FRP Needs Simultaneousness

When we finished publishing the FrTime sequence of papers, I became convinced that a notion of simultaneous events was needed to make FRP work well in practice.

When we implemented exploratory demos, the pattern that occurred frequently was decomposition-recomposition. It is a pattern which is natural when using functions which become oddly difficult in FRP.

I’ll use the most.js frp library to illustrate.

var _ = require('underscore')
var most = require('most')
// Setup
const square = x => x * x
const dbl = x => x + x
const points = [{x: 1, y: 2}, {x: 3, y: 4}, {x:5, y:6}]
const pointStream = most.from(points)

This is natural:

const squaredPoints =, ({x, y}) => ({ x: dbl(x), y: square(y) }))

This is also fine:

const pointSquaredStream ={x, y}) => ({x: dbl(x), y: square(y) }))

But once we start reconstructing, we run into trouble. So let’s deconstruct:

const xDoubleStream = => p.x).map(dbl)
const ySquareStream = => p.y).map(square)

// And then reconstruct:

const makePoint = (x, y) => ({x, y})

Reconstructing using zip, xDoubleStream, ySquareStream)

zip behaves well in the normal case. However, it fails non-gracefully in the presence of bugs.

In the case of mismatched numbers of points, zip buffers the longer stream silently, consuming memory until a long-delay out-of-memory error. This would be very hard to debug.,

Reconstructing with zip can also lead to incorrect results for simple mistakes where a fail-fast error message would have been preferable.,
         xDoubleStream.filter(x => x > 3),
         ySquareStream.filter(y => y > 3))

Here, the points { x: 6, y: 4 } and { x: 10, y: 16 } are produced, which isn’t what was intended since they pair the input points across. But the error is subtle and would be hard to track down, especially if the processing was intricate.

This case is particularly interesting since it teases apart two different notions of simultaneity. Since we defined points by invoking most.from on an array, the events are occurring at the same moment in time. That’s the first notion — time-based simultaneity. However, when the bug occurs the values being ziped are not derived from the same event via deconstruction. That’s the second notion — value-based simultaneity.

I’ll claim that in an FRP context, this second notion of simultaneousness, value-based simultaneity, is the important one.

Now let’s look at the second manner of reconstructing.

Reconstructing using combine,

javascript most.combine(makePoint, xDoubleStream, ySquareStream) This code produces three events { x: 10, y: 4 }, { x: 10, y: 16 }, and { x: 10, y: 36 }. It is only because of the notion of simultaneity that three events are produced. Without simultaneity, we should expect six events. Willingly or not, most’s implementation of combine detects that the event on xDoubleStream and ySquareStream are occurring at the same time and absorbs the glitches.

This behavior violates what is expected from combine when reading the documentation:

combine: create a new stream that emits the set of latest event values from all input streams whenever a new event arrives on any input stream.

In order to fix combine, either the documentation needs to be modified to document this behavior, which would require defining simultaneousness. In particular, it would require choosing and articulating the difference between time-based simultaneity and value-based simultaneity. Alternatively, the implementation of combine has to change to maintain the six-events output even in the presence of simultaneous events.

How to fix this

The big open question is, should the semantic of a FRP framework incorporate an explicit notion of simultaneity, along with combiners to manipulate simultaneity?

When we wrapped up the FrTime effort, if we had worked on it more I would have implemented apply and lift and then tried building some stuff with them. To be fair, Greg Cooper was skeptical. I think he had a different approach in mind.

The idea is that zip and combine are great combiners, they should not be repurposed and instead new operators should be introduced specifically to make the deconstruct-reconstruct pattern easy and reliable in the fail-fast-fail-loud sense.

The combiners could be defined as follow:

apply: same zip, but errors if the events are not simultaneous.

most.apply(makePoint, xDoubleStream, ySquareStream)

lift: consumes a function f on normal non-stream values and returns a function that applies f on the given streams, assuming all events are simultaneous. If any events are not simultaneous, it errors.

const makePointStream = most.lift(makePoint)
makePointStream(xDoubleStream, ySquareStream)

But the tricky part would be in the implementation, in particular, the handling and maintaining of the same-time versus same-value distinction.

(originally posted on Medium)