Flesh out a principled implementation of the cross-build monad

1 files changed, 9 insertions, 61 deletions

diff --git a/core/src/main/scala/mill/define/Cross.scala b/core/src/main/scala/mill/define/Cross.scala
index 1f457096..8f8c01e8 100644
--- a/core/src/main/scala/mill/define/Cross.scala
+++ b/core/src/main/scala/mill/define/Cross.scala
@@ -1,67 +1,15 @@
 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: List[(List[Any], T)]){
+  def flatMap[V](f: T => Cross[V]): Cross[V] = new Cross(
+    items.flatMap{
+      case (l, v) => f(v).items.map{case (l2, v2) => (l2 ::: l, v2)}
+    }
+  )
+  def map[V](f: T => V): Cross[V] = new Cross(items.map{case (l, v) => (l, f(v))})
+  def withFilter(f: T => Boolean): Cross[T] = new Cross(items.filter(t => f(t._2)))
 
 }
 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 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)
-  }
+  def apply[T](t: T*) = new Cross(t.map(i => List(i) -> i).toList)
 }
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