kryndex / hareactive

Pure FRP library for the web.

Hareactive

Hareactive is an FRP library for JavaScript and TypeScript. It aims to be completely pure, simple to use, powerful, and performant.

Key features

• Simple and precise semantics. This makes the library easy to use and free from surprises.
• Purely functional.
• Implements classic FRP. This means that the library makes a distinction between behaviors and streams.
• Supports continuous time for expressive and efficient creation of time-dependent behavior.
• Integrates with declarative side-effects in a way that is pure, testable and uses FRP for powerful handling of asynchronous operations.
• Great performance.

Introduction

Hareactive is simple. It aims to have an API that is understandable and easy to use. It does that by making a clear distinction by semantics and implementation details. This means that the library implements a very simple mental model. By understanding this conceptual model the entire API can be understood.

This means that to you use Hareactive you do not have to worry about things such as "lazy observables", "hot vs cold observables" and "unicast vs multicast observables". These are all unfortunate concepts that confuses people and makes reactive libraries harder to use. In Hareactive we consider such things implementation detail that users should never have to think about.

Hareactive implements what is called classic FRP. This means that it makes a distinction between two types of time dependent concepts. This makes code written in Hareactive more precise and easier to understand.

Hareactive is powerful. It features all the typical methods found in other FRP libraries. But on top of that it comes with many unique features that are rarely found elsewhere. For instance, continuous time.

Table of contents

• Installation
• Tutorial
• API documentation
• Contributing
• Benchmark

Installation

Hareactive can be installed from npm. The package ships with both CommonJS modules and ES6 modules

npm install @funkia/hareactive

Tutorial

Hareactive contains four key concepts: Future, stream, behavior and now. These are explained below.

Future

A future is a value associated with a certain point in time. For instance, the result of a HTTP-request is a future since it occurs at a specific time (when the response is received) and contains a value (the response itself).

Future has much in common with JavaScript's Promises. However, they are simpler. A future has no notion of resolution or rejection. That is, a specific future can be understood simply as a time and a value. Conceptually one can think of them as being implemented simply like this.

{time: 22, value: "Foo"}

Stream

A Stream is a list of futures. That is, a list of values where the values are each associated with a point in time.

An example could be a stream of keypresses that a user makes. Each keypress happens at a specific moment in time and with a value indicating which key was pressed.

The relationship between Future and Stream is the same as the relationship between having a variable that is a string and a variable that is a list of strings. You wouldn't store a username as ["username"] because there is always exactly one username.

Similarly in Hareactive we don't use Stream to express the result of a HTTP-request since a HTTP-request only delivers a response exactly once. Use a Future for things where there is exactly one occurrence and Stream where there may be zero or more.

Behavior

A behavior represents a value that changes over time. For instance, the current position of the mouse or the value of an input field is a behavior.

Conceptually a behavior can be thought of as a function from a point in time to a value. A behavior always has a value at any given time. This is the difference between a stream and a behavior. A behavior has a value at all points in time where a stream is a series of events that happens at specific moments in time.

Future, stream or behavior?

At first, the difference between the three things may be tricky to understand. Especially if you're used to other libraries where all three are represented as a single structure (maybe called "stream" or "observable"). The key is to understand that the three types represent things that are fundamentally different. And that expressing different things with different structures is beneficial.

You could forget about future and use a stream where you'd otherwise use a future. Because stream is more powerful than future. In the same way you could always use arrays of values instead of just single values. But you don't do that because username = "foo" expresses that only one username exists whereas username = ["foo"] gives the impression that a user can have more than one username. Similarly one could forget about numbers and just use strings instead. But saying amount = 22 is obviously better than amount = "22".

This is how to figure out if a certain thing is a future, a stream or a behavior:

1. Ask the question: "does the thing always have a current value?". If yes, you're done, the thing should be represented as a behavior.
2. Ask the question: "does the thing always happen once?". If yes, the thing should be represented as a future. If no, you should use a stream.

Below are some examples:

• The time remaining before an alarm goes off: The remaining time always have a current value, therefore it is a behavior.
• The moment where the alarm goes off: This has no current value. And since the alarm only goes off a single time this is a future.
• User clicking on a specific button: This has no notion of a current value. And the user may press the button more than once. This is a stream.
• Whether or not a button is currently pressed: This always has a current value. The button is always pressed or not pressed. This should be represented as a behavior.
• The tenth time a button is pressed: This happens once at a specific moment in time. Use a future.

Now

Now represents a computation that will be run in the present. Hence the name "now". Now is perhaps the most difficult concept in Hareactive.

Inside a Now-computation we can do two things.

• Get the current value of behavior. This is done with the sample function.
• Run side-effects.

We can do both of these in a completely pure way.

