interpreter-based `Cross` seems to work

1 files changed, 62 insertions, 13 deletions

diff --git a/core/src/main/scala/mill/define/Cross.scala b/core/src/main/scala/mill/define/Cross.scala
index c84e9b3d..1f457096 100644
--- a/core/src/main/scala/mill/define/Cross.scala
+++ b/core/src/main/scala/mill/define/Cross.scala
@@ -1,18 +1,67 @@
 package mill.define
 
+sealed trait Cross[T]{
+  def flatMap[V](f: T => Cross[V]): Cross[V] = new Cross.FlatMapping(this, f)
+  def map[V](f: T => V): Cross[V] = new Cross.Mapping(this, f)
+  def withFilter(f: T => Boolean): Cross[T] = new Cross.Filtering(this, f)
 
-case class Cross[+T](items: T*){
-  def flatMap[V](f: T => Cross[V]): Cross[(T, V)] = {
-    val flattened = for{
-      i <- items
-      k <- f(i).items
-    } yield (i, k)
-    Cross(flattened:_*)
-  }
-  def map[V](f: T => V): Cross[(T, V)] = {
-    Cross(items.map(i => i -> f(i)):_*)
+}
+object Cross{
+  class Listing[T](val items: Seq[T]) extends Cross[T]
+  class Mapping[T, V](val parent: Cross[T], val f: T => V) extends Cross[V]
+  class FlatMapping[T, V](val parent: Cross[T], val f: T => Cross[V]) extends Cross[V]
+  class Filtering[T](val parent: Cross[T], val f: T => Boolean) extends Cross[T]
+
+  def apply[T](t: T*) = new Cross.Listing(t)
+
+  def evaluate[T](c: Cross[T]): List[(List[Any], T)] = c match{
+    case c: Listing[T] =>  c.items.map(i => List(i) -> i).toList
+    case c: Mapping[_, T] => evaluate(c.parent).map{case (l, v) => (l, c.f(v))}
+    case c: FlatMapping[_, T] =>
+      evaluate(c.parent).flatMap{
+        case (l, v) => evaluate(c.f(v)).map{case (l2, v2) => (l2 ::: l, v2)}
+      }
+    case c: Filtering[T] => evaluate(c.parent).filter(t => c.f(t._2))
   }
-  def withFilter(f: T => Boolean) = {
-    Cross(items.filter(f):_*)
+  def test() = {
+    val example1 = evaluate(
+      for(a <- Cross(1, 2, 3))
+      yield a.toString
+    )
+    pprint.log(example1)
+
+    val example2 = evaluate(
+      for{
+        a <- Cross(1, 2, 3)
+        b <- Cross("A", "B", "C")
+      } yield b * a
+    )
+    pprint.pprintln(example2)
+
+    val example3 = evaluate(
+      for{
+        a <- Cross(1, 2, 3)
+        b <- Cross("A", "B", "C")
+        c <- Cross(true, false)
+      } yield b * a + c
+    )
+    pprint.log(example3)
+
+    val example4 = evaluate(
+      for{
+        a <- Cross(1, 2, 3)
+        b <- Cross("A", "B", "C")
+        if !(a == 2 && b == "B")
+      } yield b * a
+    )
+    pprint.log(example4)
+
+    val example5 = evaluate(
+      for{
+        (a, b) <- for(a <- Cross(1, 2, 3); b <- Cross("A", "B", "C")) yield (a, b)
+        c <- Cross(true, false)
+      } yield b * a + c
+    )
+    pprint.log(example5)
   }
-}
+}
\ No newline at end of file