diff --git a/.github/workflows/pr-build.yaml b/.github/workflows/pr-build.yaml
index ecb801b7..d6f0adf3 100644
--- a/.github/workflows/pr-build.yaml
+++ b/.github/workflows/pr-build.yaml
@@ -22,7 +22,11 @@ jobs:
           distribution: 'adopt'
       - name: Run mill tests
         run: ./mill __.test
-      - name: Compile with sbt
+      - name: Compile Scala 2.13 with sbt
+        run: sbt ++2.13 compile bench/compile bench/jmh:compile
+      - name: Compile Scala 2.13 with sbt
         run: sbt compile bench/compile bench/jmh:compile
-      - name: Run sbt tests
+      - name: Run sbt tests (Scala 2.13)
+        run: sbt ++2.13 test
+      - name: Run sbt tests (Scala 3)
         run: sbt test
diff --git a/sjsonnet/src/sjsonnet/BaseCharRenderer.scala b/sjsonnet/src/sjsonnet/BaseCharRenderer.scala
index 5cec5045..9c5f419a 100644
--- a/sjsonnet/src/sjsonnet/BaseCharRenderer.scala
+++ b/sjsonnet/src/sjsonnet/BaseCharRenderer.scala
@@ -28,17 +28,21 @@ class BaseCharRenderer[T <: upickle.core.CharOps.Output]
       renderIndent()
     }
   }
-  def visitArray(length: Int, index: Int) = new ArrVisitor[T, T] {
+
+  def visitArray(length: Int, index: Int): ArrVisitor[T, T] = new ArrVisitor[T, T] {
     flushBuffer()
     elemBuilder.append('[')
 
     depth += 1
     renderIndent()
-    def subVisitor = BaseCharRenderer.this
+
+    def subVisitor: BaseCharRenderer[T] = BaseCharRenderer.this
+
     def visitValue(v: T, index: Int): Unit = {
       flushBuffer()
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       depth -= 1
@@ -49,20 +53,25 @@ class BaseCharRenderer[T <: upickle.core.CharOps.Output]
     }
   }
 
-  def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[T, T] {
+  def visitJsonableObject(length: Int, index: Int): ObjVisitor[T, T] = new ObjVisitor[T, T] {
     flushBuffer()
     elemBuilder.append('{')
     depth += 1
     renderIndent()
-    def subVisitor = BaseCharRenderer.this
-    def visitKey(index: Int) = BaseCharRenderer.this
+
+    def subVisitor: BaseCharRenderer[T] = BaseCharRenderer.this
+
+    def visitKey(index: Int): BaseCharRenderer[T] = BaseCharRenderer.this
+
     def visitKeyValue(s: Any): Unit = {
       elemBuilder.append(':')
       if (indent != -1) elemBuilder.append(' ')
     }
+
     def visitValue(v: T, index: Int): Unit = {
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       depth -= 1
diff --git a/sjsonnet/src/sjsonnet/BaseRenderer.scala b/sjsonnet/src/sjsonnet/BaseRenderer.scala
index dca22588..9b52be5f 100644
--- a/sjsonnet/src/sjsonnet/BaseRenderer.scala
+++ b/sjsonnet/src/sjsonnet/BaseRenderer.scala
@@ -27,17 +27,21 @@ class BaseRenderer[T <: java.io.Writer]
       renderIndent()
     }
   }
-  def visitArray(length: Int, index: Int) = new ArrVisitor[T, T] {
+
+  def visitArray(length: Int, index: Int): ArrVisitor[T, T] = new ArrVisitor[T, T] {
     flushBuffer()
     out.append('[')
 
     depth += 1
     renderIndent()
-    def subVisitor = BaseRenderer.this
+
+    def subVisitor: BaseRenderer[T] = BaseRenderer.this
+
     def visitValue(v: T, index: Int): Unit = {
       flushBuffer()
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       depth -= 1
@@ -47,17 +51,22 @@ class BaseRenderer[T <: java.io.Writer]
     }
   }
 
