Transducist

Ergonomic JavaScript/TypeScript transducers for beginners and experts.

![Build
Status](https://travis-ci.org/dphilipson/transducist)

Table of Contents

- Introduction
- Goals
- Installation
- Basic Usage
- Iterable utilities
- Advanced Usage
- Using custom transducers
- Using custom reductions
- Creating a standalone transducer
- Bundle Size and Tree Shaking
- Benchmarks
- API

Introduction

This library will let you write code that looks like this:

``ts
// Let's find 100 people who have a parent named Brad who runs Haskell projects
// so we can ask them about their dads Brads' monads.
const result = chainFrom(haskellProjects)
.map(project => project.owner)
.filter(owner => owner.name === "Brad")
.flatMap(owner => owner.children)
.take(100)
.toArray();
`

This computation is very efficient because no intermediate arrays are created
and work stops early once 100 people are found.

You might be thinking that this looks very similar to chains in
Lodash or various other libraries that
offer a similar API. But this library is different because it's implemented with
transducers and exposes all benefits of the transducer protocol, such as being
able to easily add novel transformation types to the middle of a chain and
producing logic applicable to any data structure, not just arrays.

Never heard of a transducer? Check the links in the
transducers-js
readme for an introduction to the concept, but note that **you don't need to
understand anything about transducers to use this library**.

Goals

Provide an API for using transducers that is…

- …easy to use even without transducer knowledge or experience. If you
haven't yet wrapped your head around transducers or need to share a codebase
with others who haven't, the basic chaining API is fully usable without ever
seeing a reference to transducers or anything more advanced than map and
filter. However, it is also…

- …able to reap the full benefits of transducers for those who are
familiar with them. By using the general purpose .compose() to place
custom transducers in the middle of a chain, any kind of novel transform can
be added while still maintaining the efficiency bonuses of laziness and
short-circuiting. Further, the library can also be used to construct
standalone transducers which may be used elsewhere by other libraries that
incorporate transducers into their API.

- …fast! Transducist performs efficient computations by never creating
more objects than necessary. See the
benchmarks
for details.

- …typesafe. Transducist is written in TypeScript and is designed to be
fully typesafe without requiring you to manually specify type parameters
everywhere.

- …small. Transducist is less than 4kB gzipped, and can be made even
smaller through tree shaking.

Installation

With Yarn:

`
yarn add transducist
`

With NPM:

`
npm install transducist
`

This library, with the exception of the functions which relate to Set and
Map, works fine on ES5 without any polyfills or transpilation, but its
TypeScript definitions depend on ES6 definitions for the Iterable type. If you
use TypeScript in your project, you must make definitions for these types
available by doing one of the following:

- In tsconfig.json, set "target" to "es6" or higher.
- In tsconfig.json, set "libs" to include "es2015.iterable" or something
that includes it
- Add the definitions by some other means, such as importing types for
es6-shim.

Furthermore, the methods toSet, toMap, and toMapGroupBy assume the
presence of ES6 Set and Map classes in your environment. If you wish to use
these methods, you must ensure your environment has these classes or provide a
polyfill.

Basic Usage

Import with

`ts
import { chainFrom } from "transducist";
`

Start a chain by calling chainFrom() on any iterable, such as an array, a
string, or an ES6 Set.

`ts
const result = chainFrom(["a", "bb", "ccc", "dddd", "eeeee"]);
`

Then follow up with any number of transforms.

`ts
.map(s => s.toUpperCase())
.filter(s => s.length % 2 === 1)
.take(2)
`

To finish the chain and get a result out, call a method which terminates the
chain and produces a result.

`ts
.toArray(); // -> ["A", "CCC"]
`

Other terminating methods include .forEach(), .find(), and .toSet(), among
many others. For a particularly interesting one, see
.toMapGroupBy().

For a list of all possible transformations and terminations, see the full API
docs.

$3

Transducist also comes with a handful of iterable helpers for common sequences,
which are often useful as the starter for a chain. For example:

`ts
chainFrom(range(5))
.map(i => i * i)
.toArray(); // -> [0, 1, 4, 9, 16]

chainFrom(repeat("x", 5)).joinToString(""); // -> "xxxxx"
`

All such iterables generate values only when needed, which means they can
represent even infinite sequences:

`ts
chainFrom(range(0, Number.POSITIVE_INFINITY))
.map(n => n * n)
.takeWhile(n => n < 20)
.toArray(); // -> [0, 1, 4, 9, 16]
`

For a full list of iterables, see the Iterables
docs.

Advanced Usage

These advanced usage patterns make use of transducers. If you aren't familiar
with transducers yet, see the links in the
transducers-js
readme for an introduction.

$3

Arbitrary objects that satisfy the transducer
protocol
can be added to the chain using the .compose() method, allowing you to write
new types of transforms that can be included in the middle of the chain without
losing the benefits of early termination and no intermediate array creation.
This includes transducers defined by other libraries, so we could for instance
reuse a transducer from
transducers.js as follows:

`ts
import { chainFrom } from "transducist";
import { cat } from "transducers.js";

const result = chainFrom([[1, 2], [3, 4, 5], [6]])
.drop(1)
.compose(cat)
.map(x => 10 * x)
.toArray(); // -> [30, 40, 50, 60];
`

All of this library's transformation methods are implemented internally with
calls to .compose().

$3

Similarly, arbitrary terminating operations can be introduced using the
.reduce() method, which can accept not only a plain reducer function (that is,
a function of the form (acc, x) => acc) but also any object satisfying the
transformer
protocol.
All of this library's termination methods are implemented internally with a call
to .reduce() (with the single exception of .toIterator()).

$3

It is also possible to use a chaining API to define a transducer without using
it in a computation, so it can be passed around and consumed by other APIs which
understand the transducer protocol, such as
transduce-stream. This is done
by starting the chain by calling transducerBuilder() and calling .build()
when done, for example:

`ts
import { chainFrom, transducerBuilder } from "transducist";

const firstThreeOdds = transducerBuilder()
.filter(n => n % 2 === 1)
.take(3)
.build();
`

Since this returns a transducer, we can also use it ourselves with .compose():

`ts
const result = chainFrom([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
.compose(firstThreeOdds)
.toArray(); // -> [1, 3, 5]
`

This is a good way to factor out a transformation for reuse.

Bundle Size and Tree Shaking

If you are using a bundler which supports tree shaking (e.g. Webpack 4+, Rollup)
and are looking to decrease bundle size, Transducist also provides an alternate
API to allow you to only pay for the functions you actually use, which
incidentally is similar to the API provided by more typical transducer
libraries. All chain methods are also available as standalone functions and can
be used as follows:

`ts
import { compose, filter, map, toArray, transduce } from "transducist";

transduce(
[1, 2, 3, 4, 5],
compose(
filter(x => x > 2),
map(x => 2 * x),
),
toArray(),
); // -> [6, 8, 10]
`

which is equivalent to the fluent version:

`ts
import { chainFrom } from "transducist";

chainFrom([1, 2, 3, 4, 5])
.filter(x => x > 2)
.map(x => 2 * x)
.toArray(); // -> [6, 8, 10]
``

However, the standalone function version of this example adds a mere 1.64 kB to
bundle size (pre-gzip), compared to the chained version which adds 11.1 kB (as
of version 1.0.0). Note that after gzipping, the fluent version is below 4kB as
well.

For details, see the tree shaking
API
section of the API docs.

Benchmarks

View the
benchmarks.

API

View the full API
docs.

Copyright © 2017 David Philipson