First pass at implementing long-lived `Worker` objects. These currently are managed by the `Evaluator`, which is now a stateful object that shouldn't be thrown away every time.

We still need to update the code/test-suite to make the `Evaluator` hang around in between `evaluate` calls

3 files changed, 53 insertions, 5 deletions

diff --git a/core/src/main/scala/mill/define/Worker.scala b/core/src/main/scala/mill/define/Worker.scala
new file mode 100644
index 00000000..385d795f
--- /dev/null
+++ b/core/src/main/scala/mill/define/Worker.scala
@@ -0,0 +1,35 @@
+package mill.define
+
+import mill.util.Ctx
+
+import scala.annotation.compileTimeOnly
+
+/**
+  * Worker serves three purposes:
+  *
+  * - Cache in-memory state between tasks (e.g. object initialization)
+  *   - Including warm classloaders with isolated bytecode
+  * - Mutex to limit concurrency
+  * - Manage out-of-process subprocesses <-- skip this for now
+  *
+  * Key usage:
+  *
+  * - T{
+  *     ZincWorker().compile(a() + b())
+  *   }
+  *
+  * Desugars into:
+  *
+  * - T.zipMap(ZincWorker, a, b){ (z, a1, b1) => z.compile(a1, b1) }
+  *
+  * Workers are shoehorned into the `Task` type. This lets them fit nicely in
+  * the `T{...}` syntax, as well as being statically-inspectable before
+  * evaluating the task graph. The Worker defines how it is evaluated, but it's
+  * evaluation/caching/lifecycle are controlled by the `Evaluator`
+  */
+trait Worker[V] extends Task[V]{
+  val inputs = Nil
+  def make(): V
+  def evaluate(args: Ctx) = args.workerFor(this)
+  def path = this.getClass.getCanonicalName.filter(_ != '$').split('.')
+}
\ No newline at end of file
diff --git a/core/src/main/scala/mill/eval/Evaluator.scala b/core/src/main/scala/mill/eval/Evaluator.scala
index 9995215a..dffd609e 100644
--- a/core/src/main/scala/mill/eval/Evaluator.scala
+++ b/core/src/main/scala/mill/eval/Evaluator.scala
@@ -4,11 +4,12 @@ import java.net.URLClassLoader
 
 import ammonite.ops._
 import ammonite.runtime.SpecialClassLoader
-import mill.define.{Graph, Target, Task}
+import mill.define.{Graph, Target, Task, Worker}
 import mill.discover.Mirror
 import mill.discover.Mirror.LabelledTarget
 import mill.discover.Mirror.Segment.{Cross, Label}
 import mill.util
+import mill.util.Ctx.WorkerCtx
 import mill.util._
 
 import scala.collection.mutable
@@ -18,7 +19,7 @@ class Evaluator(workspacePath: Path,
                 log: Logger,
                 sel: List[Mirror.Segment] = List(),
                 classLoaderSig: Seq[(Path, Long)] = Evaluator.classLoaderSig){
-
+  val workerCache = mutable.Map.empty[Worker[_], _]
   def evaluate(goals: OSet[Task[_]]): Evaluator.Results = {
     mkdir(workspacePath)
 
@@ -163,7 +164,12 @@ class Evaluator(workspacePath: Path,
           val args = new Ctx(
             targetInputValues.toArray[Any],
             targetDestPath.orNull,
-            multiLogger
+            multiLogger,
+            new WorkerCtx{
+              def workerFor[T](x: mill.define.Worker[T]): T = {
+                workerCache.getOrElseUpdate(x, x.make()).asInstanceOf[T]
+              }
+            }
           )
 
           val out = System.out
diff --git a/core/src/main/scala/mill/util/Ctx.scala b/core/src/main/scala/mill/util/Ctx.scala
index 7a0a5694..3b8fb171 100644
--- a/core/src/main/scala/mill/util/Ctx.scala
+++ b/core/src/main/scala/mill/util/Ctx.scala
@@ -4,7 +4,8 @@ import java.io.PrintStream
 
 import ammonite.ops.Path
 import mill.define.Applicative.ImplicitStub
-import mill.util.Ctx.{ArgCtx, DestCtx, LogCtx}
+import mill.define.Worker
+import mill.util.Ctx.{ArgCtx, DestCtx, LogCtx, WorkerCtx}
 
 import scala.annotation.compileTimeOnly
 
@@ -22,10 +23,16 @@ object Ctx{
   trait ArgCtx{
     def args: IndexedSeq[_]
   }
+  trait WorkerCtx{
+    def workerFor[T](x: mill.define.Worker[T]): T
+  }
 }
 class Ctx(val args: IndexedSeq[_],
           val dest: Path,
-          val log: Logger) extends DestCtx with LogCtx with ArgCtx{
+          val log: Logger,
+          workerCtx0: Ctx.WorkerCtx) extends DestCtx with LogCtx with ArgCtx with WorkerCtx{
+
+  def workerFor[T](x: mill.define.Worker[T]): T = workerCtx0.workerFor(x)
   def length = args.length
   def apply[T](index: Int): T = {
     if (index >= 0 && index < args.length) args(index).asInstanceOf[T]