State made simple → Effects made easy
Derivables are an Observable-like state container with superpowers. Think MobX distilled to a potent essence, served with two heaped tablespoons of extra performance, a garnish of declarative effects management, and a healthy side-salad of immutability.
- Quick start
- Key differences with MobX
- API / Documentation
- Inspiration <3
There are two types of Derivable:
Atoms are simple mutable references to immutable values. They represent the ground truth from which all else is derived.;const name = ;name; // => 'Richard'name;name; // => 'William'
Derivations are declarative transformations of values held in atoms. You can create them with the
derivefunction.;const cyber =word;const cyberName = ;cyberName; // 'W I L L I A M'name;cyberName; // 'S A R A H'
Unlike atoms, derivations cannot be modified in-place with a
.setmethod. Their values change only when one or more of the values that they depend upon change. Here is an example with two dependencies.const transformer = ;const transformedName = ;transformedName; // => 'S A R A H'const reverse =string;transformer;transformedName; // => 'haraS'name;transformedName; // => 'naibaF'
derivetakes a function of zero arguments which should dereference one or more Derivables to compute the new derived value. DerivableJS then sneakily monitors who is dereferencing who to infer the parent-child relationships.
Declarative state management is nice in and of itself, but the real benefits come from how it enables us to more effectively manage side effects. DerivableJS has a really nice story on this front: changes in atoms or derivations can be monitored by things called Reactors, which do not themselves have any kind of 'current value', but are more like independent agents which exist solely for executing side effects.
Let's have a look at a tiny example app which greets the user:
;// global application stateconst name = ; // the name of the userconst countryCode = ; // for i18n// static constants don't need to be wrappedconst greetings =en: "Hello"de: "Hallo"es: "Hola"cn: "您好"fr: "Bonjour";// derive a greeting message based on the user's name and country.const greeting = ;const message = ;// set up a Reactor to print the message every time it changes, as long as// we know how to greet people in the current country.message;// $> Hello, World!countryCode;// $> Hallo, World!name;// $> Hallo, Dagmar!// we can avoid unwanted intermediate reactions by using transactions;// $> Bonjour, Étienne!// if we set the country code to a country whose greeting we don't know,// `greeting` becomes undefined, so the `message` reactor won't run// In fact, the value of `message` won't even be recomputed.countryCode;// ... crickets chirping
The structure of this example can be depicted as the following DAG:
Key differences with MobX
Smaller API surface area.
There are far fewer kinds of thing in DerivableJS, and therefore fewer things to learn and fewer surprising exceptions and spooky corners. This reduces noise and enhances one's ability to grok the concepts and wield the tools on offer. It also shrinks the set of tools on offer, but maybe that's not a bad thing:
It seems that perfection is attained not when there is nothing more to add, but when there is nothing more to remove.
- Antoie de Saint Exupéry
No transparent dereferencing and assignment.
It is always necessary to call
.geton derivables to find out what's inside, and you always have to call
.seton atoms to change what's inside. This provides a consistent semantic and visual distinction between ordinary values and derivable values.
No observable map and array types.
More subtle control over reactors
DerivableJS has a tidy and flexible declarative system for defining when reactors should start and stop. This is rather nice to use for managing many kinds of side effects.
DerivableJS is finely tuned, and propagates change significantly faster than MobX. [link to benchmark-results.html forthcoming]
API / Documentation
DerivableJS is fairly mature, and has been used enough in production by various people to be considered a solid beta-quality piece of kit.
The fantastic project react-derivable lets you use derivables in your render method, providing seamless interop with component-local state and props.
DerivableJS works spiffingly with Immutable, which is practically required if your app deals with medium-to-large collections.
Due to inversion of control, the stack traces you get when your derivations throw errors can be totally unhelpful. There is a nice way to solve this problem for dev time. See setDebugMode for more info.
I heartily welcome questions, feature requests, bug reports, and general suggestions/criticism on the github issue tracker. I also welcome bugfixes via pull request (please read CONTRIBUTING.md before sumbmitting).
Copyright 2015 David Sheldrick <firstname.lastname@example.org> Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.