Combining StateT IO with State

问题

If I have a function f :: State Int (), is it possible to use it within another function g :: StateT Int IO ()? Nesting it with f = do { something; g } fails to typecheck with Couldn't match type 'Data.Functor.Identity.Identity' with 'IO'.

回答1:

Yes, this operation is usually called "hoisting". For the State monad, it could be defined as

hoistState :: Monad m => State s a -> StateT s m a
hoistState = state . runState

Unfortunately, it is not defined in the Control.Monad.State module.

来源：https://stackoverflow.com/questions/17325485/combining-statet-io-with-state