mtl library in HEAD?

[Don Stewart]

malcolm.wallace:
>> The library installs fine with earlier versions of Haskell.  Is there 
>> something wrong with my installation, or is this a new  
>> incompatibility?  If the latter, is a corrected version of the mtl  
>> library going to be available soon?  Alternatively, how can I fix it?
>
> Yes, this is a new incompatibility.  The instance in question recently  
> moved from the mtl package to the base package, after discussion on this 
> list.
>
> Until the next release of ghc/base/mtl, it will be necessary to use the 
> HEAD versions of all of them (not merely the HEAD of ghc).

And indeed ,the inverse patch has been applied to mtl,

    Fri Jul 30 17:40:34 CEST 2010  Ross Paterson <ross at soi.city.ac.uk>
      *  move Monad and MonadFix instances for Either to base (proposal #4159)