I am learning reactive programming and functional reactive programming in JavaScript. I am very confused.
Wikipedia says that there are various ways to write reactive code such as imperative, OORP and functional. I want to know if event-driven is just an another way to write reactive code?
How is reactive programming related to Promises? I think promises is an alternative to event-driven and callback hell.
How is reactive programming related to Promises? I think the promise is an alternative to event-driven and callback hell.
In practice the two are related, I like to call Promises a gateway drug to functional reactive programming.
+----------------------+--------+-------------+
| | Sync | Async |
+----------------------+--------+-------------+
| Single value or null | Option | Promise |
| Multiple values | List | EventStream |
+----------------------+--------+-------------+
Promises can be thought of as EventStreams with one item, or you can think of EventStreams as multiple Promises over time.
Promises can be chained, which is getting close to reactive programming:
getUser() // return promise
.then((userId) => {
return fetch("/users/"+userId)
})
.then((user) => {
alert("Fetched user: " + user.name)
})
The same with bacon.js:
const userStream = userIdStream // EventStream of userIds
.flatMapLatest((userId) => {
return Bacon.fromPromise(fetch("/users/"+userId))
})
const userNameStream = userStream.map((user) => user.name)
userNameStream.onValue((user) => {
alert("Fetched user: " + user.name)
})
Both code snippets do the same thing, but there is a big difference in thinking: with promises you are thinking about handling a single action with async steps in a clear way - the thinking is imperative, you are doing things step by step. With FRP, you a saying "a stream of usernames is created from the stream of userIds by applying these two transformation steps". When you have a stream of usernames, without caring where they came from, and say "whenever there is a new username, display it to the user".
The FRP coding style will guide you to model your problem as a stream of values (i.e. values that change over time) and the relationships between these values. If you already know Promises, the initial learning curve will be a bit easier, but the main benefit is gained only when you start thinking and modeling the problem differently - it is possible (if not very useful) to do imperative programming with FRP libraries.
How is reactive programming different than event-driven programming?
Event driven programming revolves around so-called events, which are abstract things that programs "fire" when something happens. Other places in your code "listen" for the events and respond with what they need to do when that event happens. For instance, an event could be "user pressed this button" or "the printer is done printing your document".
Reactive programming deals with data. Ultimately this is a special case of event-driven programming. The event: data changed. The event handler: change some more data (if applicable). This concept is usually cleared up when you think of a spreadsheet. If you set cell1 = cell2 + cell3 this implicitly sets two event handlers on the data changed events of cell2 and cell3 to update cell1's data. cell1's data has no such event handler, because no cells depend on it's value.
TL;DR;
Wikipedia says that there are various ways to write reactive code such as imperative, OORP and functional. I want to know if event-driven is just an another way to write reactive code?
The idea of event-driven programming is orthogonal to the idea of imperative vs. OO vs. functional.
Event driven programming: structures your program in order to deal with ("handle") something else that happens in your program (an "event"). In other words, it structures your code logically like this
When Event1 happens
do A and B
When Event2 happens
do B and C
But there are many ways to write this code, and in fact many ways to write the code imperatively, many ways to write it functionally, etc. Here are some examples, though.
Imperatively (with an event loop):
while(true)
// some other code that you need to do...
if Event1 then
do A
do B
if Event2 then
do B
do C
Object Oriented (with background thread):
// event queue
events = new EventQueue()
handler = new EventHandler()
// creates background thread
Thread.DoInBackground(handler.listenForEvents(events))
// ... other code ...
// fire an event!
events.enqueue(new Event1())
// other file
class EventHandler
Func listenForEvents(events)
while(true)
while events.count > 0
newEvent = event.dequeue()
this.handleEvent(newEvent)
Thread.Sleep(Time.Seconds(1))
Func handleEvent(event)
if event is Event1
this.A()
this.B()
if event is Event2
this.B()
this.C()
Func A()
// do stuff
return
Func B()
// do stuff
return
Func C()
// do stuff
return
Functional (With language support for events)
on Event(1) do Event1Handler()
on Event(2) do Event2Handler()
Func Event1Handler()
do A()
do B()
Func Event2Handler()
do B()
do C()
Func A()
// do stuff
return
Func B()
// do stuff
return
Func C()
// do stuff
return
// ... some other code ...
// fire! ... some languages support features like this, and others have
// libraries with APIs that look a lot like this.
fire Event(1)
How is reactive programming related to Promises?
Promises are an abstraction of the flow of program execution which can be summed up as follows:
Nothing really special here, except it's another way to think of the order in which your code is executed. For instance, promises are useful when you make a call to a remote machine. With promises, you can say "call me back when you return from this remote call!". Whichever library you use then promises to call you back when it gets something back from the remote machine. Often, this is useful because it lets you do something else in the meantime without waiting for the call to return.
Punch line: there are a lot of different styles of code, but they don't play too big a role in the pattern of event driven and reactive programming. To my knowledge, you can do event driven and/or reactive programming in most languages.
How is reactive programming related to Promises? I think the promise is an alternative to event-driven and callback hell.
In practice the two are related, I like to call Promises a gateway drug to functional reactive programming.
