Implement Append for Function1

main-settings/src/main/scala/sbt/Append.scala

@@ -101,4 +101,13 @@ object Append {
       def appendValues(a: Seq[Source], b: Seq[File]): Seq[Source] =
         a ++ b.map(new Source(_, AllPassFilter, NothingFilter))
     }
+
+  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
+    }
+  }
 }

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"))
+}