真实世界中的MonadState实例Haskell不';不编译
从复制的MonadState实例给了我GHC 7.4.2的一个错误真实世界中的MonadState实例Haskell不';不编译,haskell,monad-transformers,Haskell,Monad Transformers,从复制的MonadState实例给了我GHC 7.4.2的一个错误 instance (MonadState s m) => MonadState s (MaybeT m) where get = lift get put k = lift (put k) 给予 如果我添加XFlexibleInstances,我会被告知添加XundecardableInstances——我认为这里不需要这些扩展。如何编译此实例?当您查看时,您会发现它也用于“Official”实现,所以我想它是必
instance (MonadState s m) => MonadState s (MaybeT m) where
get = lift get
put k = lift (put k)
给予
如果我添加XFlexibleInstances,我会被告知添加XundecardableInstances——我认为这里不需要这些扩展。如何编译此实例?当您查看时,您会发现它也用于“Official”实现,所以我想它是必需的。评论说,这与保险范围条件有关,这在以下问题中进行了解释:
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)'