-  def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[T, T] {
+  def visitJsonableObject(length: Int, index: Int): ObjVisitor[T, T] = new ObjVisitor[T, T] {
     flushBuffer()
     out.append('{')
     depth += 1
     renderIndent()
-    def subVisitor = BaseRenderer.this
-    def visitKey(index: Int) = BaseRenderer.this
+
+    def subVisitor: BaseRenderer[T] = BaseRenderer.this
+
+    def visitKey(index: Int): BaseRenderer[T] = BaseRenderer.this
+
     def visitKeyValue(s: Any): Unit = out.append(colonSnippet)
+
     def visitValue(v: T, index: Int): Unit = {
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       depth -= 1
diff --git a/sjsonnet/src/sjsonnet/Error.scala b/sjsonnet/src/sjsonnet/Error.scala
index f080f7d2..cb6ef569 100644
--- a/sjsonnet/src/sjsonnet/Error.scala
+++ b/sjsonnet/src/sjsonnet/Error.scala
@@ -78,7 +78,7 @@ class ParseError(msg: String, stack: List[Error.Frame] = Nil, underlying: Option
   extends Error(msg, stack, underlying) {
 
   override protected def copy(msg: String = msg, stack: List[Error.Frame] = stack,
-                                    underlying: Option[Throwable] = underlying) =
+                              underlying: Option[Throwable] = underlying): ParseError =
     new ParseError(msg, stack, underlying)
 }
 
@@ -86,7 +86,7 @@ class StaticError(msg: String, stack: List[Error.Frame] = Nil, underlying: Optio
   extends Error(msg, stack, underlying) {
 
   override protected def copy(msg: String = msg, stack: List[Error.Frame] = stack,
-                                    underlying: Option[Throwable] = underlying) =
+                              underlying: Option[Throwable] = underlying): StaticError =
     new StaticError(msg, stack, underlying)
 }
 
diff --git a/sjsonnet/src/sjsonnet/Evaluator.scala b/sjsonnet/src/sjsonnet/Evaluator.scala
index 28550791..859eb3f1 100644
--- a/sjsonnet/src/sjsonnet/Evaluator.scala
+++ b/sjsonnet/src/sjsonnet/Evaluator.scala
@@ -502,18 +502,18 @@ class Evaluator(resolver: CachedResolver,
   def visitMemberList(objPos: Position, e: ObjBody.MemberList, sup: Val.Obj)(implicit scope: ValScope): Val.Obj = {
     val asserts = e.asserts
     val fields = e.fields
-    var cachedSimpleScope: ValScope | Null = null
+    var cachedSimpleScope: Option[ValScope] = None
     var cachedObj: Val.Obj = null
     var asserting: Boolean = false
 
     def makeNewScope(self: Val.Obj, sup: Val.Obj): ValScope = {
       if((sup eq null) && (self eq cachedObj)) {
         cachedSimpleScope match {
-          case null =>
+          case None =>
             val cachedScope = createNewScope(self, sup)
-            cachedSimpleScope = cachedScope
+            cachedSimpleScope = Some(cachedScope)
             cachedScope
-          case cachedScope: ValScope =>
+          case Some(cachedScope) =>
             cachedScope
         }
       } else createNewScope(self, sup)
diff --git a/sjsonnet/src/sjsonnet/Format.scala b/sjsonnet/src/sjsonnet/Format.scala
index 6d742974..134e82d5 100644
--- a/sjsonnet/src/sjsonnet/Format.scala
+++ b/sjsonnet/src/sjsonnet/Format.scala
@@ -46,7 +46,7 @@ object Format{
 
 
   def plain[A: P] = P( CharsWhile(_ != '%', 0).! )
-  def format[A: P] = P( plain ~ (("%" ~/ formatSpec) ~ plain).rep ~ End)
+  def format0[A: P] = P( plain ~ (("%" ~/ formatSpec) ~ plain).rep ~ End)
 
 
 
@@ -81,7 +81,7 @@ object Format{
              values0: Val,
              pos: Position)
             (implicit evaluator: EvalScope): String = {
-    val (leading, chunks) = fastparse.parse(s, format).get.value
+    val (leading, chunks) = fastparse.parse(s, format0(using _)).get.value
     format(leading, chunks, values0, pos)
   }
 
