GenSequence

Small library to simplify working with Generators and Iterators in Javascript / Typescript

Javascript Iterators and Generators are very exciting and provide some powerful new ways to solve programming problems.

The purpose of this library is to make using the results of a generator function easier. It is not intended as a replacement for arrays and the convenient [...genFn()] notation. GenSequence is useful for cases where you might not want an array of all possible values. GenSequence deals efficiently with large sequences because only one element at a time is evaluated. Intermediate arrays are not created, saving memory and cpu cycles.

Installation

npm install -S gensequence

Usage

import { genSequence } from 'gensequence';

Examples

Fibonacci

The Fibonacci sequence can be very simply expressed using a generator. Yet using the result of a generator can be a bit convoluted. GenSequence provides a wrapper to add familiar functionality similar to arrays.

import { genSequence, Sequence } from '../index.js';

export function fibonacci(): Sequence<number> {
  function* fib() {
    let [a, b] = [0, 1];
    while (true) {
      yield b;
      [a, b] = [b, a + b];
    }
  }
  return genSequence(fib());
}

export function fib(n: number) {
  return fibonacci()
    .take(n) // Take n from the fibonacci sequence
    .toArray(); // Convert it into an array
}

export const fib5 = fib(5); // [1, 1, 2, 3, 5]

RegEx Match

Regular expressions are wonderfully powerful. Yet, working with the results can sometimes be a bit of a pain.

import { genSequence } from '../GenSequence.js';

function matchWords(text: string) {
  return genSequence(text.matchAll(/\w+/g)).map((a) => a[0]);
}

export function toSetOfWords(text: string) {
  return new Set(matchWords(text));
}

export const text = 'Some long bit of text with many words, duplicate words...';
export const setOfWords = toSetOfWords(text);
// Walk through the set of words and pull out the 4 letter one.
export const setOf4LetterWords = new Set(genSequence(setOfWords).filter((a) => a.length === 4));

Reference

• genSequence(Iterable|Array|()=>Iterable) -- generate a new Iterable from an Iterable, Array or function with the following functions.

Filters

• .filter(fn) -- just like array.filter, filters the sequence
• .skip(n) -- skip n entries in the sequence
• .take(n) -- take the next n entries in the sequence.

Extenders

• .concat(iterable) -- this will extend the current sequence with the values from iterable
• .concatMap(fnMap) -- this is used to flatten the result of a map function.

Mappers

• .combine(fnCombiner, iterable) -- is used to combine values from two different lists.
• .map(fn) -- just like array.map, allows you to convert the values in a sequence.
• .pipe(...operatorFns) -- pipe any amount of operators in sequence.
• .scan(fn, init?) -- similar to reduce, but returns a sequence of all the results of fn.

Reducers

• .all(fn) -- true if all values in the sequence return true for fn(value) or the sequence is empty.
• .any(fn) -- true if any value in the sequence exists where fn(value) returns true.
• .count() -- return the number of values in the sequence.
• .first() -- return the next value in the sequence.
• .first(fn) -- return the next value in the sequence where fn(value) return true.
• .forEach(fn) -- apply fn(value, index) to all values.
• .max() -- return the largest value in the sequence.
• .max(fn) -- return the largest value of fn(value) in the sequence.
• .min() -- return the smallest value in the sequence.
• .min(fn) -- return the smallest value of fn(value) in the sequence.
• .reduce(fn, init?) -- just like array.reduce, reduces the sequence into a single result.
• .reduceAsync(fn, init?) -- just like array.reduce, reduces promises into the sequence into a single result chaining the promises, fn/init can be async or not, it will work, the previousValue, and currentValue will never be a promise.
• .reduceToSequence(fn, init) -- return a sequence of values that fn creates from looking at all the values and the initial sequence.

Cast

• .toArray() -- convert the sequence into an array. This is the same as [...iterable].
• .toIterable() -- Casts a Sequence into an IterableIterator - used in cases where type checking is too strict.