Understanding callbags by splitting them into types with individual TypeScript definitions

Explaining callbags via TypeScript definitions

By: David

Premise

I've already written an introduction to callbags, but I wasn't completely happy - it still feels like the initial threshold to grokk what's going on is rather high. Callbags are a rather simple concept, but communicating the nuances is surprisingly difficult.

In this post I make a second attempt, this time via splitting up the lifecycle and definining precise TypeScript interfaces for the various parts. You are assumed to have read the introductory post and/or be familiar with the callbags spec.

The official callbag typing

The spec defines a callbag as...

a function of signature (TypeScript syntax:)
(type: 0 | 1 | 2, payload?: any) => void

This is further reflected in the official types.d.ts file which looks like this (slightly abbreviated):

export type START = 0;
export type DATA = 1;
export type END = 2;

export type Callbag = (type: START | DATA | END, payload?: any) => void;
export type Factory = (...args: Array<any>) => Callbag;
export type Operator = (...args: Array<any>) => (source: Callbag) => Callbag;

The trouble is that START, DATA and END are not always valid as the first parameter to a callbag. Exactly what we can send depends on where we are in the life cycle.

Or, phrased differently: it depends on from where we got the callbag we're calling. This can be surmised from the written text of the spec, but it isn't reflected in the typings.

The holy trinity of callbags

Things become more clear when you realise that there are actually three different kinds of callbags:

  • Source: The initial callbag function that a sink can subscribe to
  • Sink talkback The callbag sent to the source by the sink when subscribing
  • Source talkback The callbag sent in response to the sink talkback by the source talkback

These three kinds of callbags expect to be called in different ways. In other words, they have different signatures! Here's a lowdown in table form on how they can be called:

call source source talkback sink talkback
(0, callbag) yes no yes
(1) no yes1 no
(1, data) no no yes
(2) no yes yes
(2, error) no yes2 yes

1 If the source is pullable
2 A source likely doesn't care why the sink terminates, so you could argue for a "no" here

And, applying the notion of "three different kinds of callbags" consistently, we should split the first row in the table...

call source source talkback sink talkback
(0, callbag) yes no yes

...to this:

call source source talkback sink talkback
(0, sinkTalkback) yes no no
(0, sourceTalkback) no no yes

Let's take our three different types one by one and see if our observations match some real-world code. And, if they do, create individual type definitions for them!

Sources

According to our table, a source callbag only ever expects to be called with (0, sinkTalkback). This is reflected in the code for many source implementations in that the first parameter is called start, not t.

For example, look at Staltz' callbag-interval source:

const interval = period => (start, sink) => {
  if (start !== 0) return;
  let i = 0;
  const id = setInterval(() => {
    sink(1, i++);
  }, period);
  sink(0, t => {
    if (t === 2) clearInterval(id);
  });
};

The signature in the code is called (start, sink), and if start doesn't equal 0 we bail because nothing else makes sense at this point. This function will never receive data requests (1) or terminations (2) - those will be sent to a source talkback.

In other words, a TypeScript definition for a Source could be:

type Source = (start: START, sinkTalkback: SinkTalkback) => void;

Source talkbacks

In our table we noted (1) and (2) (and maybe (2, error)) as valid calls to the source talkback. Peeking back at the code for interval, the source talkback is implemented as:

t => {
  if (t === 2) clearInterval(id);
}

It only ever handles terminations (2). This makes sense since interval isn't pullable, so it doesn't care about data requests (1).

To take a pullable example, here's Staltz' callbag-from-pull-stream:

const fromPullStream = read => (start, sink) => {
  if (start !== 0) return;
  sink(0, (t, d) => { // <--- source talkback
    if (t === 1) read(null, (end, data) => {
      if (end === true) sink(2);
      else if (end) sink(2, end);
      else sink(1, data);
    });
    if (t === 2) read(d || true, () => {});
  });
};

Here we can see the source talkback definition dealing with both (1) and (2).

Let's now tie up our observations about the source talkback into a TypeScript definition:

type SourceTalkback = (type: DATA | END, payload: any) => void;

If we accept the notion that a source is never interested in why a sink terminates, we could skip the second parameter since it is only used for (2, error):

type SourceTalkback = (type: DATA | END) => void;

We could also potentially split this into separate interfaces for pullable and listenable sources:

type PullableSourceTalkback = (type: DATA | END) => void;
type ListenableSourceTalkback = (type: END) => void;

And then define a SourceTalkback as being one or the other:

type SourceTalkback = PullableSourceTalkback | ListenableSourceTalkback;

Sink talkbacks

Now for the third and final type - sink talkbacks! In our table we had them listed as accepting all three types START, DATA and END. This makes sense since they...

