Flyd

The modular, KISS, functional reactive programming library for JavaScript.


Table of contents

* Introduction
* Features
* Examples
* Tutorial
* API
* Modules
* Misc

Introduction

Functional reactive programming is a powerful programming paradigm for
expressing values that change over time. But existing libraries for JavaScript
are huge, complex, have a high learning curve and aren't functional enough.

Flyd is simple and expressive. It has a minimal but powerful core on top of
which new abstractions can be built modularly.

Features

__Main features__

* __Simple but powerful__. Less is more! Flyd provides combinable observable
streams as the basic building block. This minimal core is less than 200 SLOC
which makes the library transparent – end users can realistically get a full
understanding of how the library works.
* __More functional in style__. Flyd is more functional than existing FRP
libraries. Instead of methods it gives you curried functions with arguments
in the order suitable for partial application. This gives more expressive
power and modularity.
* __Modularity__. The core of the Flyd is powerful and documented. This makes
it easy for users of the library to create new FRP abstractions if existing ones
do not exist. This in turn makes it viable to capture more patterns than
otherwise because they can exist as separate modules. List of existing modules.

__Other features__

* Supports the transducer protocol. You can for instance transduce streams with
Ramda and transducers.js.
* Conforms to the fantasy land monad specification
* Atomic updates.

Examples

* Sum - A very simple example
* Multiple clicks - A remake
of the multiple clicks example from "The introduction to Reactive
Programming you've been missing". Compare it to this RxJs
implementation.
* Secret combination
* Ramda transducer
* Who to follow

For other examples check the source code of the modules.

Tutorial

This is not general introduction to functional reactive programming. For that take
a look at The introduction to Reactive Programming you've been
missing and/or this Elm
tutorial if you are comfortable
with reading Haskell-like code.

This is not a demonstration of how you would write code with Flyd on a day to
day basis. For that take a look at the examples.

This tutorial will however introduce you to the minimal but powerful core that
Flyd provides and show you how it can be used to build FRP abstractions.

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Flyd gives you streams as the building block for creating reactive dataflows.
They serve the same purpose as what other FRP libraries call Signals, Observables,
Properties and EventEmitters.

The function flyd.stream creates a representation of a value that changes
over time. The resulting stream is a function. At first sight it works a bit
like a getter-setter:

``javascript
// Create a stream with initial value 5.
var number = flyd.stream(5);
// Get the current value of the stream.
console.log(number()); // logs 5
// Update the value of the stream.
console.log(number(7));
// The stream now returns the new value.
console.log(number()); // logs 7
`

Top level streams, that is streams without dependencies, should typically
depend on the external world, like user input or fetched data.

Since streams are just functions you can easily plug them in whenever a
function is expected.

`javascript
var clicks = flyd.stream();
document.getElementById('button').addEventListener('click', clicks);
var messages = flyd.stream();
webSocket.onmessage = messages;
`

Clicks events will now flow down the clicks stream and WebSockets messages
down the messages stream.

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Streams can depend on other streams. Use var combined = flyd.combine(combineFn, [a, b, c, ...]).
The combineFn function will be called as (a, b, c, ..., self, changed) => v,
where a, b, c, ... is a spread of each dependency, self is a reference to the
combine stream itself, and changed is an array of streams that were atomically
updated.

Flyd automatically updates the stream whenever a dependency changes. This
means that the sum function below will be called whenever x and y
changes. You can think of dependent stream as streams that automatically
listens to or subscribes to their dependencies.

`javascript
// Create two streams of numbers
var x = flyd.stream(4);
var y = flyd.stream(6);
// Create a stream that depends on the two previous streams
// and with its value given by the two added together.
var sum = flyd.combine(function(x, y) {
return x() + y();
}, [x, y]);
// sum is automatically recalculated whenever the streams it depends on changes.
x(12);
console.log(sum()); // logs 18
y(8);
console.log(sum()); // logs 20
`

Naturally, a stream with dependencies can depend on other streams with dependencies.

`javascript
// Create two streams of numbers
var x = flyd.stream(4);
var y = flyd.stream(6);
var squareX = flyd.combine(function(x) {
return x() * x();
}, [x]);
var squareXPlusY = flyd.combine(function(y, squareX) {
return y() + squareX();
}, [y, squareX]);
console.log(squareXPlusY()); // logs 22
x(2);
console.log(squareXPlusY()); // logs 10
`

The body of a dependent stream is called with the spread of: each dependency, itself, and a list
of the dependencies that have changed since its last invocation (due to atomic
updates several streams could have changed).

