Copyright	(c) 2016 Allele Dev; 2017 Ixperta Solutions s.r.o.; 2017 Alexis King
License	BSD3
Maintainer	Alexis King <lexi.lambda@gmail.com>
Stability	experimental
Portability	GHC specific language extensions.
Safe Haskell	None
Language	Haskell2010

Control.Monad.Freer.State

Contents

State Effect
State Operations
State Handlers
State Utilities

Description

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.

Synopsis

data State s r where
- Get :: State s s
- Put :: !s -> State s ()
get :: forall s effs. Member (State s) effs => Eff effs s
put :: forall s effs. Member (State s) effs => s -> Eff effs ()
modify :: forall s effs. Member (State s) effs => (s -> s) -> Eff effs ()
gets :: forall s a effs. Member (State s) effs => (s -> a) -> Eff effs a
runState :: forall s effs a. s -> Eff (State s ': effs) a -> Eff effs (a, s)
evalState :: forall s effs a. s -> Eff (State s ': effs) a -> Eff effs a
execState :: forall s effs a. s -> Eff (State s ': effs) a -> Eff effs s
transactionState :: forall s effs a. Member (State s) effs => Eff effs a -> Eff effs a
transactionState' :: forall s effs a. Member (State s) effs => Proxy s -> Eff effs a -> Eff effs a

State Effect

data State s r where Source #

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 Source #

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

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

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

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

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

gets :: forall s a effs. Member (State s) effs => (s -> a) -> Eff effs a Source #

Retrieve a specific component of the current state using the provided projection function.

State Handlers

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

Handler for State effects.

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

Run a State effect, discarding the final state.

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

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 Source #

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 Source #

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