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

``sh
npm install -S gensequence
`

Usage

`ts
import { genSequence } from 'gensequence';
`

Examples

$3

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.

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

export function fibonacci(): Sequence {
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]
`

$3

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

`ts
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.

$3

- .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.

$3

- .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.

$3

- .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.

$3

- .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.

$3

- .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.