From 9cca64627d31f078c565c6865a50ae558f567d8f Mon Sep 17 00:00:00 2001
From: Li Haoyi <haoyi.sg@gmail.com>
Date: Fri, 12 Jan 2018 20:28:43 -0800
Subject: Allow implicit `crossModule()` syntax to automatically find a set of
 cross-coordinates which are compatible with a given implicit resolver

---
 core/src/main/scala/mill/define/Cross.scala        | 29 +++++++++++++++++++---
 core/src/main/scala/mill/discover/Discovered.scala |  2 +-
 scalalib/src/main/scala/mill/scalalib/Module.scala | 21 +++++++++++++++-
 3 files changed, 47 insertions(+), 5 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]"
       }
diff --git a/scalalib/src/main/scala/mill/scalalib/Module.scala b/scalalib/src/main/scala/mill/scalalib/Module.scala
index de962c9c..a6819435 100644
--- a/scalalib/src/main/scala/mill/scalalib/Module.scala
+++ b/scalalib/src/main/scala/mill/scalalib/Module.scala
@@ -3,12 +3,13 @@ package scalalib
 
 import ammonite.ops._
 import coursier.{Cache, MavenRepository, Repository}
-import mill.define.Task
+import mill.define.{Cross, Task}
 import mill.define.Task.TaskModule
 import mill.eval.{PathRef, Result}
 import mill.modules.Jvm
 import mill.modules.Jvm.{createAssembly, createJar, interactiveSubprocess, subprocess}
 import Lib._
+import mill.define.Cross.Resolver
 import sbt.testing.Status
 object TestModule{
   def handleResults(doneMsg: String, results: Seq[TestRunner.Result]) = {
@@ -375,6 +376,24 @@ trait SbtModule extends Module { outer =>
 }
 
 trait CrossSbtModule extends SbtModule { outer =>
+  implicit def crossSbtModuleResolver: Resolver[CrossSbtModule] = new Resolver[CrossSbtModule]{
+    def resolve[V <: CrossSbtModule](c: Cross[V]): V = {
+      crossScalaVersion.split('.')
+        .inits
+        .takeWhile(_.length > 1)
+        .flatMap( prefix =>
+          c.items.map(_._2).find(_.crossScalaVersion.split('.').startsWith(prefix))
+        )
+        .collectFirst{case x => x}
+        .getOrElse(
+          throw new Exception(
+            s"Unable to find compatible cross version between $crossScalaVersion and "+
+            c.items.map(_._2.crossScalaVersion).mkString(",")
+          )
+        )
+    }
+  }
+
   def crossScalaVersion: String
   def scalaVersion = crossScalaVersion
   override def sources = T.input{
-- 
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