[BUILD] Configure scalafmt. Currently, this applies only to Kernel.

.scalafmt.conf

@@ -0,0 +1,26 @@
+align = none
+align.openParenDefnSite = false
+align.openParenCallSite = false
+align.tokens = []
+importSelectors = "singleLine"
+optIn.configStyleArguments = false
+continuationIndent {
+  callSite = 2
+  defnSite = 4
+}
+danglingParentheses {
+  defnSite = false
+  callSite = false
+}
+docstrings {
+  style = Asterisk
+  wrap = no
+}
+literals.hexDigits = upper
+maxColumn = 100
+newlines {
+  beforeCurlyLambdaParams = false
+  source = keep
+}
+runner.dialect = scala212
+version = 3.8.6

build.sbt

@@ -157,11 +157,26 @@ lazy val commonSettings = Seq(
   unidocSourceFilePatterns := Nil,
 )
 
-// enforce java code style
-def javafmtCheckSettings() = Seq(
+////////////////////////////
+// START: Code Formatting //
+////////////////////////////
+
+/** Enforce java code style on compile. */
+def javafmtCheckSettings(): Seq[Def.Setting[Task[CompileAnalysis]]] = Seq(
   (Compile / compile) := ((Compile / compile) dependsOn (Compile / javafmtCheckAll)).value
 )
 
+/** Enforce scala code style on compile. */
+def scalafmtCheckSettings(): Seq[Def.Setting[Task[CompileAnalysis]]] = Seq(
+  (Compile / compile) := ((Compile / compile) dependsOn (Compile / scalafmtCheckAll)).value,
+)
+
+// TODO: define fmtAll and fmtCheckAll tasks that run both scala and java fmts/checks
+
+//////////////////////////
+// END: Code Formatting //
+//////////////////////////
+
 /**
  * Note: we cannot access sparkVersion.value here, since that can only be used within a task or
  *       setting macro.
@@ -237,7 +252,7 @@ def runTaskOnlyOnSparkMaster[T](
 }
 
 lazy val connectCommon = (project in file("spark-connect/common"))
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "delta-connect-common",
     commonSettings,
@@ -276,7 +291,7 @@ lazy val connectCommon = (project in file("spark-connect/common"))
   )
 
 lazy val connectClient = (project in file("spark-connect/client"))
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .dependsOn(connectCommon % "compile->compile;test->test;provided->provided")
   .settings(
     name := "delta-connect-client",
@@ -365,7 +380,7 @@ lazy val connectClient = (project in file("spark-connect/client"))
 lazy val connectServer = (project in file("spark-connect/server"))
   .dependsOn(connectCommon % "compile->compile;test->test;provided->provided")
   .dependsOn(spark % "compile->compile;test->test;provided->provided")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "delta-connect-server",
     commonSettings,
@@ -409,7 +424,7 @@ lazy val connectServer = (project in file("spark-connect/server"))
 lazy val spark = (project in file("spark"))
   .dependsOn(storage)
   .enablePlugins(Antlr4Plugin)
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-spark",
     commonSettings,
@@ -497,7 +512,7 @@ lazy val spark = (project in file("spark"))
 
 lazy val contribs = (project in file("contribs"))
   .dependsOn(spark % "compile->compile;test->test;provided->provided")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-contribs",
     commonSettings,
@@ -536,7 +551,7 @@ lazy val contribs = (project in file("contribs"))
 
 lazy val sharing = (project in file("sharing"))
   .dependsOn(spark % "compile->compile;test->test;provided->provided")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "delta-sharing-spark",
     commonSettings,
@@ -562,12 +577,14 @@ lazy val sharing = (project in file("sharing"))
   ).configureUnidoc()
 
 lazy val kernelApi = (project in file("kernel/kernel-api"))
+  .enablePlugins(ScalafmtPlugin)
   .settings(
     name := "delta-kernel-api",
     commonSettings,
     scalaStyleSettings,
     javaOnlyReleaseSettings,
     javafmtCheckSettings,
+    scalafmtCheckSettings,
     Test / javaOptions ++= Seq("-ea"),
     libraryDependencies ++= Seq(
       "org.roaringbitmap" % "RoaringBitmap" % "0.9.25",
@@ -642,6 +659,7 @@ lazy val kernelApi = (project in file("kernel/kernel-api"))
   ).configureUnidoc(docTitle = "Delta Kernel")
 