`javascript
// Create two streams of numbers
var x = flyd.stream(1);
var y = flyd.stream(2);
var sum = flyd.combine(function(x, y, self, changed) {
// The stream can read from itself
console.log('Last sum was ' + self());
// On the initial call no streams has changed and changed will be []
changed.map(function(s) {
var changedName = (s === y ? 'y' : 'x');
console.log(changedName + ' changed to ' + s());
});
return x() + y();
}, [x, y]);
`

Note Returning undefined in the combineFn will not trigger an update
to the stream. To trigger on undefined, update directly:
`
flyd.combine((_, self, changed) => { self(undefined); }, [depStream]);
`

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Instead of returning a value a stream can update itself by calling itself. This
is handy when working with APIs that takes callbacks.

`javascript
var urls = flyd.stream('/something.json');
var responses = flyd.combine(function(urls, self) {
makeRequest(urls(), self);
}, [urls]);
flyd.combine(function(responses) {
console.log('Received response!');
console.log(responses());
}, [responses]);
`

Note that the stream that logs the responses from the server should only be called
after an actual response has been received (otherwise responses() would return
undefined). Fortunately a stream's body will not be called before all of its declared
streams has received a value (this behaviour can be circumvented with
flyd.immediate).

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Flyd has two helpers for dealing with promises: flyd.fromPromise and flyd.flattenPromise.

Let's say you're building a filtered list. It is important to you that the latest filter always corresponds
to the latest promise and its resolution. using flyd.fromPromise guarantees the ordering, and can skip intermediate results.

`js
const filter = flyd.stream('');
const results = filter
.pipe(flyd.chain(
filter => flyd.fromPromise(requestPromise(https://example.com?q=${filter}))
));
`

On the other hand let's say you want to sum some numbers from a service you've written.
Every time someone clicks on your site you want to send a request and get back a random number to be tallied.

flyd.flattenPromise gives you the guarantee that every promise resolution will be handled, regardless of order.

`js
const clicks = flyd.stream();
const total = clicks
.map(getNumberAsync)
.pipe(flyd.flattenPromise)
.pipe(flyd.scan((acc, v)=> acc + v, 0));
`

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You've now seen most of the basic building block which Flyd provides. Let's see
what we can do with them. Let's write a function that takes a function and a
stream and returns a new stream with the function applied to every value
emitted by the stream. In short, a map function.

`javascript
var mapStream = function(f, s) {
return flyd.combine(function(s) {
return f(s());
}, [s]);
};
`

We simply create a new stream dependent on the first stream. We declare
the stream as a dependency so that our stream won't return values before
the original stream produces its first value.

Flyd includes a similar map function as part of its core.

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Lets try something else: a scan function for accumulating a stream! It could
look like this:

`javascript
var scanStream = function(f, acc, s) {
return flyd.combine(function(s) {
acc = f(acc, s());
return acc;
}, [s]);
};
`

Our scan function takes an accumulator function, an initial value and a stream.
Every time the original stream emits a value we pass it to the accumulator
function along with the accumulated value.

Flyd includes a scan function as part of its core.

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When you create a stream with flyd.stream it will have an end property
which is also a stream. That is an _end stream_:

`javascript
var s = flyd.stream();
console.log(flyd.isStream(s.end)); // logs true
`

You can end a stream by pushing true into its end stream:

`javascript
var s = flyd.stream();
s.end(true); // this ends s
`

When you create a dependent stream its end stream will initially depend on all
the end streams of its dependencies:

`javascript
var n1 = flyd.stream();
var n2 = flyd.stream();
var sum = flyd.combine(function(n1, n2) {
return n1() + n2();
}, [n1, n2]);
`

sum.end now depends on n1.end and n2.end. This means that whenever one of
the sums dependencies end sum will end as well.

You can change what a stream's end stream depends on with flyd.endsOn:

`javascript
var number = flyd.stream(2);
var killer = flyd.stream();
var square = flyd.endsOn(flyd.merge(number.end, killer), flyd.combine(function(number) {
return number() * number();
}, [number]));
`

Now square will end if either number ends or if killer emits a value.

