Use two list parameter lists.

Thus allowing embedding the error generation function in a block and making the overload seamlessly work with Scala 2.11

modules/core/shared/src/main/scala/io/circe/Decoder.scala

@@ -163,7 +163,7 @@ trait Decoder[A] extends Serializable { self =>
    * (instead only the error of the last failing `validate` in the chain will be
    * returned).
    */
-  final def validate(pred: HCursor => Boolean, message: HCursor => String): Decoder[A] = new Decoder[A] {
+  final def validate(pred: HCursor => Boolean)(message: HCursor => String): Decoder[A] = new Decoder[A] {
     final def apply(c: HCursor): Decoder.Result[A] =
       if (pred(c)) self(c) else Left(DecodingFailure(message(c), c.history))
 
@@ -182,7 +182,7 @@ trait Decoder[A] extends Serializable { self =>
     * returned).
     */
   final def validate(pred: HCursor => Boolean, message: => String): Decoder[A] =
-    validate(pred, _ => message)
+    validate(pred)(_ => message)
 
   /**
    * Convert to a Kleisli arrow.

modules/tests/shared/src/test/scala/io/circe/DecoderSuite.scala

@@ -407,16 +407,14 @@ class DecoderSuite extends CirceSuite with LargeNumberDecoderTests with TableDri
   it should "generate error messages from HCursor when a function is passed" in {
     case class Foo(x: Int, y: String)
 
-    val errorFunction: HCursor => String = { c =>
+    val decoder: Decoder[Foo] = Decoder.const(Foo(42, "meaning")).validate(_ => false) { c =>
       val maybeFieldsStr = for {
         json <- c.focus
         jsonKeys <- json.hcursor.keys
       } yield jsonKeys.mkString(",")
       maybeFieldsStr.getOrElse("")
     }
 
-    val decoder: Decoder[Foo] = Decoder.const(Foo(42, "meaning")).validate(_ => false, errorFunction)
-
     val Right(fooJson) = parse("""{"x":42, "y": "meaning"}""")
 
     assert(decoder.decodeJson(fooJson).left.get.message == "x,y")