MonadReader 类型类 class Monad m => MonadReader r m | m -> r where ask :: m r ask = reader id local :: (r -> r) -> m a -> m a reader :: (r -> a) -> m a reader f = do r <- ask return (f r) instance Monad m => MonadReader r (ReaderT…
Category class Category cat where id :: cat a a (.) :: cat b c -> cat a b -> cat a c instance Category (->) where id = GHC.Base.id (.) = (GHC.Base..) (<<<) :: Category cat => cat b c -> cat a b -> cat a c (<<<) = (.) (&…
RWST Monad转换器 RWST Monad转换器是将 ReaderT, WriterT 以及 StateT 这三个Monad转换器的功能集于一体的产物. newtype RWST r w s m a = RWST { runRWST :: r -> s -> m (a, s, w) } instance (Monoid w, Monad m) => Monad (RWST r w s m) where return a = RWST $ \ _ s -> return (a,…
MonadCont 类型类 class Monad m => MonadCont m where callCC :: ((a -> m b) -> m a) -> m a instance MonadCont (ContT r m) where callCC = ContT.callCC class Monad m => MonadCont m where MonadState 是个类型类,它为 ContT 等封装了CPS函数的 Monad 定义了通用接口. MonadSta…
MonadState 类型类 class Monad m => MonadState s m | m -> s where get :: m s get = state (\s -> (s, s)) put :: s -> m () put s = state (\_ -> ((), s)) state :: (s -> (a, s)) -> m a state f = do s <- get let ~(a, s') = f s put s' return…
MonadWriter 类型类 class (Monoid w, Monad m) => MonadWriter w m | m -> w where writer :: (a,w) -> m a writer ~(a, w) = do tell w return a tell :: w -> m () tell w = writer ((),w) listen :: m a -> m (a, w) pass :: m (a, w -> w) -> m a lis…