Validate all fragments are used

tests/Language/GraphQL/AST/EncoderSpec.hs

@@ -124,7 +124,7 @@ spec = do
             let arguments = [Argument "message" (String "line1\nline2")]
                 field = Field Nothing "field" arguments [] [] $ Location 0 0
                 operation = DefinitionOperation
-                    $ SelectionSet (pure field)
+                    $ SelectionSet (pure $ FieldSelection field)
                     $ Location 0 0
              in definition pretty operation `shouldBe` [r|{
   field(message: """

tests/Language/GraphQL/ValidateSpec.hs

@@ -373,3 +373,23 @@ spec =
                     , path = []
                     }
              in validate queryString `shouldBe` Seq.singleton expected
+
+        it "rejects unused fragments" $
+            let queryString = [r|
+              fragment nameFragment on Dog { # unused
+                name
+              }
+
+              {
+                dog {
+                  name
+                }
+              }
+            |]
+                expected = Error
+                    { message =
+                        "Fragment \"nameFragment\" is never used."
+                    , locations = [AST.Location 2 15]
+                    , path = []
+                    }
+             in validate queryString `shouldBe` Seq.singleton expected