Behavior

EXPERIMENTAL Don't use this for anything real ... yet.

Continuous time-varying values for most.js. Behaviors are the continuous complement to most.js discrete Event Streams, or, if you prefer, the "pull" complement to most.js "push" Event Streams.

Try it

Feedback welcome via gitter, but seriously, don't use it for anything real yet.

npm i --save @briancavalier/most-behavior

Behavior

A Behavior is a continuous value. In contrast to an Event Stream which has discrete occurrences at particular instants in time, a Behavior's value is defined at all real number (not integer) values of time and may vary continuously (or not) over time.

Because they are defined for all real number values of time, a Behavior must be sampled to obtain its value at specific instants in time. To sample a Behavior, pass it an Event Stream whose occurrences define all the points in time at which the behavior's value should be sampled.

Here's a simple example. Note that because clock is "pull", it does no work at the instants between clicks, where it is not being sampled.

import { time } from '@briancavalier/most-behavior'
import { click } from '@most/dom-event'

// A Behavior that always represents milliseconds since the application started
const clock = time

// Sample the clock each time the user clicks
const timeAtEachClick = sample(clock, click(document))

For now, see the examples dir for more realistic code, how to run a @most/core app that integrates behaviors, etc.

API

Creating Behaviors

time :: Behavior number

A behavior that represents the current time in milliseconds since the application started.

always :: a → Behavior a

Create a Behavior whose value is always a.

step :: a → Stream a → Behavior a

Create a Behavior that starts with an initial value and updates to each new value in the Event Stream.

Transforming Behaviors

map :: (a → b) → Behavior a → Behavior b

Apply a function to a Behavior at all points in time.

apply :: Behavior (a → b) → Behavior a → Behavior b

Apply a (time-varying) function to a Behavior at all points in time.

liftA2 :: (a → b → c) → Behavior a → Behavior b → Behavior c

Apply a function to 2 Behaviors at all points in time.

Sampling Behaviors

sample :: Behavior a → Stream b → Stream a

Sample a Behavior's value at every occurrence of an Event Stream.

snapshot :: Behavior a → Stream b → Stream [a, b]

Sample a Behavior at every occurrence of an event, and compute a new event from the (event, sample) pair.

Potential APIs

Potentially useful APIs we could add:

when :: Behavior bool → Stream a → Stream a

Allow events only when a Behavior's value is true.

accum :: a → Stream (a → a) → Behavior a

Create a Behavior with an initial value and an Event Stream carrying update functions.

scanB :: (a → b → a) → a → Stream b → Behavior a

Like scan, but produces a Behavior. Needs a helpful name ...

scanB :: (a → b → Behavior a) → Behavior a → Stream b → Behavior b

Generalized scan for Behaviors. When event occurs, sample Behavior, and apply a function that creates a new Behavior. Somewhat like switch. Needs a helpful name ...

count :: Stream a → Behavior number

Create a Behavior representing the number of event occurrences.

switch :: Behavior a → Stream (Behavior a) → Behavior a

Create a Behavior that acts like an initial Behavior and switches to act like each new Behavior that occurs in the Event Stream.