Detect duplicate fields in the parser

sjsonnet/src/sjsonnet/Parser.scala

@@ -270,7 +270,18 @@ object Parser{
 
   def objinside[_: P]: P[Expr.ObjBody] = P(
     member.rep(sep = ",") ~ ",".? ~ (forspec ~ compspec).?
-  ).map{
+  ).flatMap { case t @ (exprs, _) =>
+    val seen = collection.mutable.Set.empty[String]
+    var overlap: String = null
+    exprs.foreach {
+      case Expr.Member.Field(_, Expr.FieldName.Fixed(n), _, _, _, _) =>
+        if(seen(n)) overlap = n
+        else seen.add(n)
+      case _ =>
+    }
+    if (overlap == null) Pass(t)
+    else Fail.opaque("no duplicate field: " + overlap)
+  }.map{
     case (exprs, None) => Expr.ObjBody.MemberList(exprs)
     case (exprs, Some(comps)) =>
       val preLocals = exprs

sjsonnet/test/src/sjsonnet/ParserTests.scala

@@ -1,8 +1,10 @@
 package sjsonnet
 import utest._
 import Expr._
+import fastparse.Parsed
 object ParserTests extends TestSuite{
   def parse(s: String) = fastparse.parse(s, Parser.document(_)).get.value._1
+  def parseErr(s: String) = fastparse.parse(s, Parser.document(_), verboseFailures = true).asInstanceOf[Parsed.Failure].msg
   def tests = Tests{
     test("hello") {
       parse("true") ==> True(0)
@@ -42,5 +44,8 @@ object ParserTests extends TestSuite{
 //      parse("[1, 2]") ==> Arr(Seq(Num(1), Num(2)))
 //      parse("[1, [2, 3], 4]") ==> Arr(Seq(Num(1), Arr(Seq(Num(2), Num(3))), Num(4)))
     }
+    test("duplicateFields") {
+      parseErr("{ a: 1, a: 2 }") ==> """Expected no duplicate field: a:1:14, found "}""""
+    }
   }
 }