@@ -257,7 +257,7 @@ object Format{
   }
 
   class PartialApplyFmt(fmt: String) extends Val.Builtin1("values") {
-    val (leading, chunks) = fastparse.parse(fmt, format).get.value
+    val (leading, chunks) = fastparse.parse(fmt, format0(using _)).get.value
     def evalRhs(values0: Val, ev: EvalScope, pos: Position): Val =
       Val.Str(pos, format(leading, chunks, values0, pos)(ev))
   }
diff --git a/sjsonnet/src/sjsonnet/PrettyYamlRenderer.scala b/sjsonnet/src/sjsonnet/PrettyYamlRenderer.scala
index fb89aa25..076f98fd 100644
--- a/sjsonnet/src/sjsonnet/PrettyYamlRenderer.scala
+++ b/sjsonnet/src/sjsonnet/PrettyYamlRenderer.scala
@@ -138,18 +138,20 @@ class PrettyYamlRenderer(out: Writer = new java.io.StringWriter(),
     newlineBuffered = false
     dashBuffered = false
   }
-  override def visitArray(length: Int, index: Int) = new ArrVisitor[Writer, Writer] {
+
+  override def visitArray(length: Int, index: Int): ArrVisitor[Writer, Writer] = new ArrVisitor[Writer, Writer] {
     var empty = true
     val dedentInObject = afterKey && !indentArrayInObject
-    def subVisitor = {
-      if (empty){
+
+    def subVisitor: PrettyYamlRenderer = {
+      if (empty) {
         afterColon = false
         flushBuffer()
         val outerFirstElementInArray = firstElementInArray
         firstElementInArray = true
         if (!topLevel) {
           depth += 1
-          if (!firstElementInArray || !outerFirstElementInArray)  newlineBuffered = true
+          if (!firstElementInArray || !outerFirstElementInArray) newlineBuffered = true
         }
         topLevel = false
 
@@ -161,6 +163,7 @@ class PrettyYamlRenderer(out: Writer = new java.io.StringWriter(),
       leftHandPrefixOffset = 0
       PrettyYamlRenderer.this
     }
+
     def visitValue(v: Writer, index: Int): Unit = {
       firstElementInArray = true
       empty = false
@@ -169,6 +172,7 @@ class PrettyYamlRenderer(out: Writer = new java.io.StringWriter(),
 
       dashBuffered = true
     }
+
     def visitEnd(index: Int) = {
       firstElementInArray = false
       if (!dedentInObject) depth -= 1
@@ -182,16 +186,19 @@ class PrettyYamlRenderer(out: Writer = new java.io.StringWriter(),
       out
     }
   }
-  override def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[Writer, Writer] {
+
+  override def visitJsonableObject(length: Int, index: Int): ObjVisitor[Writer, Writer] = new ObjVisitor[Writer, Writer] {
     firstElementInArray = false
     var empty = true
     flushBuffer()
     if (!topLevel) depth += 1
     topLevel = false
-    def subVisitor = PrettyYamlRenderer.this
-    def visitKey(index: Int) = {
 
-      if (empty){
+    def subVisitor: PrettyYamlRenderer = PrettyYamlRenderer.this
+
+    def visitKey(index: Int): PrettyYamlRenderer = {
+
+      if (empty) {
         leftHandPrefixOffset = 0
 
         afterColon = false
@@ -200,12 +207,13 @@ class PrettyYamlRenderer(out: Writer = new java.io.StringWriter(),
       }
       PrettyYamlRenderer.this
     }
+
     def visitKeyValue(s: Any): Unit = {
       empty = false
       flushBuffer()
       out.append(":")
       saveCurrentPos()
-      if (bufferedComment != null){
+      if (bufferedComment != null) {
         out.append(bufferedComment)
         bufferedComment = null
       }
@@ -213,10 +221,12 @@ class PrettyYamlRenderer(out: Writer = new java.io.StringWriter(),
       afterColon = true
       newlineBuffered = false
     }
+
     def visitValue(v: Writer, index: Int): Unit = {
       newlineBuffered = true
       afterKey = false
     }
+
     def visitEnd(index: Int) = {
       if (empty) {
         addSpaceAfterColon()
@@ -473,7 +483,7 @@ object PrettyYamlRenderer{
       yamlPunctuation | (&(CharIn(".0-9\\-")) ~ (yamlTime | yamlDate | yamlNumber) | yamlKeyword) ~ End
     )
 
