monads-tf- Monad classes, using type families

Portabilitynon-portable (type families)
Safe HaskellSafe-Inferred



MonadState class.

This module is inspired by the paper /Functional Programming with Overloading and Higher-Order Polymorphism/, Mark P Jones ( Advanced School of Functional Programming, 1995.



class Monad m => MonadState m whereSource

get returns the state from the internals of the monad.

put replaces the state inside the monad.

Associated Types

type StateType m Source


get :: m (StateType m)Source

put :: StateType m -> m ()Source


modify :: MonadState m => (StateType m -> StateType m) -> m ()Source

Monadic state transformer.

Maps an old state to a new state inside a state monad. The old state is thrown away.

      Main> :t modify ((+1) :: Int -> Int)
      modify (...) :: (MonadState Int a) => a ()

This says that modify (+1) acts over any Monad that is a member of the MonadState class, with an Int state.

gets :: MonadState m => (StateType m -> a) -> m aSource

Gets specific component of the state, using a projection function supplied.