A function to execute a transaction on a microstate substate.

example.js

let next = over(inventory.item.price, price => price.add(2).add(2).add(2));

over.js

import { create, valueOf } from 'microstates';

export default function over(microstate, fn) {
  let Type = typeOf(microstate);
  let value = valueOf(microstate);

  let updated = fn(create(Type, value));

  return microstate.set(valueOf(updated));
}


// this will probably become part of the public API of
// microstates at some point.
function typeOf(microstate) {
  return microstate.constructor.Type;
}