-    fastparse.parse(str, parser).isSuccess ||
+    fastparse.parse(str, parser(using _)).isSuccess ||
     str.contains(": ") || // Looks like a key-value pair
     str.contains(" #") || // Comments
     str.charAt(str.length - 1) == ':' || // Looks like a key-value pair
diff --git a/sjsonnet/src/sjsonnet/ReadWriter.scala b/sjsonnet/src/sjsonnet/ReadWriter.scala
index d7667bea..01e109a9 100644
--- a/sjsonnet/src/sjsonnet/ReadWriter.scala
+++ b/sjsonnet/src/sjsonnet/ReadWriter.scala
@@ -10,34 +10,34 @@ sealed abstract class ReadWriter[T] {
 object ReadWriter{
   implicit object StringRead extends ReadWriter[String]{
     def apply(t: Val) = t.asString
-    def write(pos: Position, t: String) = Val.Str(pos, t)
+    def write(pos: Position, t: String): Val.Str = Val.Str(pos, t)
   }
   implicit object BooleanRead extends ReadWriter[Boolean]{
     def apply(t: Val) = t.asBoolean
-    def write(pos: Position, t: Boolean) = Val.bool(pos, t)
+    def write(pos: Position, t: Boolean): Val.Bool = Val.bool(pos, t)
   }
   implicit object IntRead extends ReadWriter[Int]{
     def apply(t: Val) = t.asInt
-    def write(pos: Position, t: Int) = Val.Num(pos, t)
+    def write(pos: Position, t: Int): Val.Num = Val.Num(pos, t)
   }
   implicit object DoubleRead extends ReadWriter[Double]{
     def apply(t: Val) = t.asDouble
-    def write(pos: Position, t: Double) = Val.Num(pos, t)
+    def write(pos: Position, t: Double): Val.Num = Val.Num(pos, t)
   }
   implicit object ValRead extends ReadWriter[Val]{
     def apply(t: Val) = t
-    def write(pos: Position, t: Val) = t
+    def write(pos: Position, t: Val): Val = t
   }
   implicit object ObjRead extends ReadWriter[Val.Obj]{
     def apply(t: Val) = t.asObj
-    def write(pos: Position, t: Val.Obj) = t
+    def write(pos: Position, t: Val.Obj): Val.Obj = t
   }
   implicit object ArrRead extends ReadWriter[Val.Arr]{
     def apply(t: Val) = t.asArr
-    def write(pos: Position, t: Val.Arr) = t
+    def write(pos: Position, t: Val.Arr): Val.Arr = t
   }
   implicit object FuncRead extends ReadWriter[Val.Func]{
     def apply(t: Val) = t.asFunc
-    def write(pos: Position, t: Val.Func) = t
+    def write(pos: Position, t: Val.Func): Val.Func = t
   }
 }
diff --git a/sjsonnet/src/sjsonnet/Renderer.scala b/sjsonnet/src/sjsonnet/Renderer.scala
index df9ab922..c3636408 100644
--- a/sjsonnet/src/sjsonnet/Renderer.scala
+++ b/sjsonnet/src/sjsonnet/Renderer.scala
@@ -46,19 +46,23 @@ class Renderer(out: Writer = new java.io.StringWriter(),
     newlineBuffered = false
     commaBuffered = false
   }
-  override def visitArray(length: Int, index: Int) = new ArrVisitor[Writer, Writer] {
+
+  override def visitArray(length: Int, index: Int): ArrVisitor[Writer, Writer] = new ArrVisitor[Writer, Writer] {
     var empty = true
     flushBuffer()
     elemBuilder.append('[')
     newlineBuffered = true
 
     depth += 1
-    def subVisitor = Renderer.this
+
+    def subVisitor: Renderer = Renderer.this
+
     def visitValue(v: Writer, index: Int): Unit = {
       empty = false
       flushBuffer()
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       newlineBuffered = false
@@ -72,23 +76,28 @@ class Renderer(out: Writer = new java.io.StringWriter(),
     }
   }
 
