我想让我的 monad 转换器成为一个实例MonadError如果转换后的单子是一个实例。基本上我希望我的变压器的行为与内置变压器一样，例如有一个MonadError实例为StateT:
MonadError e m => MonadError e (StateT s m)
我尝试这样做：
instance MonadError e m => MonadError e (MyMonadT m)
但是 GHC 开始抱怨不可判定的实例，显然 MTL 库只是启用了不可判定的实例，但是有什么方法可以避免这种情况吗？或者在这种情况下就可以了，不会造成任何问题？
This is basically fine. UndecidableInstances isn't all that scary; all it means is that the compiler could, instead of finding an instance, go into an infinite loop. That sounds pretty bad, until you realise that GHC actually has a limit to the number of steps it'll take to find an instance; nothing will go wrong unless you write a bad instance, and the error messages you get will generally make it very obvious what's gone wrong.1 Certainly it's a lot less scary than things like OverlappingInstances (or worse, IncoherentInstances).
它抱怨的原因是因为MonadError具有函数依赖性m to e。这意味着选择m决定什么e必须是;即每个m仅与一个相关联e。对此（覆盖条件）的检查是保守的，因此很容易遇到这样的问题，您尝试“向下递归一个级别”来指定e.
1 It'll list all the instances it looked at to try and find the one it's looking for, so you'll see a bunch of repeating lines. But usually you won't even run into trouble like this in the first place.
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