这个MonadState实例是从http://book.realworldhaskell.org/read/monad-transformers.html复制的,但在GHC 7.4.2中会出现错误。
instance (MonadState s m) => MonadState s (MaybeT m) where
get = lift get
put k = lift (put k)
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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,然后被告知要添加XUndecidableInstances - 我不认为我需要在这里使用这些扩展。如何让此实例编译通过?