-  override def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[Writer, Writer] {
+  override def visitJsonableObject(length: Int, index: Int): ObjVisitor[Writer, Writer] = new ObjVisitor[Writer, Writer] {
     var empty = true
     flushBuffer()
     elemBuilder.append('{')
     newlineBuffered = true
     depth += 1
-    def subVisitor = Renderer.this
-    def visitKey(index: Int) = Renderer.this
+
+    def subVisitor: Renderer = Renderer.this
+
+    def visitKey(index: Int): Renderer = Renderer.this
+
     def visitKeyValue(v: Any): Unit = {
       empty = false
       //flushBuffer()
       elemBuilder.append(':')
       elemBuilder.append(' ')
     }
+
     def visitValue(v: Writer, index: Int): Unit = {
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       newlineBuffered = false
@@ -141,21 +150,26 @@ class PythonRenderer(out: Writer = new java.io.StringWriter(),
     out
   }
 
-  override def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[Writer, Writer] {
+  override def visitJsonableObject(length: Int, index: Int): ObjVisitor[Writer, Writer] = new ObjVisitor[Writer, Writer] {
     flushBuffer()
     elemBuilder.append('{')
     depth += 1
     renderIndent()
-    def subVisitor = PythonRenderer.this
-    def visitKey(index: Int) = PythonRenderer.this
+
+    def subVisitor: PythonRenderer = PythonRenderer.this
+
+    def visitKey(index: Int): PythonRenderer = PythonRenderer.this
+
     def visitKeyValue(s: Any): Unit = {
       elemBuilder.ensureLength(2)
       elemBuilder.append(':')
       elemBuilder.append(' ')
     }
+
     def visitValue(v: Writer, index: Int): Unit = {
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       depth -= 1
@@ -181,18 +195,21 @@ class PythonRenderer(out: Writer = new java.io.StringWriter(),
 case class MaterializeJsonRenderer(indent: Int = 4, escapeUnicode: Boolean = false, out: StringWriter = new StringWriter())
   extends BaseCharRenderer(out, indent, escapeUnicode) {
 
-  override def visitArray(length: Int, index: Int) = new ArrVisitor[StringWriter, StringWriter] {
+  override def visitArray(length: Int, index: Int): ArrVisitor[StringWriter, StringWriter] = new ArrVisitor[StringWriter, StringWriter] {
     flushBuffer()
     elemBuilder.append('[')
 
     depth += 1
     // account for rendering differences of whitespaces in ujson and jsonnet manifestJson
-    if(length == 0) elemBuilder.append('\n') else renderIndent()
-    def subVisitor = MaterializeJsonRenderer.this
+    if (length == 0) elemBuilder.append('\n') else renderIndent()
+
+    def subVisitor: MaterializeJsonRenderer = MaterializeJsonRenderer.this
+
     def visitValue(v: StringWriter, index: Int): Unit = {
       flushBuffer()
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       depth -= 1
@@ -203,21 +220,26 @@ case class MaterializeJsonRenderer(indent: Int = 4, escapeUnicode: Boolean = fal
     }
   }
 
