Abstract Suspense

suspense.js

/** effectful expression throws this object if it requires suspension */
const token = {};

/** Pointer to mutable data used to record effectful computations */
let context;

/** Runs `thunk()` as an effectful expression with `of` and `chain` as Monad's definition */
const run = (of, chain) => thunk => {
  /** here it caches effects requests */
  const trace = [];
  const ctx = {trace};
  return step();
  function step() {
    const savedContext = context;
    ctx.pos = 0;
    try {
      context = ctx;
      return of(thunk());
    } catch(e) {
      /** re-throwing other exceptions */
      if (e !== token)
        throw e;
      const {pos} = ctx;
      return chain(ctx.effect,
                   (value) => {
                     trace.length = pos;
                     /* recording the resolved value */
                     trace[pos] = value;
                     ctx.pos = pos + 1;
                     /** replay */
                     return step(value);
                   })
    } finally {
      context = savedContext;
    }
  }
}

/** marks effectful expression */
const M = eff => {
  /* if the execution is in a replay stage the value will be cached */
  if (context.pos < context.trace.length)
    return context.trace[context.pos++];
  /* saving the expression to resolve in `run` */
  context.effect = eff;
  throw token;
}