On Thu, Apr 30, 2009 at 10:20:44AM -0400, Brandon S. Allbery KF8NH wrote:

http://www.haskell.org/pipermail/haskell-cafe/2008-April/041397.html http://hackage.haskell.org/trac/ghc/ticket/1897

Thanks for your answer and these links. Although the issue discussed there is more related to type families which were not used in my example, it was still helpful for me. At least now I know about the "show . read" problem ;-) Jan. -- Heriot-Watt University is a Scottish charity registered under charity number SC000278.