Validate single root field in subscriptions

tests/Language/GraphQL/AST/EncoderSpec.hs

@@ -123,7 +123,9 @@ spec = do
         it "indents block strings in arguments" $
             let arguments = [Argument "message" (String "line1\nline2")]
                 field = Field Nothing "field" arguments [] []
-                operation = DefinitionOperation $ SelectionSet $ pure field
+                operation = DefinitionOperation
+                    $ SelectionSet (pure field)
+                    $ Location 0 0
              in definition pretty operation `shouldBe` [r|{
   field(message: """
     line1

tests/Language/GraphQL/ValidateSpec.hs

@@ -148,7 +148,7 @@ validate queryString =
 
 spec :: Spec
 spec =
-    describe "document" $
+    describe "document" $ do
         it "rejects type definitions" $
             let queryString = [r|
               query getDogName {
@@ -169,3 +169,43 @@ spec =
                     , path = []
                     }
              in validate queryString `shouldBe` Seq.singleton expected
+
+        it "rejects multiple subscription root fields" $
+            let queryString = [r|
+              subscription sub {
+                newMessage {
+                  body
+                  sender
+                }
+                disallowedSecondRootField
+              }
+            |]
+                expected = Error
+                    { message =
+                        "Subscription sub must select only one top level field."
+                    , locations = [AST.Location 2 15]
+                    , path = []
+                    }
+             in validate queryString `shouldBe` Seq.singleton expected
+
+        it "rejects multiple subscription root fields coming from a fragment" $
+            let queryString = [r|
+              subscription sub {
+                ...multipleSubscriptions
+              }
+
+              fragment multipleSubscriptions on Subscription {
+                newMessage {
+                  body
+                  sender
+                }
+                disallowedSecondRootField
+              }
+            |]
+                expected = Error
+                    { message =
+                        "Subscription sub must select only one top level field."
+                    , locations = [AST.Location 2 15]
+                    , path = []
+                    }
+             in validate queryString `shouldBe` Seq.singleton expected