  • must be greeted back by the source with a source talkback, which means START
  • will receive data from the source, which means DATA
  • can be terminated by the source, which means END

Here is Staltz' callbag-for-each:

const forEach = operation => source => {
  let talkback;
  source(0, (t, d) => { // <--- sink talkback
    if (t === 0) talkback = d;
    if (t === 1) operation(d);
    if (t === 1 || t === 0) talkback(1);
  });
};

It doesn't handle (2), but it doesn't need to - if there's no more data coming in, nothing more will happen. There's nothing going on downstream from forEach and nothing to clean up, so we don't need to act.

To see termination handling we must look at something that needs to clean up. Or something that isn't at the end of the chain - in other words, a sink used by an operator to make a new source! As an example, here is my callbag-latest which turns a listenable source into a pullable one:

const latest = listenable => (start, sink) =>  {
  if (start !== 0) return;
  let ltalkback;
  let latestValue;
  let hasLatestValue = false;
  listenable(0, (lt, ld) => { // <--- sink talkback
    if (lt === 0) {
      ltalkback = ld;
      sink(0, (st, sd) => {
        if (st === 1 && hasLatestValue) sink(1, latestValue);
        if (st === 2) ltalkback(2, sd);
      });
    }
    if (lt === 1) {
      latestValue = ld;
      hasLatestValue = true;
    }
    if (lt === 2) sink(2, ld);
  });
};

Here we can clearly see the sink talkback handling all three cases including terminations (2) - although, as is often the case, the only thing it does with the termination is to pass it down the chain.

Since sink talkbacks actually handle all three cases, they are the first kind of callbag that matches the official callbag typing!

type SinkTalkback = (type: START | DATA | END, payload?: any) => void;

We can clarify the second parameter by splitting this into three different types and joining them via an intersection type using &:

type SinkTalkback = 
  & ((start: START, sourceTalkback: SourceTalkback) => void)
  & ((deliver: DATA, data: any) => void)
  & ((terminate: END, error?: any) => void);

The extra parenthesises around the three function types are needed for TypeScript not to throw a fit.

This gives us rather nice intellisense help in a TypeScript setting:

In fact, that's so nice that we should split up SourceTalkback in the same way so that it too gets separate parameter names per call type:

type SourceTalkback = 
  & ((request: DATA) => void)
  & ((terminate: END) => void);

Source VS source talkback

Our tour of the three different kinds of callbags - source, source talkback and sink talkback - is now concluded!

But let's zoom out and discuss the difference between source and source talkback for a bit. Our TypeScript definition for Source was:

type Source = (start: START, sinkTalkback: SinkTalkback) => void;

...and Source talkback:

type SourceTalkback = 
  & ((request: DATA) => void)
  & ((terminate: END) => void);

But what exactly does the spec say about source? Well:

a callbag which is expected to deliver data

Hmm. Who is it that delivers data - is it the Source or the SourceTalkback? Well, that actually depends!

In a pullable source it is the source talkback. Check again at the source for fromPullStream:

const fromPullStream = read => (start, sink) => {
  if (start !== 0) return;
  sink(0, (t, d) => { // <--- source talkback
    if (t === 1) read(null, (end, data) => {
      if (end === true) sink(2);
      else if (end) sink(2, end);
      else sink(1, data); // <--- delivery call
    });
    if (t === 2) read(d || true, () => {});
  });
};

The sink(1, data) call takes place inside the source talkback. But if we look at a listenable source like interval...

const interval = period => (start, sink) => {
  if (start !== 0) return;
  let i = 0;
  const id = setInterval(() => {
    sink(1, i++); // <---- delivery call
  }, period);
  sink(0, t => { // <---- source talkback
    if (t === 2) clearInterval(id);
  });
};

...you'll note the sink(1, data) call is taking place inside the source.

This means that the word source as defined in the spec encompasses both Source and SourceTalkback. The concept includes both typings. Or you could argue that SourceTalkback is an implementation detail of Source.

Either way, using the name Source for a type seems like needless confusion potential. To mitigate that, let's rename what we've defined as Source and call it SourceInitiator instead:

type SourceInitiator = (start: START, sinkTalkback: SinkTalkback) => void;

Now the words make more sense - the definition of source in the spec includes both concepts of SourceInitiator and SourceTalkback.

Sink VS sink talkback

The obvious next question - what, then, is the difference between a sink and a sink talkback? What even is a sink?

The spec simply says...

a callbag which is expected to be delivered data

...which clearly fits the sink talkback. That will always be what receives the data. However, that doesn't answer our question of what a sink actually is.

But if a SourceInitiator is what provides a SourceTalkback, an interesting thing might be to look at what provides the SinkTalkback! Let's look again at forEach:

const forEach = operation => source => {
  let talkback;
  source(0, (t, d) => { // <--- sink talkback
    if (t === 0) talkback = d;
    if (t === 1) operation(d);
    if (t === 1 || t === 0) talkback(1);
  });
};

The sourceInitiator(0, sinkTalkback) call takes place inside a function that takes a source parameter and returns nothing. Is perhaps that a sink? Or, by the previous logic, a sink initiator?

type SinkInitiator = (source: SourceInitiator) => void;

I'd say yes, that is a sink (initiator)!