The fact that a stream's ending is itself a stream is a very powerful concept.
It means that we can use the full expressiveness of Flyd to control when a stream
ends. For an example, take a look at the implementation of
takeUntil.

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You're done! To learn more check out the API, the examples
and the source of the modules.

API

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Creates a new top level stream.

__Signature__

a -> Stream a

__Example__
`javascript
var n = flyd.stream(1); // Stream with initial value 1
var s = flyd.stream(); // Stream with no initial value
`

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Creates a new dependent stream.

__Signature__

(...Stream -> Stream b -> b) -> [Stream ] -> Stream b

__Example__
`javascript
var n1 = flyd.stream(0);
var n2 = flyd.stream(0);
var max = flyd.combine(function(n1, n2, self, changed) {
return n1() > n2() ? n1() : n2();
}, [n1, n2]);
`

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Returns true if the supplied argument is a Flyd stream and false otherwise.

__Signature__

* -> Boolean

__Example__

`javascript
var s = flyd.stream(1);
var n = 1;
flyd.isStream(s); //=> true
flyd.isStream(n); //=> false
`

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By default the body of a dependent stream is only called when all the streams
upon which it depends has a value. immediate can circumvent this behaviour.
It immediately invokes the body of a dependent stream.

__Signature__

Stream a -> Stream a

__Example__

`javascript
var s = flyd.stream();
var hasItems = flyd.immediate(flyd.combine(function(s) {
return s() !== undefined && s().length > 0;
}, [s]);
console.log(hasItems()); // logs false. Had immediate not been
// used hasItems() would've returned undefined
s([1]);
console.log(hasItems()); // logs true.
s([]);
console.log(hasItems()); // logs false.
`

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Changes which endsStream should trigger the ending of s.

__Signature__

Stream a -> Stream b -> Stream b

__Example__

`javascript
var n = flyd.stream(1);
var killer = flyd.stream();
// double ends when n ends or when killer emits any value
var double = flyd.endsOn(flyd.merge(n.end, killer), flyd.combine(function(n) {
return 2 * n();
}, [n]);
`

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Returns a new stream consisting of every value from s passed through fn. I.e. map creates
a new stream that listens to s and applies fn to every new value.

__Signature__

(a -> result) -> Stream a -> Stream result

__Example__
`javascript
var numbers = flyd.stream(0);
var squaredNumbers = flyd.map(function(n) { return n*n; }, numbers);
`


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fn must return a stream.

fn is run every time a value is pushed into s.
Returns a single stream of merged values from the created streams.

Ends when every created stream and the main stream ends

__Signature__

(a -> Stream b) -> Stream a -> Stream b

__Example__
`javascript
var filter = flyd.stream('filter');
var search_results = flyd.chain(function(filter){
return flyd.stream(getResults(filter));
}, filter);
`

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Applies the value in valueStream to the function in functionStream

__Signature__
Stream a -> Stream (a -> b) -> Stream b

__Example__
`javascript
function add3(x) { return x + 3; }
flyd.ap(flyd.stream(5), flyd.stream(add3)) // stream(8);
`

while it can not seem useful immediately consider this example

`javascript
var get_results = function (filter, sortProperty, sortDirection) {
return flyd.stream(fetch(${base_url}/search?q=${filter}&sort=${sortProperty} ${sortDirection}))
};

// this would eventually be linked to an input field
var filter = flyd.stream('');
var sortProperty = flyd.stream('name');
var sortDirection = flyd.stream('descending');

var results = flyd.stream(flyd.curryN(3, get_results))
.pipe(flyd.ap(filter))
.pipe(flyd.ap(sortProperty))
.pipe(flyd.ap(sortDirection))
.pipe(flyd.map(function(d){ return d; }));
`

In the above example you have a stream of results that triggers a call for get_results
every time filter, sortProperty, or sortDirection is changed.

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Similar to map except that the returned stream is empty. Use on for doing
side effects in reaction to stream changes. Use the returned stream only if you
need to manually end it.

__Signature__

(a -> result) -> Stream a -> Stream undefined

__Example__
`javascript
var numbers = flyd.stream(0);
flyd.on(function(n) { console.log('numbers changed to', n); }, numbers);
`

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Creates a new stream with the results of calling the function on every incoming
stream with an accumulator and the incoming value.

