ghci 5.00 instance declaration error

[Martin Sulzmann]

Consider the following program:

--------------------

module MZip where

class MZip a where mzip :: a

instance (MZip (as->bs->ds), MZip(ds->cs->r)) => MZip (as->bs->cs->r) where
   mzip as bs cs = mzip (mzip as bs) cs 

----------------------------

I get the following error message:

Compiling MZip             ( MZip2.hs, interpreted )

MZip2.hs:9:
    Could not deduce `MZip (t -> cs -> r)'
        from the context (MZip (as -> bs -> cs -> r),
                          MZip (as -> bs -> ds),
                          MZip (ds -> cs -> r))
    Probable fix:
        Add `MZip (t -> cs -> r)' to the instance declaration context
        Or add an instance declaration for `MZip (t -> cs -> r)'
    arising from use of `mzip' at MZip2.hs:9
    in the definition of function `mzip': mzip (mzip as bs) cs

MZip2.hs:9:
    Could not deduce `MZip (as -> bs -> t)'
        from the context (MZip (as -> bs -> cs -> r),
                          MZip (as -> bs -> ds),
                          MZip (ds -> cs -> r))
    Probable fix:
        Add `MZip (as -> bs -> t)' to the instance declaration context
        Or add an instance declaration for `MZip (as -> bs -> t)'
    arising from use of `mzip' at MZip2.hs:9
    In the first argument of `mzip', namely `(mzip as bs)'
    in the definition of function `mzip': mzip (mzip as bs) cs


I used ghci -fglasgow-exts -fallow-undecidable-instances


Note that If I use only (empty instance declaration)

instance (MZip (as->bs->ds), MZip(ds->cs->r)) => MZip (as->bs->cs->r) 

everything works fine.

Why do I get an error message of I include the body of the where
statement? Is there a form of skolemization happening?
It seems to me a limitation of the implementation. 

Martin