来自Real World Haskell的MonadState实例无法编译

Question

从http://book.realworldhaskell.org/read/monad-transformers.html复制的这个MonadState实例给出了GHC 7.4.2的错误

instance (MonadState s m) => MonadState s (MaybeT m) where
  get = lift get
  put k = lift (put k)

给

    Illegal instance declaration for `MonadState s (MaybeT m)'
  (All instance types must be of the form (T a1 ... an)
   where a1 ... an are *distinct type variables*,
   and each type variable appears at most once in the instance head.
   Use -XFlexibleInstances if you want to disable this.)
In the instance declaration for `MonadState s (MaybeT m)'

如果我添加XFlexibleInstances,我会被告知添加XUndecidableInstances - 我不认为我应该在这里需要这些扩展.如何编译这个实例？

Answer 1

当您查看http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/src/Control-Monad-State-Class.html#MonadState时,您会看到它也用于"官方的"实施,所以我想这是必要的.评论说它与覆盖条件有关,这在这些stackoverflow问题中有解释:

• 什么是"覆盖条件"？
• 覆盖条件失败

在这种情况下,变量s不在右侧,并且功能依赖性从右向左,因此您的实例无效.(没有UndecidableInstances)