__Signature__

((a, b) -> a) -> a -> Stream b -> Stream a

__Example__
`javascript
var numbers = flyd.stream();
var sum = flyd.scan(function(sum, n) { return sum+n; }, 0, numbers);
numbers(2)(3)(5);
sum(); // 10
`

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Creates a new stream down which all values from both stream1 and stream2
will be sent.

__Signature__

Stream a -> Stream a -> Stream a

__Example__
`javascript
var btn1Clicks = flyd.stream();
button1Elm.addEventListener(btn1Clicks);
var btn2Clicks = flyd.stream();
button2Elm.addEventListener(btn2Clicks);
var allClicks = flyd.merge(btn1Clicks, btn2Clicks);
`

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Creates a new stream resulting from applying transducer to stream.

__Signature__

Transducer -> Stream a -> Stream b

__Example__

`javascript
var t = require('transducers.js');

var results = [];
var s1 = flyd.stream();
var tx = t.compose(
t.map(function(x) { return x * 2; }),
t.dedupe()
);
var s2 = flyd.transduce(tx, s1);
flyd.combine(function(s2) { results.push(s2()); }, [s2]);
s1(1)(1)(2)(3)(3)(3)(4);
results; // [2, 4, 6, 8]
`

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Returns fn curried to n. Use this function to curry functions exposed by
modules for Flyd.

__Signature__

Integer -> ( -> a) -> ( -> a)

__Example__

`javascript
function add(x, y) { return x + y; };
flyd.curryN(2, add);
var add
`

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Returns the last value of the stream.

__Signature__

a

__Example__

`javascript
var names = flyd.stream('Turing');
names(); // 'Turing'
`

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Pushes a value down the stream.

__Signature__

a -> Stream a

__Example__

`javascript
names('Bohr');
names(); // 'Bohr'
`

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A stream that emits true when the stream ends. If true is pushed down the
stream the parent stream ends.

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Returns the result of applying function fn to the stream.

__Signature__
Called bound to Stream a: (Stream a -> Stream b) -> Stream b

__Example__
`javascript
// map a stream
var numbers = flyd.stream(0);
var squaredNumbers = numbers
.pipe(flyd.map(function(n) { return n*n; }));

// Chain a stream
var filter = flyd.stream('filter');
var search_results = filter
.pipe(flyd.chain(function(filter){
return flyd.stream(getResults(filter));
}));

// use with a flyd module
var filter = require('flyd/module/filter');
var numbers = flyd.stream(0);
var isEven = function(x){ return x % 2 === 0; };
var evenNumbers = numbers
.pipe(filter(isEven));
`

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Returns a new stream identical to the original except every
value will be passed through f.

_Note:_ This function is included in order to support the fantasy land
specification.

__Signature__

Called bound to Stream a: (a -> b) -> Stream b

__Example__

`javascript
var numbers = flyd.stream(0);
var squaredNumbers = numbers.map(function(n) { return n*n; });
`

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stream1 must be a stream of functions.

Returns a new stream which is the result of applying the
functions from stream1 to the values in stream2.

_Note:_ This function is included in order to support the fantasy land
specification.

__Signature__

Called bound to Stream (a -> b): a -> Stream b

__Example__

`javascript
var add = flyd.curryN(2, function(x, y) { return x + y; });
var numbers1 = flyd.stream();
var numbers2 = flyd.stream();
var addToNumbers1 = flyd.map(add, numbers1);
var added = addToNumbers1.ap(numbers2);
`

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Returns a new stream with value as its initial value. It is identical to
calling flyd.stream with one argument.

__Signature__

Called bound to Stream (a): b -> Stream b

__Example__
`javascript
var n = flyd.stream(1);
var m = n.of(1);
`

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If you've created a module for Flyd, open an issue or send a pull request, and it
will be added to this list.

