How to debug in pipeline easily?

Strict evaluation

In strictly-evaluated pipeline, you can track changing of the value in the middle of the pipeline by using the tap function. If you want to know more about the tap function, see here.

pipe(
  "2021/11/25",
  (str) => str.split("/"),
  tap((a) => console.log(a)), // ['2021', '11', '25']
  (date) => date.map(Number),
  tap((a) => console.log(a)), // [2021, 11, 25]
  (date) => date.map((n) => (n === 1 ? 1 : n - 1)),
  tap((a) => console.log(a)), // [2020, 10, 24]
  (date) => new Date(...date),
);

Lazy evaluation

The code below creates a list of the 13th of Fridays, starting from January 1, 2000. Each function in the pipeline returns an IterableIterator, Which means nothing happens until the toArray function iterates through the iterable and evaluates the value.

const addDate = (from: Date, n: number) => {
  const clone = new Date(from);
  clone.setDate(n);
  return clone;
};
const addDateFrom = (from: Date) => (n: number) => addDate(from, n);
const is13thOfFriday = (date: Date) =>
  date.getDate() === 13 && date.getDay() === 5;
const formatYYYYMMDD = (date: Date) => {
  return `${date.getFullYear()}-${date.getMonth() + 1}-${date.getDate()}`;
};

pipe(
  range(1, Infinity),
  map(addDateFrom(new Date(2000, 0, 1))),
  filter(is13thOfFriday),
  map(formatYYYYMMDD),
  take(5),
  toArray,
  console.log,
);
// ['2000-10-13', '2001-4-13', '2001-7-13', '2002-9-13', '2002-12-13']

Therefore, even if the logs are in the middle of the pipeline using the tap function, It is hard to trace the actual evaluation process. Maybe you will only know that type of value is IterableIterator.

pipe(
  range(1, Infinity),
  map(addDateFrom(new Date(2000, 0, 1))),
  filter(is13thOfFriday),
  tap(console.log), // IterableIterator
  map(formatYYYYMMDD),
  tap(console.log), // IterableIterator
  take(5),
  toArray,
);

If you want to track the progress of this lazy evaluation, you can use the peek function.

pipe(
  range(1, Infinity),
  map(addDateFrom(new Date(2000, 0, 1))),
  filter(is13thOfFriday),
  peek(console.log),
  map(formatYYYYMMDD),
  peek(console.log),
  take(5),
  toArray,
);

The entire code is here.

To check for lazy evaluation in FxTS, see the Lazy evaluation