[Haskell-cafe] creating a type based on a string

I have a user input (string) and need to select one of two types. depending what the input is. is this possible? data A data B data X n = X String op :: String -> X n op "a" = X "a" :: X A op "b" = X "b" :: X B this does obviously not compile. is there a way to achieve that the type X A is produced when the input is "a" and X B when the input is "b"? thank you for help! andrew