Allow implicit `crossModule()` syntax to automatically find a set of cross-coordinates which are compatible with a given implicit resolver

2 files changed, 27 insertions, 4 deletions

diff --git a/core/src/main/scala/mill/define/Cross.scala b/core/src/main/scala/mill/define/Cross.scala
index 6226590d..2dd4d2fe 100644
--- a/core/src/main/scala/mill/define/Cross.scala
+++ b/core/src/main/scala/mill/define/Cross.scala
@@ -26,10 +26,15 @@ object Cross{
       reify { mill.define.Cross.Factory[T](instance.splice) }
     }
   }
+
+  trait Resolver[-T]{
+    def resolve[V <: T](c: Cross[V]): V
+  }
 }
+
 class Cross[T](cases: Any*)
-              (implicit ci: Cross.Factory[T],
-               val ctx: Module.Ctx){
+               (implicit ci: Cross.Factory[T],
+                val ctx: Module.Ctx){
 
   val items = for(c0 <- cases.toList) yield{
     val c = c0 match{
@@ -47,5 +52,23 @@ class Cross[T](cases: Any*)
     (crossValues, sub)
   }
   val itemMap = items.toMap
-  def apply(args: Any*) = itemMap(args.toList)
+
+  /**
+    * Fetch the cross module corresponding to the given cross values
+    */
+  def get(args: Seq[Any]) = itemMap(args.toList)
+
+  /**
+    * Fetch the cross module corresponding to the given cross values
+    */
+  def apply(arg0: Any, args: Any*) = itemMap(arg0 :: args.toList)
+
+  /**
+    * Fetch the relevant cross module given the implicit resolver you have in
+    * scope. This is often the first cross module whose cross-version is
+    * compatible with the current module.
+    */
+  def apply[V >: T]()(implicit resolver: Cross.Resolver[V]): T = {
+    resolver.resolve(this.asInstanceOf[Cross[V]]).asInstanceOf[T]
+  }
 }
\ No newline at end of file
diff --git a/core/src/main/scala/mill/discover/Discovered.scala b/core/src/main/scala/mill/discover/Discovered.scala
index 0728a027..c5ca4843 100644
--- a/core/src/main/scala/mill/discover/Discovered.scala
+++ b/core/src/main/scala/mill/discover/Discovered.scala
@@ -112,7 +112,7 @@ object Discovered {
         val base = q"${TermName(c.freshName())}"
         val ident = segments.reverse.zipWithIndex.foldLeft[Tree](base) {
           case (prefix, (Some(name), i)) => q"$prefix.${TermName(name)}"
-          case (prefix, (None, i)) => q"$prefix.apply($crossName($i):_*)"
+          case (prefix, (None, i)) => q"$prefix.get($crossName($i))"
         }
         q"($base: $baseType, $crossName: List[List[Any]]) => $ident.asInstanceOf[$t]"
       }