 lazy val kernelDefaults = (project in file("kernel/kernel-defaults"))
+  .enablePlugins(ScalafmtPlugin)
   .dependsOn(kernelApi)
   .dependsOn(kernelApi % "test->test")
   .dependsOn(storage)
@@ -654,6 +672,7 @@ lazy val kernelDefaults = (project in file("kernel/kernel-defaults"))
     scalaStyleSettings,
     javaOnlyReleaseSettings,
     javafmtCheckSettings,
+    scalafmtCheckSettings,
     Test / javaOptions ++= Seq("-ea"),
     libraryDependencies ++= Seq(
       "org.apache.hadoop" % "hadoop-client-runtime" % hadoopVersion,
@@ -686,7 +705,7 @@ lazy val kernelDefaults = (project in file("kernel/kernel-defaults"))
 // TODO unidoc
 // TODO(scott): figure out a better way to include tests in this project
 lazy val storage = (project in file("storage"))
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-storage",
     commonSettings,
@@ -711,7 +730,7 @@ lazy val storage = (project in file("storage"))
 lazy val storageS3DynamoDB = (project in file("storage-s3-dynamodb"))
   .dependsOn(storage % "compile->compile;test->test;provided->provided")
   .dependsOn(spark % "test->test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-storage-s3-dynamodb",
     commonSettings,
@@ -737,7 +756,7 @@ val icebergSparkRuntimeArtifactName = {
 lazy val testDeltaIcebergJar = (project in file("testDeltaIcebergJar"))
   // delta-iceberg depends on delta-spark! So, we need to include it during our test.
   .dependsOn(spark % "test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "test-delta-iceberg-jar",
     commonSettings,
@@ -767,7 +786,7 @@ val deltaIcebergSparkIncludePrefixes = Seq(
 // scalastyle:off println
 lazy val iceberg = (project in file("iceberg"))
   .dependsOn(spark % "compile->compile;test->test;provided->provided")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-iceberg",
     commonSettings,
@@ -837,7 +856,7 @@ lazy val generateIcebergJarsTask = TaskKey[Unit]("generateIcebergJars", "Generat
 
 lazy val icebergShaded = (project in file("icebergShaded"))
   .dependsOn(spark % "provided")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "iceberg-shaded",
     commonSettings,
@@ -868,7 +887,7 @@ lazy val icebergShaded = (project in file("icebergShaded"))
 
 lazy val hudi = (project in file("hudi"))
   .dependsOn(spark % "compile->compile;test->test;provided->provided")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-hudi",
     commonSettings,
@@ -920,7 +939,7 @@ lazy val hudi = (project in file("hudi"))
 
 lazy val hive = (project in file("connectors/hive"))
   .dependsOn(standaloneCosmetic)
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-hive",
     commonSettings,
@@ -937,7 +956,7 @@ lazy val hive = (project in file("connectors/hive"))
 
 lazy val hiveAssembly = (project in file("connectors/hive-assembly"))
   .dependsOn(hive)
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "delta-hive-assembly",
     Compile / unmanagedJars += (hive / Compile / packageBin / packageBin).value,
@@ -964,7 +983,7 @@ lazy val hiveAssembly = (project in file("connectors/hive-assembly"))
 
 lazy val hiveTest = (project in file("connectors/hive-test"))
   .dependsOn(goldenTables % "test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "hive-test",
     // Make the project use the assembly jar to ensure we are testing the assembly jar that users
@@ -997,7 +1016,7 @@ lazy val hiveTest = (project in file("connectors/hive-test"))
 
 lazy val hiveMR = (project in file("connectors/hive-mr"))
   .dependsOn(hiveTest % "test->test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "hive-mr",
     commonSettings,
@@ -1024,7 +1043,7 @@ lazy val hiveMR = (project in file("connectors/hive-mr"))
 
 lazy val hiveTez = (project in file("connectors/hive-tez"))
   .dependsOn(hiveTest % "test->test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "hive-tez",
     commonSettings,
@@ -1068,7 +1087,7 @@ lazy val hiveTez = (project in file("connectors/hive-tez"))
 
 lazy val hive2MR = (project in file("connectors/hive2-mr"))
   .dependsOn(goldenTables % "test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "hive2-mr",
     commonSettings,
@@ -1099,7 +1118,7 @@ lazy val hive2MR = (project in file("connectors/hive2-mr"))
 
 lazy val hive2Tez = (project in file("connectors/hive2-tez"))
   .dependsOn(goldenTables % "test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "hive2-tez",
     commonSettings,
@@ -1166,7 +1185,7 @@ lazy val hive2Tez = (project in file("connectors/hive2-tez"))
  */
 lazy val standaloneCosmetic = project
   .dependsOn(storage) // this doesn't impact the output artifact (jar), only the pom.xml dependencies
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "delta-standalone",
     commonSettings,
@@ -1186,7 +1205,7 @@ lazy val standaloneCosmetic = project
 lazy val testStandaloneCosmetic = (project in file("connectors/testStandaloneCosmetic"))
   .dependsOn(standaloneCosmetic)
   .dependsOn(goldenTables % "test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "test-standalone-cosmetic",
     commonSettings,
@@ -1203,7 +1222,7 @@ lazy val testStandaloneCosmetic = (project in file("connectors/testStandaloneCos
  * except `ParquetSchemaConverter` are working without `parquet-hadoop` in testStandaloneCosmetic`.
  */
 lazy val testParquetUtilsWithStandaloneCosmetic = project.dependsOn(standaloneCosmetic)
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "test-parquet-utils-with-standalone-cosmetic",
     commonSettings,
@@ -1227,7 +1246,7 @@ def scalaCollectionPar(version: String) = version match {
  * create a separate project to skip the shading.
  */
 lazy val standaloneParquet = (project in file("connectors/standalone-parquet"))
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .dependsOn(standaloneWithoutParquetUtils)
   .settings(
     name := "delta-standalone-parquet",
@@ -1242,7 +1261,7 @@ lazy val standaloneParquet = (project in file("connectors/standalone-parquet"))
 
 /** A dummy project to allow `standaloneParquet` depending on the shaded standalone jar. */
 lazy val standaloneWithoutParquetUtils = project
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "delta-standalone-without-parquet-utils",
     commonSettings,
@@ -1255,7 +1274,7 @@ lazy val standaloneWithoutParquetUtils = project
 lazy val standalone = (project in file("connectors/standalone"))
   .dependsOn(storage % "compile->compile;provided->provided")
   .dependsOn(goldenTables % "test")
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "delta-standalone-original",
     commonSettings,
@@ -1380,7 +1399,7 @@ lazy val compatibility = (project in file("connectors/oss-compatibility-tests"))
 
 lazy val goldenTables = (project in file("connectors/golden-tables"))
   .dependsOn(spark % "test") // depends on delta-spark
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings(
     name := "golden-tables",
     commonSettings,
@@ -1407,7 +1426,7 @@ def sqlDeltaImportScalaVersion(scalaBinaryVersion: String): String = {
 
 lazy val sqlDeltaImport = (project in file("connectors/sql-delta-import"))
   .dependsOn(spark)
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "sql-delta-import",
     commonSettings,
@@ -1439,7 +1458,7 @@ lazy val flink = (project in file("connectors/flink"))
   .dependsOn(standaloneCosmetic % "provided")
   .dependsOn(kernelApi)
   .dependsOn(kernelDefaults)
-  .disablePlugins(JavaFormatterPlugin)
+  .disablePlugins(JavaFormatterPlugin, ScalafmtPlugin)
   .settings (
     name := "delta-flink",
     commonSettings,

kernel/README.md

@@ -74,6 +74,9 @@ We welcome contributions to Delta Lake and we accept contributions via Pull Requ
 Java code adheres to the [Google style](https://google.github.io/styleguide/javaguide.html), which is verified via `build/sbt javafmtCheckAll` during builds.
 In order to automatically fix Java code style issues, please use `build/sbt javafmtAll`.
 
+Scala code adheres to the styled defined in the root `.scalafmt.conf` file, which is verified via `build/sbt scalafmtCheckAll` during builds. You should configure your IDE to use the `Scalafmt` formatter and point it to the right configuration file. 
+In order to automatically fix Scala code style issues, please use `build/sbt scalafmtAll`.
+
 ## Configuring Code Formatter for Eclipse/IntelliJ
 
 Follow the instructions for [Eclipse](https://github.com/google/google-java-format#eclipse) or