Table of Contents
- Basic Usage
- Advanced Usage
- Bundle Size and Tree Shaking
This library will let you write code that looks like this:
// 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..mapproject.owner.filterowner.name === "Brad".flatMapowner.children.take100.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.
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
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.
yarn add transducist
npm install transducist
This library, with the exception of the functions which relate to
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:
"es2015.iterable"or something that includes it
- Add the definitions by some other means, such as importing types for
Furthermore, the methods
toMapGroupBy assume the
presence of ES6
Map classes in your environment. If you wish to use
these methods, you must ensure your environment has these classes or provide a
Start a chain by calling
chainFrom() on any iterable, such as an array, a
string, or an ES6
Then follow up with any number of transforms.
.maps.toUpperCase.filters.length % 2 === 1.take2
To finish the chain and get a result out, call a method which terminates the chain and produces a result.
.toArray; // -> ["A", "CCC"]
Other terminating methods include
many others. For a particularly interesting one, see
For a list of all possible transformations and terminations, see the full API docs.
Transducist also comes with a handful of iterable helpers for common sequences, which are often useful as the starter for a chain. For example:
chainFromrange5.mapi * i.toArray; // -> [0, 1, 4, 9, 16]chainFromrepeat"x", 5.joinToString""; // -> "xxxxx"
All such iterables generate values only when needed, which means they can represent even infinite sequences:
chainFromrange0, Number.POSITIVE_INFINITY.mapn * n.takeWhilen < 20.toArray; // -> [0, 1, 4, 9, 16]
For a full list of iterables, see the Iterables docs.
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.
Using custom transducers
Arbitrary objects that satisfy the transducer
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:
;;.drop1.composecat.map10 * x.toArray; // -> [30, 40, 50, 60];
All of this library's transformation methods are implemented internally with
Using custom reductions
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
All of this library's termination methods are implemented internally with a call
.reduce() (with the single exception of
Creating a standalone transducer
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
when done, for example:
;.filtern % 2 === 1.take3.build;
Since this returns a transducer, we can also use it ourselves with
.composefirstThreeOdds.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:
;transduce,composefilterx > 2,map2 * x,,toArray,; // -> [6, 8, 10]
which is equivalent to the fluent version:
;chainFrom.filterx > 2.map2 * 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.
Copyright © 2017 David Philipson