Hello, please suppose the following setting: (see [1] too) {-# OPTIONS_GHC -fglasgow-exts #-} module C where class C a where name :: a -> String ; pre :: a -> a data Cs = forall a . (C a) => Cs a instance C Cs where name (Cs a) = name a ; pre (Cs a) = Cs (pre a) mkCs :: C a => a -> Cs mkCs = Cs instance C Int where name = show ; pre = \ _ -> 0 instance C Char where name = return ; pre = \ _ -> 'A' all_Cs = [ mkCs (undefined :: Int), mkCs (undefined :: Char) ] Note that despite I served undefined values only I can type *C> map (name . pre) all_Cs ["0","A"] to extract some information. But, when several instances spreads over some modules, writing down 'all_Cs' is an error-prone task, in particular when using some third party modules. So my question is: Is it possible to construct 'all_Cs' automatically? I think such a list cannot be constructed at compile-time, but at link- and run-time a complete list of instantiated types should be available. But is this list accessible somehow? Is there a possibility to write foreach type t that is an instance of C: return (mkCs (undefined :: t)) What is the general problem? Thanks, MR [1] http://www.haskell.org/pipermail/haskell-cafe/2006-March/014947.html -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ ---