extensible-effects- An Alternative to Monad Transformers

Safe HaskellSafe



Strict state effect

Example: implementing Fresh

runFresh' :: (Typeable i, Enum i, Num i) => Eff (Fresh i :> r) w -> i -> Eff r w
runFresh' m s = fst <$> runState s (loop $ admin m)
  loop (Val x) = return x
  loop (E u)   = case decomp u of
    Right (Fresh k) -> do
                      n <- get
                      put (n + 1)
                      loop (k n)
    Left u' -> send (\k -> unsafeReUnion $ k <$> u') >>= loop



data State s w Source

Strict state effect


State (s -> s) (s -> w) 


get :: (Typeable e, Member (State e) r) => Eff r e Source

Return the current value of the state.

put :: (Typeable e, Member (State e) r) => e -> Eff r () Source

Write a new value of the state.

modify :: (Typeable s, Member (State s) r) => (s -> s) -> Eff r () Source

Transform the state with a function.

runState Source


:: Typeable s 
=> s

Initial state

-> Eff (State s :> r) w

Effect incorporating State

-> Eff r (s, w)

Effect containing final state and a return value

Run a State effect.

evalState :: Typeable s => s -> Eff (State s :> r) w -> Eff r w Source

Run a State effect, discarding the final state.

execState :: Typeable s => s -> Eff (State s :> r) w -> Eff r s Source

Run a State effect and return the final state.