Safe Haskell | Trustworthy |
---|---|

Language | Haskell2010 |

Lazy state effect

## Synopsis

- data State s v where
- withState :: Monad m => a -> s -> m (a, s)
- get :: Member (State s) r => Eff r s
- put :: Member (State s) r => s -> Eff r ()
- runState :: s -> Eff (State s ': r) a -> Eff r (a, s)
- modify :: Member (State s) r => (s -> s) -> Eff r ()
- evalState :: s -> Eff (State s ': r) a -> Eff r a
- execState :: s -> Eff (State s ': r) a -> Eff r s
- data TxState s v where
- type TxStateT s = TxState s s
- withTxState :: Member (State s) r => a -> s -> Eff r a
- transactionState :: forall s r a. Member (State s) r => TxStateT s -> Eff r a -> Eff r a
- runStateR :: s -> Eff (Writer s ': (Reader s ': r)) a -> Eff r (a, s)

# Documentation

State, lazy

Initial design: The state request carries with it the state mutator function We can use this request both for mutating and getting the state. But see below for a better design!

data State s v where State :: (s->s) -> State s s

In this old design, we have assumed that the dominant operation is modify. Perhaps this is not wise. Often, the reader is most nominant.

See also below, for decomposing the State into Reader and Writer!

The conventional design of State

withState :: Monad m => a -> s -> m (a, s) Source #

Embed a pure value in a stateful computation, i.e., given an initial state, how to interpret a pure value in a stateful computation.

get :: Member (State s) r => Eff r s Source #

Return the current value of the state. The signatures are inferred

:: s | Initial state |

-> Eff (State s ': r) a | Effect incorporating State |

-> Eff r (a, s) | Effect containing final state and a return value |

Run a State effect

evalState :: s -> Eff (State s ': r) a -> Eff r a Source #

Run a State effect, discarding the final state.

execState :: s -> Eff (State s ': r) a -> Eff r s Source #

Run a State effect and return the final state.

data TxState s v where Source #

An encapsulated State handler, for transactional semantics The global state is updated only if the transactionState finished successfully

withTxState :: Member (State s) r => a -> s -> Eff r a Source #

Embed Transactional semantics to a stateful computation.