Thanks Stephen, On 09/09/2010 18:07, Stephen Tetley wrote:
What package are you trying build? - at a pinch it looks like it has conflicting dependencies for the packages 'transformers' and mtl.
It was a (cabalized) project of mine. I finally solved the problem. My project depends on transformers and HTTP packages. When you import Network.HTTP, it brings into scope: instance Monad (Either e) What I discovered is that: - Not only the instance is in scope in the module that imports HTTP but in all project modules. That means that I cannot use Either as Monad Error in my modules (since that would trigger the overlapping instances error because it would conflict with transformers' instance). - you cannot hide class instances for imported modules - I was getting the overalapping instance error in a module that didn't import Network.HTTP. This is somewhat misleading because the imports in that module seemed safe. - There is no way to discover quickly / automatically which import is causing the overlap. The best that I could do was to check the imports one by one in ghci and check with :info Either Either's instances. Is there a better/quicker way? I'm reporting my finding to ask for a confirmation or get advices on how to generally cope with this kind of errors. Thanks Paolo