How stateful behaviors work

A notorious problem in FRP is how to implement stateful behaviors in a pure way.

Understanding stateful behaviors

FRP has a notorious problem with regards to functions that return behaviors or streams that depends on the past. Such behaviors or streams are sometimes called "stateful". For instance scan creates a behavior that accumulates values over time. Clearly such a behavior depends on the past. Thus we say that scan returns a stateful behavior.

Implementing stateful methods such as scan in a way that is both intuitive to use, pure and memory safe is very tricky.

When implementing functions such as scan most reactive libraries in JavaScript does one of these two things:

• Calling scan doesn't begin accumulating state at all. Only when someone starts observing the result of scan is state accumulated. This is very counter intuitive behavior.
• Calling scan starts accumulating state from when scan is called. This is pretty easy to understand. But it makes scan impure as it will not return the same behavior when called at different time.

To solve this problem Hareactive uses a solution invented by Atze van der Ploeg and presented in his paper "Principled Practical FRP". His brilliant idea gives Hareactive the best of both worlds. Intuitive behavior and purity.

In Hareactive some functions returns a value that, compared to what you might expect, is wrapped in an "extra" behavior.

This "behavior wrapping" is applied to all functions that returns a result that depends on the past. For instance scan creates a behavior that accumulates values over time. Clearly such a behavior depends on the past. When implementing a function such as scan most reactive libraries does one of these two:

API

Future

Future#listen(o: Consumer<A>): void

Adds a consumer as listener to a future. If the future has already occurred the consumer is immediately pushed to.

fromPromise<A>(p: Promise<A>): Future<A>

Converts a promise to a future.

Stream

apply<A, B>(behavior: Behavior<(a: A) => B>, stream: Stream<A>): Stream<B>

Applies a function-valued behavior to a stream. Whenever the stream has an occurrence the value is passed through the current function of the behavior.

filter<A>(predicate: (a: A) => boolean, s: Stream<A>): Stream<A>

Returns a stream with all the occurrences from s for which predicate returns true.

const stream = testStreamFromArray([1, 3, 2, 4, 1]);
const filtered = stream.filter((n) => n > 2);
filtered.semantic() //=> [{ time: 1, value: 3 }, { time: 3, value: 4 }]

split<A>(predicate: (a: A) => boolean, stream: Stream<A>): [Stream<A>, Stream<A>]

Returns a pair of streams. The first contains all occurrences from stream for which predicate returns true and the other the occurrences for which predicate returns false.

const whereTrue = stream.filter(predicate);
const whereFalse = stream.filter((v) => !predicate(v));
// is equivalent to 
const [whereTrue, whereFalse] = split(predicate, stream);

filterApply<A>(predicate: Behavior<(a: A) => boolean>, stream: Stream<A>): Stream<A>

Filters a stream by applying the predicate-valued behavior to all occurrences.

keepWhen<A>(stream: Stream<A>, behavior: Behavior<boolean>): Stream<A>

Whenever stream has an occurrence the current value of behavior is considered. If it is true then the returned stream also has the occurrence—otherwise it doesn't. The behavior works as a filter that decides whether or not values are let through.

scanS<A, B>(fn: (a: A, b: B) => B, startingValue: B, stream: Stream<A>): Behavior<Stream<B>>

A stateful scan.

snapshot<B>(b: Behavior<B>, s: Stream<any>): Stream<B>

Creates a streams that occurs exactly when s occurs. Every time the stream s has an occurrence the current value of b is sampled. The value in the occurrence is then replaced with the sampled value.

const stream = testStreamFromObject({
  1: 0, 4: 0, 8: 0, 12: 0
});
const shot = snapshot(time, stream);
const result = testStreamFromObject({
  1: 1, 4: 4, 8: 8, 12: 12
});
// short == result

snapshotWith<A, B, C>(f: (a: A, b: B) => C, b: Behavior<B>, s: Stream<A>): Stream<C>

Returns a stream that occurs whenever s occurs. At each occurrence the value from s and the value from b is passed to f and the return value is the value of the returned streams occurrence.

switchStream<A>(b: Behavior<Stream<A>>): Stream<A>

Takes a stream valued behavior and returns a stream that emits values from the current stream at the behavior. I.e. the returned stream always "switches" to the current stream at the behavior.

changes<A>(b: Behavior<A>): Stream<A>

Takes a behavior and returns a stream that has an occurrence whenever the behavior changes.

combine<A, B>(a: Stream<A>, b: Stream<B>): Stream<(A|B)>

Combines two streams into a single stream that contains the occurrences of both a and b sorted by the time of their occurrences. If two occurrences happens at the exactly same time then the occurrence from a comes first.

