Scalafmt support (#308)

* add scalafmt module, that formats all sources files on every run

* scalafmt worker that internally chaches reformatted files

* move jvm process call to helper method

* use scala 2.12.4 to resolve scalafmt deps; check for config file existence; add quiet flags to scalafmt CLI

* make a scalafmt worker a singleton

* add tests for scalafmt module

* add reformatAll command

* tests for reformatAll command

* add docs about scalafmt support

2 files changed, 116 insertions, 0 deletions

diff --git a/scalalib/src/mill/scalalib/scalafmt/ScalafmtModule.scala b/scalalib/src/mill/scalalib/scalafmt/ScalafmtModule.scala
new file mode 100644
index 00000000..3b325b4c
--- /dev/null
+++ b/scalalib/src/mill/scalalib/scalafmt/ScalafmtModule.scala
@@ -0,0 +1,58 @@
+package mill.scalalib.scalafmt
+
+import ammonite.ops.{exists, ls, pwd}
+import mill._
+import mill.define._
+import mill.scalalib._
+
+trait ScalafmtModule extends JavaModule {
+
+  def reformat(): Command[Unit] = T.command {
+    ScalafmtWorkerModule
+      .worker()
+      .reformat(
+        filesToFormat(sources()),
+        scalafmtConfig().head,
+        scalafmtDeps().map(_.path)
+      )
+  }
+
+  def scalafmtVersion: T[String] = "1.5.1"
+
+  def scalafmtConfig: Sources = T.sources(pwd / ".scalafmt.conf")
+
+  def scalafmtDeps: T[Agg[PathRef]] = T {
+    Lib.resolveDependencies(
+      ScalaWorkerModule.repositories,
+      Lib.depToDependency(_, "2.12.4"),
+      Seq(ivy"com.geirsson::scalafmt-cli:${scalafmtVersion()}")
+    )
+  }
+
+  protected def filesToFormat(sources: Seq[PathRef]) = {
+    for {
+      pathRef <- sources if exists(pathRef.path)
+      file <- ls.rec(pathRef.path) if file.isFile && file.ext == "scala"
+    } yield PathRef(file)
+  }
+
+}
+
+object ScalafmtModule extends ExternalModule with ScalafmtModule {
+
+  def reformatAll(sources: mill.main.Tasks[Seq[PathRef]]): Command[Unit] =
+    T.command {
+      val files = Task.sequence(sources.value)().flatMap(filesToFormat)
+      ScalafmtWorkerModule
+        .worker()
+        .reformat(
+          files,
+          scalafmtConfig().head,
+          scalafmtDeps().map(_.path)
+        )
+    }
+
+  implicit def millScoptTargetReads[T] = new mill.main.Tasks.Scopt[T]()
+
+  lazy val millDiscover = Discover[this.type]
+}
diff --git a/scalalib/src/mill/scalalib/scalafmt/ScalafmtWorker.scala b/scalalib/src/mill/scalalib/scalafmt/ScalafmtWorker.scala
new file mode 100644
index 00000000..d9921e9d
--- /dev/null
+++ b/scalalib/src/mill/scalalib/scalafmt/ScalafmtWorker.scala
@@ -0,0 +1,58 @@
+package mill.scalalib.scalafmt
+
+import ammonite.ops.{Path, exists}
+import mill._
+import mill.define.{Discover, ExternalModule, Worker}
+import mill.modules.Jvm
+import mill.util.Ctx
+
+import scala.collection.mutable
+
+object ScalafmtWorkerModule extends ExternalModule {
+  def worker: Worker[ScalafmtWorker] = T.worker { new ScalafmtWorker() }
+
+  lazy val millDiscover = Discover[this.type]
+}
+
+private[scalafmt] class ScalafmtWorker {
+  private val reformatted: mutable.Map[Path, Int] = mutable.Map.empty
+  private var configSig: Int = 0
+
+  def reformat(input: Seq[PathRef],
+               scalafmtConfig: PathRef,
+               scalafmtClasspath: Agg[Path])(implicit ctx: Ctx): Unit = {
+    val toFormat =
+      if (scalafmtConfig.sig != configSig) input
+      else
+        input.filterNot(ref => reformatted.get(ref.path).contains(ref.sig))
+
+    if (toFormat.nonEmpty) {
+      ctx.log.info(s"Formatting ${toFormat.size} Scala sources")
+      reformatAction(toFormat.map(_.path),
+                     scalafmtConfig.path,
+                     scalafmtClasspath)
+      reformatted ++= toFormat.map { ref =>
+        val updRef = PathRef(ref.path)
+        updRef.path -> updRef.sig
+      }
+      configSig = scalafmtConfig.sig
+    } else {
+      ctx.log.info(s"Everything is formatted already")
+    }
+  }
+
+  private val cliFlags = Seq("--non-interactive", "--quiet")
+
+  private def reformatAction(toFormat: Seq[Path],
+                             config: Path,
+                             classpath: Agg[Path])(implicit ctx: Ctx) = {
+    val configFlags =
+      if (exists(config)) Seq("--config", config.toString) else Seq.empty
+    Jvm.subprocess(
+      "org.scalafmt.cli.Cli",
+      classpath,
+      mainArgs = toFormat.map(_.toString) ++ configFlags ++ cliFlags
+    )
+  }
+
+}