haskell - Associated data families and overlapping instances -

i'd 'generic' map data structure can efficiently specialized providing custom instances, in the ghc manual section on type families.

{-# language flexibleinstances    #-} {-# language typefamilies         #-} {-# language undecidableinstances #-}  module mapkey  import           data.map.strict    (map) import qualified data.map.strict    map  class mapkey k   data mmap k :: * -> *  instance {-# overlapping #-} mapkey ()   newtype mmap () v = unitmap (maybe v)  instance {-# overlappable #-} ord k => mapkey k   newtype mmap k v = ordmap (map k v) 

sadly, doesn't work. ghc (8.2.1) complains:

    conflicting family instance declarations:       mmap () = unitmap (maybe v)       mmap = ordmap (map k v)    | 14 |   newtype mmap () v = unitmap (maybe v)    | 

is there language extension allows this? otherwise there way make easy users define 'default' instance ord?

one solution relinquishes overlapping instances use default associated injective type family (quite mouthful). attached methods default implementations default mmap synonym:

{-# language defaultsignatures      #-} {-# language typefamilies           #-} {-# language typefamilydependencies #-}  module mapkey  import           data.map.strict    (map) import qualified data.map.strict    map  class mapkey k   type mmap k v = r | r -> k v   type mmap k v = map k v   empty :: mmap k v   default empty :: (mmap k v ~ map k v) => mmap k v   empty = map.empty   insert :: k -> v -> mmap k v -> mmap k v   default insert :: (mmap k v ~ map k v, ord k) => k -> v -> mmap k v -> mmap k v   insert = map.insert   lookuple :: k -> mmap k v -> [(k, v)]   default lookuple :: (mmap k v ~ map k v, ord k) => k -> mmap k v -> [(k, v)]   lookuple k m =     case map.lookuple k m of       nothing -> []       e -> [e]  instance mapkey ()   type mmap () v = maybe v   empty = nothing   insert _ v _ = v   lookuple _ m =     case m of       nothing  -> []       (just v) -> [((), v)] 

this means client code still has define boilerplate orphan instances like

instance mapkey int 

i'd rather see solution uses overlapping instances instead.