My early intention was to limit the EW data type to showable/readable values. It wanted it to only accepting one of these.
Now I have even two alternatives to choose :-)
Thanks
Hartmut

On Mon, Aug 8, 2011 at 12:02 AM, David Place <d@vidplace.com> wrote:

On Aug 7, 2011, at 5:54 PM, Hartmut wrote:

David,
that was fast :-) Thank you for your help!
Hartmut

Your welcome.  ;-)  I needed the break from what I was doing.  I think I actually would like to add that normally i wouldn't put the class constraints in the data type definition.

module Main where

data EW a = EW a

x01 = EW 20
x02 = EW "Test"

instance (Show a) => Show (EW a) where
  show (EW x) = show "EW:" ++ show x

instance (Read a) =>  Read (EW a) where
    read (etc...)

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David Place   
Owner, Panpipes Ho! LLC