-  override def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[StringWriter, StringWriter] {
+  override def visitJsonableObject(length: Int, index: Int): ObjVisitor[StringWriter, StringWriter] = new ObjVisitor[StringWriter, StringWriter] {
     flushBuffer()
     elemBuilder.append('{')
     depth += 1
     // account for rendering differences of whitespaces in ujson and jsonnet manifestJson
-    if(length == 0) elemBuilder.append('\n') else renderIndent()
-    def subVisitor = MaterializeJsonRenderer.this
-    def visitKey(index: Int) = MaterializeJsonRenderer.this
+    if (length == 0) elemBuilder.append('\n') else renderIndent()
+
+    def subVisitor: MaterializeJsonRenderer = MaterializeJsonRenderer.this
+
+    def visitKey(index: Int): MaterializeJsonRenderer = MaterializeJsonRenderer.this
+
     def visitKeyValue(s: Any): Unit = {
       elemBuilder.append(':')
       if (indent != -1) elemBuilder.append(' ')
     }
+
     def visitValue(v: StringWriter, index: Int): Unit = {
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       depth -= 1
diff --git a/sjsonnet/src/sjsonnet/Std.scala b/sjsonnet/src/sjsonnet/Std.scala
index 19a97a0f..097a702a 100644
--- a/sjsonnet/src/sjsonnet/Std.scala
+++ b/sjsonnet/src/sjsonnet/Std.scala
@@ -1272,7 +1272,7 @@ class Std {
             Val.Str(self.pos, fs.currentFile.relativeToString(ev.wd))
         }
       )
-    )*
+    ): _*
   )
 
   def builtin[R: ReadWriter, T1: ReadWriter](name: String, p1: String)
diff --git a/sjsonnet/src/sjsonnet/ValVisitor.scala b/sjsonnet/src/sjsonnet/ValVisitor.scala
index e6af5a66..2f2e68ca 100644
--- a/sjsonnet/src/sjsonnet/ValVisitor.scala
+++ b/sjsonnet/src/sjsonnet/ValVisitor.scala
@@ -21,7 +21,7 @@ class ValVisitor(pos: Position) extends JsVisitor[Val, Val] { self =>
     val allKeys = new util.LinkedHashMap[String, java.lang.Boolean]
     var key: String = null
     def subVisitor: Visitor[?, ?] = self
-    def visitKey(index: Int) = upickle.core.StringVisitor
+    def visitKey(index: Int): Visitor[?, ?] = upickle.core.StringVisitor
     def visitKeyValue(s: Any): Unit = key = s.toString
     def visitValue(v: Val, index: Int): Unit = {
       cache.put(key, v)
diff --git a/sjsonnet/src/sjsonnet/YamlRenderer.scala b/sjsonnet/src/sjsonnet/YamlRenderer.scala
index 1f6945e8..ae55600a 100644
--- a/sjsonnet/src/sjsonnet/YamlRenderer.scala
+++ b/sjsonnet/src/sjsonnet/YamlRenderer.scala
@@ -8,7 +8,7 @@ import upickle.core.{ArrVisitor, ObjVisitor}
 
 
 class YamlRenderer(_out: StringWriter = new java.io.StringWriter(), indentArrayInObject: Boolean = false,
-                   indent: Int = 2) extends BaseCharRenderer(_out, indent){
+                   indent: Int = 2) extends BaseCharRenderer[StringWriter](_out, indent){
   var newlineBuffered = false
   var dashBuffered = false
   var afterKey = false
@@ -81,7 +81,7 @@ class YamlRenderer(_out: StringWriter = new java.io.StringWriter(), indentArrayI
     dashBuffered = false
   }
 
-  override def visitArray(length: Int, index: Int) = new ArrVisitor[StringWriter, StringWriter] {
+  override def visitArray(length: Int, index: Int): ArrVisitor[StringWriter, StringWriter] = new ArrVisitor[StringWriter, StringWriter] {
     var empty = true
     flushBuffer()
 
@@ -96,13 +96,15 @@ class YamlRenderer(_out: StringWriter = new java.io.StringWriter(), indentArrayI
     if (dedentInObject) depth -= 1
     dashBuffered = true
 
