具有类约束的GHC重写规则

Question

我已经将以下重写规则添加到管道而没有问题:

{-# RULES "ConduitM: lift x >>= f" forall m f.
    lift m >>= f = ConduitM (PipeM (liftM (unConduitM . f) m))
  #-}

我想增加一个类似重写规则liftIO以及

{-# RULES "ConduitM: liftIO x >>= f" forall m f.
    liftIO m >>= f = ConduitM (PipeM (liftM (unConduitM . f) (liftIO m)))
  #-}

但是,当我尝试这样做时,我从GHC收到以下错误消息:

Data/Conduit/Internal/Conduit.hs:1025:84:
    Could not deduce (Monad m) arising from a use of ‘liftM’
    from the context (Monad (ConduitM i o m), MonadIO (ConduitM i o m))
      bound by the RULE "ConduitM: liftIO x >>= f"
      at Data/Conduit/Internal/Conduit.hs:1025:11-118
    Possible fix:
      add (Monad m) to the context of the RULE "ConduitM: liftIO x >>= f"
    In the first argument of ‘PipeM’, namely
      ‘(liftM (unConduitM . f) (liftIO m))’
    In the first argument of ‘ConduitM’, namely
      ‘(PipeM (liftM (unConduitM . f) (liftIO m)))’
    In the expression:
      ConduitM (PipeM (liftM (unConduitM . f) (liftIO m)))

Data/Conduit/Internal/Conduit.hs:1025:108:
    Could not deduce (MonadIO m) arising from a use of ‘liftIO’
    from the context (Monad (ConduitM i o m), MonadIO (ConduitM i o m))
      bound by the RULE "ConduitM: liftIO x >>= f"
      at Data/Conduit/Internal/Conduit.hs:1025:11-118
    Possible fix:
      add (MonadIO m) to the context of
        the RULE "ConduitM: liftIO x >>= f"
    In the second argument of ‘liftM’, namely ‘(liftIO m)’
    In the first argument of ‘PipeM’, namely
      ‘(liftM (unConduitM . f) (liftIO m))’
    In the first argument of ‘ConduitM’, namely
      ‘(PipeM (liftM (unConduitM . f) (liftIO m)))’

我不知道任何语法可以让我为重写规则指定这样的上下文.有没有办法实现这个目标？

Answer 1

您可以在规则中指定具有约束的参数类型,例如

{-# RULES "ConduitM: liftIO x >>= f" forall m (f :: (Monad n, MonadIO n) => CounduitM i o n r).
    liftIO m >>= f = ConduitM (PipeM (liftM (unConduitM . f) (liftIO m)))
  #-}

(我没有测试它,因为我没有安装相关的软件包,但据我所知,所涉及的类型,我觉得应该可以工作.)