+----------------------+--------+-------------+
| | Sync | Async |
+----------------------+--------+-------------+
| Single value or null | Option | Promise |
| Multiple values | List | EventStream |
+----------------------+--------+-------------+
Promises can be thought of as EventStreams with one item, or you can think of EventStreams as multiple Promises over time.
Promises can be chained, which is getting close to reactive programming:
getUser() // return promise
.then((userId) => {
return fetch("/users/"+userId)
})
.then((user) => {
alert("Fetched user: " + user.name)
})
The same with bacon.js:
const userStream = userIdStream // EventStream of userIds
.flatMapLatest((userId) => {
return Bacon.fromPromise(fetch("/users/"+userId))
})
const userNameStream = userStream.map((user) => user.name)
userNameStream.onValue((user) => {
alert("Fetched user: " + user.name)
})
Both code snippets do the same thing, but there is a big difference in thinking: with promises you are thinking about handling a single action with async steps in a clear way - the thinking is imperative, you are doing things step by step. With FRP, you a saying "a stream of usernames is created from the stream of userIds by applying these two transformation steps". When you have a stream of usernames, without caring where they came from, and say "whenever there is a new username, display it to the user".
The FRP coding style will guide you to model your problem as a stream of values (i.e. values that change over time) and the relationships between these values. If you already know Promises, the initial learning curve will be a bit easier, but the main benefit is gained only when you start thinking and modeling the problem differently - it is possible (if not very useful) to do imperative programming with FRP libraries.
The difference is mostly related to how you "configure" (or declare) things convention: what happens to something when something other happens.
In reactive programming, you declare a reaction to a change. You don't have to forsee this reaction needed to that change upfront, you may add - declare - this reaction anytime later. Therefore, it might be considered a "pull" or "watch" strategy.
Therefore, in reactive programming, you hook up to / watch data which you know that exist. Data is crucial here.
Example: a user clicked an item on the page -> update the counter how many clicks user made.
Example Calculator App: the calculator display is bound to all buttons, and reacts with any change (clicks on the buttons) with its own change on the display. Buttons don't have awareness that their clicks can be utilized by any other parts.
In event-driven programming, you trigger an event in a certain situation in the imperative-written code. You need to be explicit upfront here, because the event needs to be triggered first in order to be received later - because basically you push the event in the "change happening" part of code. So it is a "push" strategy.
Therefore, in event-driven programming, you push an event in a certain situation that maybe would be received by some other parts of code. Situation is important here, data does not matter.
Example: someone has visited the contact page -> trigger an event (which might not be received at all in the end by any listener, which is typical case for many modules and libraries).
Example Calculator App: the calculator display is just a listener, and the buttons trigger events. Buttons need to know that they exist in a certain context (but - thanks to the event-listener pattern - don't have to know what exactly is that context), and therefore they are required to trigger an event.
So in most, they are just different conventions. Look at this simple example. The imperative approach example:
event: perform some operation on a, e.g. a += value, and trigger the event
listener: counter++
And the reactive declarative approach example:
counter: whenever an operation on a occurs, react with this: counter++
No need in the last example to trigger anything - you just "hook up" with a reaction to anything that may happen.
You may say, then, that reaction is bound to a in reactive approach, while in imperative event-driven approach you push an event that can be later received by a listener - and since this type of approach is not related by any means to data, you may change this: a += value to anything else later, even removing a completely.
Event driven approach has nothing to do with data essentially. Reactive programmin is basically about data.
So as you see, reactive programming is data-oriented (change in data reacts with triggering other code), while event-driven programming is process-oriented (it doesn't matter if and what data change, if any - you just trigger an event that would be received by some other parts of code). In the latter case, you need to know that this "informing" the other parts of code is required, and you have to then foresee that the event should be triggered. In the former case, you don't have to do that, you can do it any time, or not at all - no triggering events required - but the trick here is that there must be "something" that you can hook up to with your reaction declaration, kind of watchers which allow you to react to the watched changes.
Reactive programming is all about streams, it could be streams of events, or anything else. It is on emitting/announcing these streams or subscribing/watching these streams or stream transformations that lead to some events. So both programming paradigms are related.
To me it's like comparing oranges to apples. Let's try to define in a simple way what is what and so distinguish the things:
Reactive programming is a programming paradigm that is applied when one wants to achieve a functionality akin to data binding in libraries like KnockoutJS. Also an example would be Excel formulas: all cells are like variables in memory. There are those that simply hold some data and those which are computed from that data. If the former changes, so does the latter. Pay attention that the paradigm is about lower-level implementation; when someone is talking about reactive programming they are referring to data, its changes and what happens when it mutates.
On the other hand, event-driven programming is about system architecture. According to that paradigm events and event handlers are the basis of a system and everything is built upon and around them. Common examples would be UI and web server multiplexing. Do you feel how all this is different? The paradigm is applied on the level of a whole system or a subsystem.
How is reactive programming related to Promises? I think promises is an alternative to event-driven and callback hell.
Promise is a tool to achieve concurrency and specific order of execution. It can be used in any paradigm.
In practice the paradigms serve different purposes and at different levels. You can have event-driven design with some bits of reactive code. You can have distributed system that uses reactive design patterns. However, events are ultimately higher-level concept. Reactive is about data and its re-evaluation, an approach for implementation or its detail, and events are something that naturally come up from a case and drive your design.
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