23 Mar
2007
23 Mar
'07
8:48 p.m.
Hello, What Ian suggested is a very GHC 6.6 specific solution that uses much more that simply rank-2 types. Here is another solution that uses just rank-2 types (and, by the way, all type signatures are optional, as in ordinary Haskell): module Value where class SqlBind a where fromSqlValue :: String -> a data Field data Value emptyValue :: Field -> Value emptyValue _ = undefined data Binder = Binder (forall s. SqlBind s => s) readValue :: Field -> Binder -> Value readValue _ (Binder _) = undefined readOptValue :: Field -> Maybe Binder -> Value readOptValue f x = maybe (emptyValue f) (readValue f) x Hope this helps -Iavor