-    def subVisitor = YamlRenderer.this
+    def subVisitor: YamlRenderer = YamlRenderer.this
+
     def visitValue(v: StringWriter, index: Int): Unit = {
       empty = false
       flushBuffer()
       newlineBuffered = true
       dashBuffered = true
     }
+
     def visitEnd(index: Int) = {
       if (!dedentInObject) depth -= 1
       if (empty) {
@@ -116,7 +118,8 @@ class YamlRenderer(_out: StringWriter = new java.io.StringWriter(), indentArrayI
       _out
     }
   }
-  override def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[StringWriter, StringWriter] {
+
+  override def visitJsonableObject(length: Int, index: Int): ObjVisitor[StringWriter, StringWriter] = new ObjVisitor[StringWriter, StringWriter] {
     var empty = true
     flushBuffer()
     if (!topLevel) depth += 1
@@ -124,8 +127,10 @@ class YamlRenderer(_out: StringWriter = new java.io.StringWriter(), indentArrayI
 
     if (afterKey) newlineBuffered = true
 
-    def subVisitor = YamlRenderer.this
-    def visitKey(index: Int) = YamlRenderer.this
+    def subVisitor: YamlRenderer = YamlRenderer.this
+
+    def visitKey(index: Int): YamlRenderer = YamlRenderer.this
+
     def visitKeyValue(s: Any): Unit = {
       empty = false
       flushBuffer()
@@ -136,10 +141,12 @@ class YamlRenderer(_out: StringWriter = new java.io.StringWriter(), indentArrayI
       afterKey = true
       newlineBuffered = false
     }
+
     def visitValue(v: StringWriter, index: Int): Unit = {
       newlineBuffered = true
       afterKey = false
     }
+
     def visitEnd(index: Int) = {
       if (empty) {
         elemBuilder.ensureLength(2)
diff --git a/sjsonnet/test/src/sjsonnet/DummyPath.scala b/sjsonnet/test/src/sjsonnet/DummyPath.scala
index 2b76dac1..79c80197 100644
--- a/sjsonnet/test/src/sjsonnet/DummyPath.scala
+++ b/sjsonnet/test/src/sjsonnet/DummyPath.scala
@@ -15,7 +15,7 @@ case class DummyPath(segments: String*) extends Path{
 
   def last: String = segments.last
 
-  def /(s: String): Path = DummyPath(segments :+ s*)
+  def /(s: String): Path = DummyPath(segments :+ s:_*)
 
   def renderOffsetStr(offset: Int, loadedFileContents: mutable.HashMap[Path, Array[Int]]): String = {
     segments.mkString("/") + ":" + offset
diff --git a/sjsonnet/test/src/sjsonnet/FormatTests.scala b/sjsonnet/test/src/sjsonnet/FormatTests.scala
index a3e4f4a4..77dfe910 100644
--- a/sjsonnet/test/src/sjsonnet/FormatTests.scala
+++ b/sjsonnet/test/src/sjsonnet/FormatTests.scala
@@ -10,7 +10,7 @@ object FormatTests extends TestSuite{
     val json = ujson.read(jsonStr)
     val formatted = Format.format(fmt, Materializer.reverse(null, json), dummyPos)(
       new EvalScope{
-        def extVars = _ => None
+        def extVars: String => Option[Expr] = _ => None
         def wd: Path = DummyPath()
         def visitExpr(expr: Expr)(implicit scope: ValScope): Val = ???
         def materialize(v: Val): Value = ???
diff --git a/sjsonnet/test/src/sjsonnet/OldRenderer.scala b/sjsonnet/test/src/sjsonnet/OldRenderer.scala
index f9cf5045..98156129 100644
--- a/sjsonnet/test/src/sjsonnet/OldRenderer.scala
+++ b/sjsonnet/test/src/sjsonnet/OldRenderer.scala
@@ -38,19 +38,23 @@ class OldRenderer(out: Writer = new java.io.StringWriter(),
     newlineBuffered = false
     commaBuffered = false
   }
-  override def visitArray(length: Int, index: Int) = new ArrVisitor[Writer, Writer] {
+
+  override def visitArray(length: Int, index: Int): ArrVisitor[Writer, Writer] = new ArrVisitor[Writer, Writer] {
     var empty = true
     flushBuffer()
     out.append('[')
     newlineBuffered = true
 
     depth += 1
-    def subVisitor = OldRenderer.this
+
+    def subVisitor: OldRenderer = OldRenderer.this
+
     def visitValue(v: Writer, index: Int): Unit = {
       empty = false
       flushBuffer()
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       newlineBuffered = false
@@ -63,22 +67,27 @@ class OldRenderer(out: Writer = new java.io.StringWriter(),
     }
   }
 
