As a side note i'd like to point out that introspectData has a problem with constructors containing Strings because show (x::String) /= x:
data Foo = Foo { bar :: String } deriving (Typeable,Data)
introspectData (Foo "quux") --> [("bar","\"quux\"")]
Those extras \" don't look very nice in the xml.. (the output of introspectData is also wrong in the article's example )
you should probably use a modified gshow:
gshow' :: Data a => a -> String
gshow' x = fromMaybe (gshow x) (cast x)
which is id for Strings.