Modules listed with names in the format flyd/module/filter are builtin to the main flyd module and can be required with require('flyd/module/filter'). Other modules must be installed first with npm.

| Module | Description |
| --- | --- |
| flyd/module/filter | Filter values from stream based on predicate. |
| flyd/module/lift | Maps a function taking _n_ parameters over _n_ streams. |
| flyd/module/switchlatest | Flattens a stream of streams. The result stream reflects changes from the last stream only. |
| flyd/module/keepwhen | Keep values from one stream only when another stream is true. |
| flyd/module/obj | Functions for working with stream in objects. |
| flyd/module/sampleon | Samples from a stream every time an event occurs on another stream. |
| flyd/module/scanmerge | Merge and scan several streams into one. |
| flyd/module/mergeall | Merge a list of streams. |
| flyd/module/takeuntil | Emit values from a stream until a second stream emits a value. |
| flyd/module/forwardto | Create a new stream that passes all values through a function and forwards them to a target stream. |
| flyd/module/droprepeats | Drop repeated values from a stream. |
| flyd-cacheUntil | Cache a stream's output until triggered by another stream. |
| flyd-keyboard | Keyboard events as streams. |
| flyd-glob | File glob and watch for Flyd. |
| flyd-skip | Skip function for flyd. |
| flyd-until | only accept n event values - mirror function to flyd-skip. |
| flyd-bufferCount | Buffers the source stream and emits all values together. |
| flyd-mergeAll (with high order streams) | rxjs-like implementation of mergeAll for flyd. |
| flyd-once | Only emits the first value of the source stream. |
| flyd-withLatestFrom | When the source observable emits, the value also contains the latest value from withLatestFrom parameter stream. |
| flyd-zip | Zip streams together into arrays of values |
| flyd-undo | An undo/redo utility for saving and restoring state in flyd |
| flyd-ajax | An ajax utility that returns flyd streams |
| flyd-xstate | Integration of flyd with xstate Harel Statecharts |
| flyd-windowresize | Get a stream for the window size |
| flyd-stream-querystring | Manage the URL query params using flyd streams |
| Time related |
| flyd/module/every | Takes a number of milliseconds t and creates a stream of the current time updated every t. |
| flyd/module/aftersilence | Buffers values from a source stream in an array and emits it after a specified duration of silence from the source stream. |
| flyd/module/inlast | Creates a stream that emits a list of all values from the source stream that were emitted in a specified duration. |
| flyd-onAnimationFrame | Emits values from a source stream on successive animation frames. |
| flyd-timeInterval | Records the time interval between consecutive values emitted from a stream. |
| flyd-debounceTime | Like aftersilence, but only emits the latest value of the stream. |
| flyd-group-within | buffers values within x millisecond. |

Misc

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The name Flyd was chosen since the author of Flyd is danish and Flyd is a
danish word meaning float, afloat or flow. It is furthermore short and not too
bad for searching.

For most native English speakers "flyd" is impossible to pronounce like a
dane would do it. The "d" is soft like "th" in "the". The "y" is a vocal sound
unknown to the English language. If you're curious Google Translates listening feature
provides an accurate pronounciation..

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Consider the following example:

`javascript
var a = flyd.stream(1);
var b = flyd.combine(function(a) { return a() * 2; }, [a]);
var c = flyd.combine(function(a) { return a() + 4; }, [a]);
var d = flyd.combine(function(b, c, self, ch) {
result.push(b() + c());
}, [b, c]);
`

The dependency graph looks like this.

`
a
/ \
b c
\ /
d
`

Now, when a value flows down a, both b and c will change because they
depend on a. If you merely consider streams as being event emitters you'd expect d
to be updated twice. Because a triggers b triggers d after which a also
triggers c which _again_ triggers d.

But Flyd handles such cases optimally. Since only one value entered the
system d will only be updated once with the changed values of b and c.

Flyd guarantees that when a single value enters the system every stream will
only be updated once, along with their dependencies in their most recent state.

This avoids superfluous updates of your streams and intermediate states when
several streams change at the same time.

Flyd implements atomic updates with a _O(n)_ topological sort where _n_
is number of streams that directly or indirectly depends on the updated
stream.

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Flyd works in all ECMAScript 5 environments. It works in older environments
with polyfills for Array.prototype.filter and Array.prototype.map.

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To run the test, clone this repository and:

`bash
npm install
npm test
`

The npm test command run three tests: a eslint js style checker test, the test of the core library and the test of the modules. If you want to run only the test of the library npm run test.

The API.md file is generated using npm run docs (it assumes it has documentation installed globally: npm i -g documentation`)