-  override def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[Writer, Writer] {
+  override def visitJsonableObject(length: Int, index: Int): ObjVisitor[Writer, Writer] = new ObjVisitor[Writer, Writer] {
     var empty = true
     flushBuffer()
     out.append('{')
     newlineBuffered = true
     depth += 1
-    def subVisitor = OldRenderer.this
-    def visitKey(index: Int) = OldRenderer.this
+
+    def subVisitor: OldRenderer = OldRenderer.this
+
+    def visitKey(index: Int): OldRenderer = OldRenderer.this
+
     def visitKeyValue(v: Any): Unit = {
       empty = false
       flushBuffer()
       out.append(colonSnippet)
     }
+
     def visitValue(v: Writer, index: Int): Unit = {
       commaBuffered = true
     }
+
     def visitEnd(index: Int) = {
       commaBuffered = false
       newlineBuffered = false
diff --git a/sjsonnet/test/src/sjsonnet/OldYamlRenderer.scala b/sjsonnet/test/src/sjsonnet/OldYamlRenderer.scala
index b335b505..659fc045 100644
--- a/sjsonnet/test/src/sjsonnet/OldYamlRenderer.scala
+++ b/sjsonnet/test/src/sjsonnet/OldYamlRenderer.scala
@@ -54,7 +54,8 @@ class OldYamlRenderer(out: StringWriter = new java.io.StringWriter(), indentArra
     newlineBuffered = false
     dashBuffered = false
   }
-  override def visitArray(length: Int, index: Int) = new ArrVisitor[StringWriter, StringWriter] {
+
+  override def visitArray(length: Int, index: Int): ArrVisitor[StringWriter, StringWriter] = new ArrVisitor[StringWriter, StringWriter] {
     var empty = true
     flushBuffer()
 
@@ -69,13 +70,15 @@ class OldYamlRenderer(out: StringWriter = new java.io.StringWriter(), indentArra
     if (dedentInObject) depth -= 1
     dashBuffered = true
 
-    def subVisitor = OldYamlRenderer.this
+    def subVisitor: OldYamlRenderer = OldYamlRenderer.this
+
     def visitValue(v: StringWriter, index: Int): Unit = {
       empty = false
       flushBuffer()
       newlineBuffered = true
       dashBuffered = true
     }
+
     def visitEnd(index: Int) = {
       if (!dedentInObject) depth -= 1
       if (empty) out.append("[]")
@@ -84,7 +87,8 @@ class OldYamlRenderer(out: StringWriter = new java.io.StringWriter(), indentArra
       out
     }
   }
-  override def visitJsonableObject(length: Int, index: Int) = new ObjVisitor[StringWriter, StringWriter] {
+
+  override def visitJsonableObject(length: Int, index: Int): ObjVisitor[StringWriter, StringWriter] = new ObjVisitor[StringWriter, StringWriter] {
     var empty = true
     flushBuffer()
     if (!topLevel) depth += 1
@@ -92,8 +96,10 @@ class OldYamlRenderer(out: StringWriter = new java.io.StringWriter(), indentArra
 
     if (afterKey) newlineBuffered = true
 
-    def subVisitor = OldYamlRenderer.this
-    def visitKey(index: Int) = OldYamlRenderer.this
+    def subVisitor: OldYamlRenderer = OldYamlRenderer.this
+
+    def visitKey(index: Int): OldYamlRenderer = OldYamlRenderer.this
+
     def visitKeyValue(s: Any): Unit = {
       empty = false
       flushBuffer()
@@ -101,10 +107,12 @@ class OldYamlRenderer(out: StringWriter = new java.io.StringWriter(), indentArra
       afterKey = true
       newlineBuffered = false
     }
+
     def visitValue(v: StringWriter, index: Int): Unit = {
       newlineBuffered = true
       afterKey = false
     }
+
     def visitEnd(index: Int) = {
       if (empty) out.append("{}")
       newlineBuffered = false