Sometimes the simple atomic action creators - update, set, clear... - won't feel sufficient.

The general purpose do can help with flexibility: it takes a callback of (leafState, treeState) => newLeafState.

const pizzaShopState = {
  stock: {
    margherita: 10,
    pepperoni: 20
  },
  isOpen: {
    forEatIn: false,
    forTakeOut: true
  }
}

const [reducer, actions] = riduce(pizzaShopState)
const { getState, dispatch } = createStore(reducer)

const squareMargheritaStock = actions.stock.margherita.create.do(leafState => leafState ** 2)

dispatch(squareMargheritaStock)
getState().stock // => { margherita: 100, pepperoni: 20 }

const openIfSurplusStock = actions.isOpen.create.do(
  (leafState, treeState) => {
    const hasEnoughStock = treeState.stock.margherita > 10
    return {
      forEatIn: leafState.forEatIn || hasEnoughStock,
      forTakeOut: leafState.forTakeOut || hasEnoughStock
    }
  }
)

getState().isOpen // => { forEatIn: true, forTakeOut: true }