Add scala.Tuple.Fold

Author: nicolasstucki

library/src/scala/Tuple.scala

@@ -103,6 +103,11 @@ object Tuple {
     case x *: xs => S[Size[xs]]
   }
 
+  /** Fold a tuple `(T1, ..., Tn)` into `F[T1, F[... F[Tn, Z]...]]]` */
+  type Fold[T <: Tuple, Z, F[_, _]] = T match
+    case EmptyTuple => Z
+    case h *: t => F[h, Fold[t, Z, F]]
+
   /** Converts a tuple `(T1, ..., Tn)` to `(F[T1], ..., F[Tn])` */
   type Map[Tup <: Tuple, F[_]] <: Tuple = Tup match {
     case EmptyTuple => EmptyTuple

tests/pos/tuple-fold.scala

@@ -0,0 +1,8 @@
+
+type Empty[X] = EmptyTuple
+type Twice[X] = (X, X)
+
+def test =
+  val a1: EmptyTuple = ??? : Tuple.Fold[EmptyTuple, Nothing, Tuple2]
+  val a2: (Int, (String, Nothing)) = ??? : Tuple.Fold[(Int, String), Nothing, Tuple2]
+  val a3: Int | String | Char = ??? : Tuple.Fold[(Int, String, Char), Nothing, [X, Y] =>> X | Y]