[Haskell-cafe] Transforming AST parametrized with type

Hi. Pat asked a question [1] about AST parametrized with types. People suggest to use Functor machinery if possible. Have anything changed since them? Do we have a way to safely transform the tree like data Expr a = Id { id :: Id a } | Op { op :: Char, expra :: (Expr a) , exprb :: (Expr a) } data Id a = Id { name :: a } from Expr String to, say, Expr Int where Int names may represent some keys in the hash map? Regards, Sergey [1] - http://stackoverflow.com/questions/5434889/is-it-possible-to-use-syb-to-tran... PS sclv suggested to use synthesize from SYB to solve the problem. Could anyone provide me with example of this method?