Hi all, We've had a discussion on #haskell about how we can make a function that reads in serialized values of an open data type, such as class (Show a, Read a) => MyClass a where typeTag :: a -> String ... operations on the open data type... data Obj = forall a. MyClass a => Obj { unObj :: a } We would like to write a function like, oread :: String -> Obj The problem here is that unlike with 'read,' the calling context of 'oread' does not fix the implementation of Read that we want to use. The best we've been able to come up with was to build a table that maps type names to types: type TypeTable = Map String Obj oread :: TypeTable -> String -> Obj oread types s = Obj $ read repr `asTypeOf` unObj (types ! tag) where tag = takeWhile (/= ' ') s repr = drop 1 $ dropWhile (/= ' ') s instance Show Obj where show (Obj x) = typeTag x ++ " " ++ show x As far as we can see, we'll have to create this table manually, without support from the compiler. My suggestion would be to put the code constructing the table for each module at the top of that module, to keep it together with the exports and imports, like this: module Foo.Foo (export1, export2, fooFooTypes) where import Foo.Bar import Foo.Baz import Data.Map fooFooTypes = fooBarTypes `union` fooBazTypes `union` fromList [ ("Foo.Foo.Ty1", Obj (undefined :: Ty1)) , ("Foo.Foo.Ty2", Obj (undefined :: Ty2)) ] However, this is still kind of boring. Is there a better way? If not, would it be a good idea to have compiler support for building this kind of type table? Thanks, - Benja