T): Sequence[Seq[T], Seq[V], V] =
     new Sequence[Seq[T], Seq[V], V] {
-      def appendValues(a: Seq[T], b: Seq[V]): Seq[T] =
-        a ++ (b map { x =>
-          (x: T)
-        })
+      def appendValues(a: Seq[T], b: Seq[V]): Seq[T] = a ++ b.map(x => (x: T))
       def appendValue(a: Seq[T], b: V): Seq[T] = a :+ (b: T)
     }
+
   @compileTimeOnly("This can be used in += only.")
   implicit def appendTaskValueSeq[T, V <: T]: Value[Seq[Task[T]], Initialize[Task[V]]] =
-    new Value[Seq[Task[T]], Initialize[Task[V]]] {
-      def appendValue(a: Seq[Task[T]], b: Initialize[Task[V]]): Seq[Task[T]] = ???
-    }
+    (_, _) => ??? // SAM conversion.  This implementation is rewritten by sbt's macros too.
+
   @compileTimeOnly("This can be used in += only.")
-  implicit def appendTaskKeySeq[T, V <: T]: Value[Seq[Task[T]], TaskKey[V]] =
-    new Value[Seq[Task[T]], TaskKey[V]] {
-      def appendValue(a: Seq[Task[T]], b: TaskKey[V]): Seq[Task[T]] = ???
-    }
+  implicit def appendTaskKeySeq[T, V <: T]: Value[Seq[Task[T]], TaskKey[V]] = (_, _) => ??? // SAM
+
   implicit def appendList[T, V <: T]: Sequence[List[T], List[V], V] =
     new Sequence[List[T], List[V], V] {
       def appendValues(a: List[T], b: List[V]): List[T] = a ::: b
       def appendValue(a: List[T], b: V): List[T] = a :+ b
     }
+
   implicit def appendListImplicit[T, V](implicit ev: V => T): Sequence[List[T], List[V], V] =
     new Sequence[List[T], List[V], V] {
-      def appendValues(a: List[T], b: List[V]): List[T] =
-        a ::: (b map { x =>
-          (x: T)
-        })
+      def appendValues(a: List[T], b: List[V]): List[T] = a ::: b.map(x => (x: T))
       def appendValue(a: List[T], b: V): List[T] = a :+ (b: T)
     }
+
   implicit def appendVectorImplicit[T, V](implicit ev: V => T): Sequence[Vector[T], Seq[V], V] =
     new Sequence[Vector[T], Seq[V], V] {
-      def appendValues(a: Vector[T], b: Seq[V]): Vector[T] =
-        a ++ (b map { x =>
-          (x: T)
-        })
+      def appendValues(a: Vector[T], b: Seq[V]): Vector[T] = a ++ b.map(x => (x: T))
       def appendValue(a: Vector[T], b: V): Vector[T] = a :+ (b: T)
     }
-  implicit def appendString: Value[String, String] = new Value[String, String] {
-    def appendValue(a: String, b: String) = a + b
-  }
-  implicit def appendInt = new Value[Int, Int] {
-    def appendValue(a: Int, b: Int) = a + b
-  }
-  implicit def appendLong = new Value[Long, Long] {
-    def appendValue(a: Long, b: Long) = a + b
-  }
-  implicit def appendDouble = new Value[Double, Double] {
-    def appendValue(a: Double, b: Double) = a + b
-  }
+
+  // psst... these are implemented with SAM conversions
+  implicit def appendString: Value[String, String] = _ + _
+  implicit def appendInt: Value[Int, Int] = _ + _
+  implicit def appendLong: Value[Long, Long] = _ + _
+  implicit def appendDouble: Value[Double, Double] = _ + _
+
   implicit def appendClasspath: Sequence[Classpath, Seq[File], File] =
     new Sequence[Classpath, Seq[File], File] {
       def appendValues(a: Classpath, b: Seq[File]): Classpath = a ++ Attributed.blankSeq(b)
       def appendValue(a: Classpath, b: File): Classpath = a :+ Attributed.blank(b)
     }
+
   implicit def appendSet[T, V <: T]: Sequence[Set[T], Set[V], V] =
     new Sequence[Set[T], Set[V], V] {
       def appendValues(a: Set[T], b: Set[V]): Set[T] = a ++ b
       def appendValue(a: Set[T], b: V): Set[T] = a + b
     }
+
   implicit def appendMap[A, B, X <: A, Y <: B]: Sequence[Map[A, B], Map[X, Y], (X, Y)] =
     new Sequence[Map[A, B], Map[X, Y], (X, Y)] {
       def appendValues(a: Map[A, B], b: Map[X, Y]): Map[A, B] = a ++ b
       def appendValue(a: Map[A, B], b: (X, Y)): Map[A, B] = a + b
     }
+
   implicit def appendOption[T]: Sequence[Seq[T], Option[T], Option[T]] =
     new Sequence[Seq[T], Option[T], Option[T]] {
       def appendValue(a: Seq[T], b: Option[T]): Seq[T] = b.fold(a)(a :+ _)
       def appendValues(a: Seq[T], b: Option[T]): Seq[T] = b.fold(a)(a :+ _)
     }
+
   implicit def appendSource: Sequence[Seq[Source], Seq[File], File] =
     new Sequence[Seq[Source], Seq[File], File] {
-      def appendValue(a: Seq[Source], b: File): Seq[Source] =
-        appendValues(a, Seq(b))
+      def appendValue(a: Seq[Source], b: File): Seq[Source] = appendValues(a, Seq(b))
       def appendValues(a: Seq[Source], b: Seq[File]): Seq[Source] =
         a ++ b.map(new Source(_, AllPassFilter, NothingFilter))
     }
+
+  // Implemented with SAM conversion short-hand
+  implicit def appendFunction[A, B]: Value[A => A, A => A] = _.andThen(_)
+
+  implicit def appendSideEffectToFunc[A, B]: Value[A => B, () => Unit] = (f, sideEffect) => {
+    f.andThen { b =>
+      sideEffect()
+      b
+    }
+  }
 }
diff --git a/main-settings/src/test/scala/sbt/AppendSpec.scala b/main-settings/src/test/scala/sbt/AppendSpec.scala
new file mode 100644
index 000000000..a9c0e035e
--- /dev/null
+++ b/main-settings/src/test/scala/sbt/AppendSpec.scala
@@ -0,0 +1,31 @@
+/*
+ * sbt
+ * Copyright 2011 - 2018, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package sbt
+
+object AppendSpec {
+  val onLoad = SettingKey[State => State]("onLoad")
+
+  import Scope.Global
+  import SlashSyntax0._
+
+  def doSideEffect(): Unit = ()
+
+  Global / onLoad := (Global / onLoad).value.andThen { s =>
+    doSideEffect()
+    s
+  }
+
+  Global / onLoad += { (s: State) =>
+    doSideEffect()
+    s
+  }
+
+  Global / onLoad += doSideEffect _
+  Global / onLoad += (() => doSideEffect())
+  Global / onLoad += (() => println("foo"))
+}