Add scala.Tuple.FlatMap

Author: nicolasstucki

library/src/scala/Tuple.scala

@@ -109,6 +109,12 @@ object Tuple {
     case h *: t => F[h] *: Map[t, F]
   }
 
+  /** Converts a tuple `(T1, ..., Tn)` to a flattened `(..F[T1], ..., ..F[Tn])` */
+  type FlatMap[Tup <: Tuple, F[_] <: Tuple] <: Tuple = Tup match {
+    case EmptyTuple => EmptyTuple
+    case h *: t => Concat[F[h], FlatMap[t, F]]
+  }
+
   /** Given two tuples, `A1 *: ... *: An * At` and `B1 *: ... *: Bn *: Bt`
    *  where at least one of `At` or `Bt` is `EmptyTuple` or `Tuple`,
    *  returns the tuple type `(A1, B1) *: ... *: (An, Bn) *: Ct`

tests/pos/tuple-flatmap.scala

@@ -0,0 +1,14 @@
+
+type Empty[X] = EmptyTuple
+type Twice[X] = (X, X)
+
+def test =
+  val a1: EmptyTuple = ??? : Tuple.FlatMap[EmptyTuple, Empty]
+  val a2: EmptyTuple = ??? : Tuple.FlatMap[(Int, String), Empty]
+
+  val b1: EmptyTuple = ??? : Tuple.FlatMap[EmptyTuple, Tuple1]
+  val b2: (Int, String) = ??? : Tuple.FlatMap[(Int, String), Tuple1]
+
+  val c1: EmptyTuple = ??? : Tuple.FlatMap[EmptyTuple, Twice]
+  val c2: (Int, Int, String, String) = ??? : Tuple.FlatMap[(Int, String), Twice]
+  val c3: (Int, List[Int], String, List[String]) = ??? : Tuple.FlatMap[(Int, String), [X] =>> (X, List[X])]