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

[Andrew U. Frank]

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