const s1 = testStreamFromObject({ 0: "#1", 2: "#3" });
const s2 = testStreamFromObject({ 1: "#2", 2: "#4", 3: "#5" });
const combined = combine(s1, s2);
assert.deepEqual(
  combined.semantic(),
  [
    { time: 0, value: "#1" }, { time: 1, value: "#2" },
    { time: 2, value: "#3" }, { time: 2, value: "#4" },
    { time: 3, value: "#5" }
  ]
);

isStream(obj: any): boolean

Returns true if obj is a stream and false otherwise.

isStream(empty); //=> true
isStream(12); //=> false

delay<A>(ms: number, s: Stream<A>): Stream<A>

Returns a stream that occurs ms milliseconds after s occurs.

throttle<A>(ms: number, s: Stream<A>): Stream<A>

Returns a stream that after occurring, ignores the next occurrences in ms milliseconds.

Behavior

when(b: Behavior<boolean>): Behavior<Future<{}>>

Takes a boolean valued behavior an returns a behavior that at any point in time contains a future that occurs in the next moment where b is true.

snapshot<A>(b: Behavior<A>, f: Future<any>): Behavior<Future<A>>

Creates a future than on occurence samples the current value of the behavior and occurs with that value. That is, the original value of the future is overwritten with the behavior value at the time when the future occurs.

switchTo<A>(init: Behavior<A>, next: Future<Behavior<A>>): Behavior<A>

Creates a new behavior that acts exactly like initial until next occurs after which it acts like the behavior it contains.

switcher<A>(init: Behavior<A>, s: Stream<Behavior<A>>): Behavior<Behavior<A>>

A behavior of a behavior that switches to the latest behavior from s.

stepper<B>(initial: B, steps: Stream<B>): Behavior<B>

Creates a behavior whose value is the last occurrence in the stream.

scan<A, B>(fn: (a: A, b: B) => B, init: B, source: Stream<A>): Behavior<Behavior<B>>

The returned behavior initially has the initial value, on each occurrence in source the function is applied to the current value of the behaviour and the value of the occurrence, the returned value becomes the next value of the behavior.

fromFunction<B>(fn: () => B): Behavior<B>

This takes an impure function that varies over time and returns a pull-driven behavior. This is particularly useful if the function is contionusly changing, like Date.now.

isBehavior(b: any): b is Behavior<any>

Returns true if b is a behavior and false otherwise.

time: Behavior<Time>

A behavior whose value is the number of milliseconds elapsed win UNIX epoch. I.e. its current value is equal to the value got by calling Date.now.

timeFrom: Behavior<Behavior<Time>>

A behavior giving access to continous time. When sampled the outer behavior gives a behavior with values that contain the difference between the current sample time and the time at which the outer behavior was sampled.

integrate(behavior: Behavior<number>): Behavior<Behavior<number>>

Integrate behavior with respect to time.

Now

The Now monad represents a computation that takes place in a given moment and where the moment will always be now when the computation is run.

async<A>(comp: IO<A>): Now<Future<A>>

Run an asynchronous IO action and return a future in the Now monad that resolves with the eventual result of the IO action once it completes. This function is what allows the Now monad to execute imperative actions in a way that is pure and integrated with FRP.

sample<A>(b: Behavior<A>): Now<A>

Returns the current value of a behavior in the Now monad. This is possible because computations in the Now monad have an associated point in time.

performStream<A>(s: Stream<IO<A>>): Now<Stream<A>>

Takes a stream of IO actions and return a stream in a now computation. When run the now computation executes each IO action and delivers their result into the created stream.

performStreamLatest<A>(s: Stream<IO<A>>): Now<Stream<A>>

A variant of performStream where outdated IO results are ignored.

performStreamOrdered<A>(s: Stream<IO<A>>): Now<Stream<A>>

A variant of performStream where IO results occur in the same order.

plan<A>(future: Future<Now<A>>): Now<Future<A>>

Convert a future now computation into a now computation of a future. This function is what allows a Now-computation to reach beyond the current moment that it is running in.

runNow<A>(now: Now<Future<A>>): Promise<A>

Run the given Now-computation. The returned promise resolves once the future that is the result of running the now computation occurs. This is an impure function and should not be used in normal application code.

Contributing

Contributions are very welcome. Development happens as follows:

Install dependencies.

npm install

Run tests.

npm test

Running the tests will generate an HTML coverage report in ./coverage/.

Continuously run the tests with

npm run test-watch

We also use tslint for ensuring a coherent code-style.

Benchmark

Get set up to running the benchmarks:

npm run build
./benchmark/prepare-benchmarks.sh

Run all benchmarks with:

npm run bench

Run a single benchmark with:

node benchmark/<name-of-benchmark>

For example

node benchmark/scan.suite