LoneAnonymousOperation rule implemented

src/main/scala/sangria/validation/Violation.scala

@@ -161,3 +161,6 @@ case class FieldsConflictViolation(outputName: String, reason: Either[String, Ve
   }
 }
 
+case class AnonOperationNotAloneViolation(sourceMapper: Option[SourceMapper], positions: List[Position]) extends AstNodeViolation {
+  lazy val errorMessage = s"This anonymous operation must be the only defined operation.$astLocation"
+}

src/main/scala/sangria/validation/rules/LoneAnonymousOperation.scala

@@ -0,0 +1,30 @@
+package sangria.validation.rules
+
+import sangria.ast
+import sangria.ast.AstVisitorCommand._
+import sangria.validation._
+
+import scala.language.postfixOps
+
+/**
+ * Lone anonymous operation
+ *
+ * A GraphQL document is only valid if when it contains an anonymous operation
+ * (the query short-hand) that it contains only that one operation definition.
+ */
+class LoneAnonymousOperation extends ValidationRule {
+  override def visitor(ctx: ValidationContext) = new AstValidatingVisitor {
+    var operationCount = 0
+
+    override val onEnter: ValidationVisit = {
+      case ast.Document(definitions, _, _) =>
+        operationCount = definitions.count(_.isInstanceOf[ast.OperationDefinition])
+        Right(Continue)
+      case op: ast.OperationDefinition =>
+        if (op.name.isEmpty && operationCount > 1)
+          Left(Vector(AnonOperationNotAloneViolation(ctx.sourceMapper, op.position.toList)))
+        else
+          Right(Continue)
+    }
+  }
+}

src/test/scala/sangria/validation/rules/LoneAnonymousOperationSpec.scala

@@ -0,0 +1,73 @@
+package sangria.validation.rules
+
+import org.scalatest.WordSpec
+import sangria.util.{Pos, ValidationSupport}
+
+class LoneAnonymousOperationSpec extends WordSpec with ValidationSupport {
+
+  override val defaultRule = Some(new LoneAnonymousOperation)
+
+  "Validate: Anonymous operation must be alone" should {
+    "no operations" in expectPasses(
+      """
+        fragment fragA on Type {
+          field
+        }
+      """)
+
+    "one anon operation" in expectPasses(
+      """
+        {
+          field
+        }
+      """)
+
+    "multiple named operations" in expectPasses(
+      """
+        query Foo {
+          field
+        }
+
+        query Bar {
+          field
+        }
+      """)
+
+    "anon operation with fragment" in expectPasses(
+      """
+        {
+          ...Foo
+        }
+        fragment Foo on Type {
+          field
+        }
+      """)
+
+    "multiple anon operations" in expectFails(
+      """
+        {
+          fieldA
+        }
+        {
+          fieldB
+        }
+      """,
+      List(
+        "This anonymous operation must be the only defined operation." -> Some(Pos(2, 9)),
+        "This anonymous operation must be the only defined operation." -> Some(Pos(5, 9))
+      ))
+
+    "anon operation with another operation" in expectFails(
+      """
+        {
+          fieldA
+        }
+        mutation Foo {
+          fieldB
+        }
+      """,
+      List(
+        "This anonymous operation must be the only defined operation." -> Some(Pos(2, 9))
+      ))
+  }
+}