cognitive

A signals library for functional reactive programming. Author: Tim Farland

Inspired by Elm and Bacon.js. Written without the use of this, new, or prototype - only simple objects and functions. Miniscule size - ~1kb minified/gzipped. For use as an ES6 module.

Install

npm install --save tote

Test

Requires global webpack installation.

npm test

Api

Signal type

Signal A :: {
	listeners: [(A -> _)],
	active: boolean,
	value: A || null,
	error: error || null
}

Creating signals

Capture events on a dom node.

// DomNode -> String -> Signal DomEvent
const clicks = fromDomEvent(document.body, "click", evt => console.log(evt.target))

A signal that will emit one value, then terminate.

// (A -> _) -> Signal A
const later = fromCallback(callback => setTimeout(() => callback("Finished"), 1000))

A signal that will emit one value or an error from a Promise, then terminate.

// Promise A -> Signal A
const wait = fromPromise(new Promise(resolve => setTimeout(() => resolve("Finished"), 1000)))

A signal that emits an integer count of millisecond intervals since it was started.

// Int -> Signal Int
const seconds = fromInterval(1000)

Low-level signal creation.

// Signal A
const rawSignal = create()

Interacting with signals

Subscribe to values emitted by the signal.

// Signal A -> (A -> _) -> Signal A
listen(clicks, e => console.log(e))

Send a value to a signal.

// Signal A -> A -> Signal A
send(rawSignal, "value")

Stop a signal - no more values will be emitted.

// Signal A -> Signal A
stop(rawSignal)

Transforming signals

Map values of a signal

// (... _ -> B) -> ... Signal _ -> Signal B
const values = map(evt => evt.target.value, fromDomEvent(input, "keydown"))

Map (zip) the latest value of multiple signals

// (... _ -> B) -> ... Signal _ -> Signal B
const areas = map((x, y) => x * y, widthSignal, heightSignal)

Filter a signal, will only emit event that pass the test

// (A -> Bool) -> Signal A -> Signal A 
const evens = filter(n => n % 2 === 0, numberSignal)

Only emit if the current value is different to the previous (as compared by ===). Not a full deduplication.

// Signal A -> Signal A
dropRepeats(numbers)

Fold a signal over an initial seed value.

// (A -> B -> B) -> B -> Signal A -> Signal B
const sum = fold((a, b) => a + b, 0, numbersStream)

Merge many signals into one that emits values from all.

// ... Signal _ -> Signal _
const events = merge(clicks, keypresses)

Take the last value of a stream when another stream emits.

// Signal A -> Signal B -> Signal A
const mousePositionsBySeconds = sampleOn(mousePosition, fromInterval(1000))

Emit an array of the last n values of a signal.

// Int -> Signal A -> Signal [A]
const trail = slidingWindow(5, mousePosition)

Map values of a signal to a new signal, then flatten the results of all emitted into one signal.

// (A -> Signal B) -> Signal A -> Signal B
const responses = flatMap(evt => fromPromise(ajaxGet("/" + evt.target.value)), keyPresses)

The same as above, but only emits values from the latest child signal.

// (A -> Signal B) -> Signal A -> Signal B
flatMapLatest(v => fromPromise(promiseCreator(v)), valueSignal)