haskell – 依赖类约束的不明确类型变量

在这样做时,我遇到了模糊类型约束的问题.在下面的代码中,对我来说很明显,返回的类型只能是函数类型中的类型变量b,但GHC抱怨类型不明确.

如何更改以下代码,使背面的类型为b,而不使用例如ScopedTypeVariables(因为问题在于约束,而不是具有太一般的类型)？某处需要功能依赖吗？

相关类型类：

data AuthSnaplet b u =
  AuthSnaplet
  { _backend    :: b,_activeUser :: Maybe u
  }
-- data-lens-template:Data.Lens.Template.makeLens
-- data-lens:Data.Lens.Common.Lens
-- generates: backend :: Lens (AuthSnaplet b u) b
makeLens ''AuthSnaplet

-- Some encrypted password
newtype Password =
  Password
  { passwordData :: ByteString
  }

-- data-default:Data.Default.Default
class Default u => AuthUser u where
  userLogin :: Lens u Text
  userPassword :: Lens u Password

class AuthUser u => AuthBackend b u where
  save :: MonadIO m => b -> u -> m u
  lookupByLogin :: MonadIO m => b -> Text -> m (Maybe u)
  destroy :: MonadIO m => b -> u -> m ()

-- snap:Snap.Snaplet.Snaplet
class AuthBackend b u => HasAuth s b u where
  authSnaplet :: Lens s (Snaplet (AuthSnaplet b u))

失败的代码：

-- snap:Snap.Snaplet.with :: Lens v (Snaplet v') -> m b v' a -> m b v a
-- data-lens-fd:Data.Lens.access :: MonadState a m => Lens a b -> m b
loginUser :: HasAuth s b u
          => Text -> Text -> Handler a s (Either AuthFailure u)
loginUser uname passwd = with authSnaplet $do
  back <- access backend
  maybeUser <- lookupByLogin back uname -- !!! type of back is ambiguous !!!
  -- ... For simplicity's sake,let's say the function ends like this:
  return . Right . fromJust $maybeUser

完整错误：

src/Snap/Snaplet/Authentication.hs:105:31:
    Ambiguous type variables `b0',`u0' in the constraint:
      (HasAuth s b0 u0) arising from a use of `authSnaplet'
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `with',namely `authSnaplet'
    In the expression: with authSnaplet
    In the expression:
        with authSnaplet
      $do { back <- access backend;
             maybeUser <- lookupByLogin back uname;
               ... }

src/Snap/Snaplet/Authentication.hs:107:16:
    Ambiguous type variable `b0' in the constraint:
      (AuthBackend b0 u) arising from a use of `lookupByLogin'
    Probable fix: add a type signature that fixes these type variable(s)
    In a stmt of a 'do' expression:
        maybeUser <- lookupByLogin back uname
    In the second argument of `($)',namely
      `do { back <- access backend;
            maybeUser <- lookupByLogin back uname;
              ... }'
    In the expression:
        with authSnaplet
      $do { back <- access backend;
             maybeUser <- lookupByLogin back uname;
               ... }