instance overlap in 6.6 candidate

To my request on overlapping instances in 6.6 candidate Simon Peyton-Jones writes on 4 Sep 2006

[..]

GHC 6.6's story is that an instance declaration can only be overlapped if you compile that module with -fallow-overlapping-instances. Since the list instance for Show was not compiled in this way, you can't overlap it.

You may think this is annoying -- but it does mean that you can look at an instance declaration and see whether it might be overlapped, just by look at the flags for that module.

But why to restrict the user, why one needs to detect which library instance can be overlapped? Why not to allow all of them to overlap? Here is an example of how I alayws was using overlaps with standard instances. ------------------------------------------------ data Equation = ... instance Show Equation where ... instance Show [Equation] where showsPrec _ eqs = ------------------------------------------------ This gives the user possibility to redefine a certain part of the library instance. Is not ghc-6.4.1 better at this point? ----------------- Serge Mechveliani mechvel@botik.ru