Except this time the typescript definition isn't as helpful, since it doesn't encode the fact that to be a SinkInitiator you must also call the passed source in a specific way. It is kind of implied through the type of the passed in SourceInitiator, but, still.

Perhaps we can make the implication stronger still by calling it SinkConnector instead of SinkInitiator? Yeah, let's do that!

type SinkConnector = (source: SourceInitiator) => void;

However, we can note that every single usage of the word "sink" in the spec fits what we've labeled as a SinkTalkback. In other words, we could have called that a Sink. But it seems nice to be constistent in suffixing the type for a callbag that is passed in to another callbag with -Talkback.

Operators and sinks

In the official typings, operators are defined as:

type Operator = (...args: Array<any>) => (source: Callbag) => Callbag;

We can replace Callbag with what we've called SourceInitiator:

type Operator = (...args: Array<any>) => (source: SourceInitiator) => SourceInitiator;

So what's returned from calling the operators with some arguments is almost a sink connector, except it returns SourceInitiator instead of void!

type SinkConnector = (source: SourceInitiator) => void;
type ReturnedFromOp = (source: SourceInitiator) => SourceInitiator;

Do we want to be able to say that an operator returns a sink connector? Or should we invent a new concept for this?

I'm voting for the former - it makes sense to say that an operator returns a sink connector. We can do that by merging the two types above thusly:

type SinkConnector = (source: SourceInitiator) => SourceInitiator | void;

This has four benefits:

  • no need for a separate type to describe what is returned by an operator
  • we can type operators who don't return anything (in other words, an operator for a sink)
  • we can type sinks who returns a source initiator, allowing for the chain to continue
  • the chainability is now visible in the typings!

So our revised type for Operator is now:

type Operator = (...args: Array<any>) => SinkConnector

Factories

The official typings also had a typing for Factory:

type Factory = (...args: Array<any>) => Callbag;

We can now clarify that into:

type SourceFactory = (...args: Array<any>) => SourceInitiator;

What about a SinkFactory? It turns out we don't need one - a SinkFactory is simply an Operator that returns a SinkConnector that doesn't return a SourceInitiator!

The new typings

Here are all our new typings collected:

// Source
type SourceFactory = (...args: Array<any>) => SourceInitiator;

type SourceInitiator = (start: START, sinkTalkback: SinkTalkback) => void;

type SourceTalkback = 
  & ((request: DATA) => void)
  & ((terminate: END) => void);

// Sink
type SinkConnector = (source: SourceInitiator) => SourceInitiator | void;

type SinkTalkback = 
  & ((start: START, sourceTalkback: SourceTalkback) => void)
  & ((deliver: DATA, data: any) => void)
  & ((terminate: END, error?: any) => void);

// Other
type Operator = (...args: Array<any>) => SinkConnector;

However, if we want to avoid using types that haven't been defined yet, we can look at the dependency graph of the types...

...and reorder them backwards according to that:

type SourceTalkback = 
  & ((request: DATA) => void)
  & ((terminate: END) => void);

type SinkTalkback = 
  & ((start: START, sourceTalkback: SourceTalkback) => void)
  & ((deliver: DATA, data: any) => void)
  & ((terminate: END, error?: any) => void);

type SourceInitiator = (type: START, payload: SinkTalkback) => void;

type SinkConnector = (source: SourceInitiator) => SourceInitiator | void;

type SourceFactory = (...args: Array<any>) => SourceInitiator;

type Operator = (...args: Array<any>) => SinkConnector;

Reaping the rewards

With these typings down we can use them as a very precise vocabulary for explaining the process of using callbags!

Here's the setup and handshake:

  1. We call a SinkConnector passing in a SourceInitiator: sinkConnector(sourceInitiator)
  2. The SourceInitiator is called with a SinkTalkback: sourceInitiator(0,sinkTalkback)
  3. The SinkTalkback is called with a SourceTalkback: sinkTalkback(0,sourceTalkback)

And data passing:

  1. The SourceTalkback is called with a request for data (if pullable): sourceTalkback(1)
  2. The SinkTalkback is called with data delivery: sinkTalkback(1,data)

And termination:

  • The SourceTalkback can be called to terminate the relationship: sourceTalkback(2)
  • The SinkTalkback can be called to terminate the relationship: sinkTalkback(2)
  • The SinkTalkback can be called to terminate the relationship due to an error: sinkTalkback(2,error)

Wrapping up

Creating the types definitely helped me cement my own understanding of callbags, but I'm not sure if they have a pedagogic power in general. I can only hope you found this instructive, otherwise you just made a very long journey for nothing. :)

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