Event streams of stateful operations with shared accumulated state.

Sodium.FRP.State.hs

{-# LANGUAGE RecursiveDo #-}
module FRP.Sodium.State
       ( runAccum
       , snapshotState
       ) where

import Control.Monad.State
import FRP.Sodium

type AccumE s = State (Behaviour s, Event s)

-- | Build accumulated state operation streams with an initial state.
runAccum :: s -> AccumE s a -> Reactive a
runAccum initValue op = do
  rec
    let (result, (_, asChanges)) = runState op (behaviour, never)
    behaviour <- hold initValue asChanges
  return result

-- | Snapshot stateful events with the accumulated state.
snapshotState :: Event (State s a) -> AccumE s (Event a)
snapshotState op = do
  st <- fst <$> get
  let (result, changes) = splitE $ snapshot (\f b -> runState f b) op st
  modify $ \(b, stream) -> (b, stream `merge` changes)
  return result