Control.Monad.State.Class

monads-tf-0.1.0.3: Monad classes, using type families

Copyright | (c) Andy Gill 2001 (c) Oregon Graduate Institute of Science and Technology 2001
License | BSD-style (see the file LICENSE)
Maintainer | ross@soi.city.ac.uk
Stability | experimental
Portability | non-portable (type families)
Safe Haskell | Safe
Language | Haskell98

Control.Monad.State.Class

Description

MonadState class.

This module is inspired by the paper /Functional Programming with Overloading and Higher-Order Polymorphism/, Mark P Jones (http://web.cecs.pdx.edu/~mpj/) Advanced School of Functional Programming, 1995.

Synopsis

- class Monad m => MonadState m where
  - type StateType m
- modify :: MonadState m => (StateType m -> StateType m) -> m ()
- gets :: MonadState m => (StateType m -> a) -> m a

# Documentation

class Monad m => MonadState m where #

get returns the state from the internals of the monad.

put replaces the state inside the monad.

Minimal complete definition

get, put

Associated Types

type StateType m #

Methods

get :: m (StateType m) #

put :: StateType m -> m () #

Instances

MonadState m => MonadState (MaybeT m) # |
Associated Types type StateType (MaybeT m :: * -> *) :: * # Methods get :: MaybeT m (StateType (MaybeT m)) # put :: StateType (MaybeT m) -> MaybeT m () #
MonadState m => MonadState (ListT m) # |
Associated Types type StateType (ListT m :: * -> *) :: * # Methods get :: ListT m (StateType (ListT m)) # put :: StateType (ListT m) -> ListT m () #
(Monoid w, MonadState m) => MonadState (WriterT w m) # |
Associated Types type StateType (WriterT w m :: * -> *) :: * # Methods get :: WriterT w m (StateType (WriterT w m)) # put :: StateType (WriterT w m) -> WriterT w m () #
(Monoid w, MonadState m) => MonadState (WriterT w m) # |
Associated Types type StateType (WriterT w m :: * -> *) :: * # Methods get :: WriterT w m (StateType (WriterT w m)) # put :: StateType (WriterT w m) -> WriterT w m () #
Monad m => MonadState (StateT s m) # |
Associated Types type StateType (StateT s m :: * -> *) :: * # Methods get :: StateT s m (StateType (StateT s m)) # put :: StateType (StateT s m) -> StateT s m () #
Monad m => MonadState (StateT s m) # |
Associated Types type StateType (StateT s m :: * -> *) :: * # Methods get :: StateT s m (StateType (StateT s m)) # put :: StateType (StateT s m) -> StateT s m () #
MonadState m => MonadState (IdentityT * m) # |
Associated Types type StateType (IdentityT * m :: * -> *) :: * # Methods get :: IdentityT * m (StateType (IdentityT * m)) # put :: StateType (IdentityT * m) -> IdentityT * m () #
(Error e, MonadState m) => MonadState (ErrorT e m) # |
Associated Types type StateType (ErrorT e m :: * -> *) :: * # Methods get :: ErrorT e m (StateType (ErrorT e m)) # put :: StateType (ErrorT e m) -> ErrorT e m () #
MonadState m => MonadState (ReaderT * r m) # |
Associated Types type StateType (ReaderT * r m :: * -> *) :: * # Methods get :: ReaderT * r m (StateType (ReaderT * r m)) # put :: StateType (ReaderT * r m) -> ReaderT * r m () #
MonadState m => MonadState (ContT * r m) # |
Associated Types type StateType (ContT * r m :: * -> *) :: * # Methods get :: ContT * r m (StateType (ContT * r m)) # put :: StateType (ContT * r m) -> ContT * r m () #
(Monad m, Monoid w) => MonadState (RWST r w s m) # |
Associated Types type StateType (RWST r w s m :: * -> *) :: * # Methods get :: RWST r w s m (StateType (RWST r w s m)) # put :: StateType (RWST r w s m) -> RWST r w s m () #
(Monad m, Monoid w) => MonadState (RWST r w s m) # |
Associated Types type StateType (RWST r w s m :: * -> *) :: * # Methods get :: RWST r w s m (StateType (RWST r w s m)) # put :: StateType (RWST r w s m) -> RWST r w s m () #

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

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 a #

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