Control.Monad.Freer.State

Composable handler for State effects. Handy for passing an updatable state through a computation.

Some computations may not require the full power of State effect:

• For a read-only state, see Control.Monad.Freer.Reader.
• To accumulate a value without using it on the way, see Control.Monad.Freer.Writer.

Using http://okmij.org/ftp/Haskell/extensible/Eff1.hs as a starting point.

State Effect

data State s r where

Strict State effects: one can either Get values or Put them.

Constructors

Get :: State s s

Put :: !s -> State s ()

State Operations

get :: forall s effs. Member (State s) effs => Eff effs s

Retrieve the current value of the state of type s :: *.

put :: forall s effs. Member (State s) effs => s -> Eff effs ()

Set the current state to a specified value of type s :: *.

modify :: forall s effs. Member (State s) effs => (s -> s) -> Eff effs ()

Modify the current state of type s :: * using provided function (s -> s).

State Handlers

runState :: forall s effs a. s -> Eff (State s ': effs) a -> Eff effs (a, s)

Handler for State effects.

evalState :: forall s effs a. s -> Eff (State s ': effs) a -> Eff effs a

Run a State effect, discarding the final state.

execState :: forall s effs a. s -> Eff (State s ': effs) a -> Eff effs s

Run a State effect, returning only the final state.

State Utilities

transactionState :: forall s effs a. Member (State s) effs => Eff effs a -> Eff effs a

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

GHC cannot infer the s type parameter for this function, so it must be specified explicitly with TypeApplications. Alternatively, it can be specified by supplying a Proxy to transactionState'.

transactionState' :: forall s effs a. Member (State s) effs => Proxy s -> Eff effs a -> Eff effs a

Like transactionState, but s is specified by providing a Proxy instead of requiring TypeApplications.