Merge pull request #750 from haskell-servant/enter-tweak

Add functional dependency back to Enter

servant/src/Servant/Utils/Enter.hs

@@ -27,41 +27,46 @@ import           Prelude.Compat
 
 import           Servant.API
 
+-- | Helper type family to state the 'Enter' symmetry.
 type family Entered m n api where
-    Entered m n (m a)            = n a
     Entered m n (a -> api)       = a -> Entered m n api
+    Entered m n (m a)            = n a
     Entered m n (api1 :<|> api2) = Entered m n api1 :<|> Entered m n api2
 
-class ( Entered m n typ ~ ret
+class
+    ( Entered m n typ ~ ret
     , Entered n m ret ~ typ
-      )
-    => Enter typ m n ret
+    ) => Enter typ m n ret | typ m n ->  ret, ret m n -> typ, ret typ m -> n, ret typ n -> m
   where
+    -- | Map the leafs of an API type.
     enter :: (m :~> n) -> typ -> ret
 
 -- ** Servant combinators
 
 instance
-    ( Entered m1 n1 (typ1 :<|> typ2) ~ (ret1 :<|> ret2)
-    , Entered n1 m1 (ret1 :<|> ret2) ~ (typ1 :<|> typ2)
-    , Enter typ1 m1 n1 ret1, Enter typ2 m2 n2 ret2
+    ( Enter typ1 m1 n1 ret1, Enter typ2 m2 n2 ret2
     , m1 ~ m2, n1 ~ n2
+    , Entered m1 n1 (typ1 :<|> typ2) ~ (ret1 :<|> ret2)
+    , Entered n1 m1 (ret1 :<|> ret2) ~ (typ1 :<|> typ2)
     ) => Enter (typ1 :<|> typ2) m1 n1 (ret1 :<|> ret2)
   where
     enter e (a :<|> b) = enter e a :<|> enter e b
 
 instance
-    ( Entered m n (a -> typ) ~ (a -> ret)
+    ( Enter typ m n ret
+    , Entered m n (a -> typ) ~ (a -> ret)
     , Entered n m (a -> ret) ~ (a -> typ)
-    , Enter typ m n ret
-    )
-    => Enter (a -> typ) m n (a -> ret)
+    ) => Enter (a -> typ) m n (a -> ret)
   where
     enter arg f a = enter arg (f a)
 
--- ** Useful instances
+-- ** Leaf instances
 
-instance Enter (m a) m n (n a) where
+instance
+    ( Entered m n (m a) ~ n a
+    , Entered n m (n a) ~ m a
+    ) => Enter (m a) m n (n a)
+  where
     enter (NT f) = f
 
 -- | Like `lift`.