Editing Class system extension proposal (section)

==Explicit import/export of instances==
This is needed so that large programs can be built without fear of colliding instance declarations between different packages. A possible syntax could be:
<haskell>
    module M
         -- exported instances 
       ( instance Monad T
       , instance Functor (F a) hiding (Functor (F Int), Functor (F Char))
       , F(..)
       ) where

    import Foo (instance Monad a hiding Monad Maybe)

    data T a
    data F a b
</haskell>
where the context is elided because this isn't used in instance selection (at the moment). The import directive tells the compiler to use all <hask>Monad</hask> instances exported by <hask>Foo</hask> except for the <hask>Monad Maybe</hask> instance (it doesn't matter whether or not <hask>Foo</hask> actually does export a <hask>Monad Maybe</hask> instance - all that matters here is that we don't want it if there is one).