Hi, My attempt which compiles some files with stub failed. Can you tell me how to compile them? Thanks. PrettyJSON.hs: renderJValue :: JValue -> Doc renderJValue (JBool True) = text "true" renderJValue (JBool False) = text "false" renderJValue JNull = text "null" renderJValue (JNumber num) = double num renderJValue (JString str) = string str PrettyStub.hs: import SimpleJSON data Doc = ToBeDefined deriving (Show) string :: String -> Doc string str = undefined text :: String -> Doc text str = undefined double :: Double -> Doc double num = undefined Result: bogon% ghc -c PrettyJSON.hs PrettyStub.hs SimpleJSON.o Warning: the following files would be used as linker inputs, but linking is not being done: SimpleJSON.o PrettyJSON.hs:1:16: Not in scope: type constructor or class `JValue' PrettyJSON.hs:1:26: Not in scope: type constructor or class `Doc' PrettyJSON.hs:2:14: Not in scope: data constructor `JBool' PrettyJSON.hs:2:29: Not in scope: `text' PrettyJSON.hs:3:14: Not in scope: data constructor `JBool' PrettyJSON.hs:3:29: Not in scope: `text' PrettyJSON.hs:4:13: Not in scope: data constructor `JNull' PrettyJSON.hs:4:29: Not in scope: `text' PrettyJSON.hs:5:14: Not in scope: data constructor `JNumber' PrettyJSON.hs:5:29: Not in scope: `double' PrettyJSON.hs:6:14: Not in scope: data constructor `JString' PrettyJSON.hs:6:29: Not in scope: `string' Best regards, Zhi-Qiang Lei zhiqiang.lei@gmail.com