What is reactive programming?

http://paulstovell.com/blog/reactive-programming

by outdooricon • 12 years ago

67 52 12 years ago

davexunit• 12 years ago

I've been slowly implementing my own functional reactive programming system over for a game engine I'm writing. A few months ago, I wrote a short blog post on the topic that includes a screencast to see how it all works. Maybe it will help someone understand (F)RP a bit better.

http://dthompson.us/functional-reactive-programming-in-schem...

Vaskivo• 12 years ago

Thanks. I've been reading about FRP, and I think games or GUI centric applications are a great use-case for it.

ef44• 12 years ago

The "destiny" operator is just function definition. It might seem different if you're only used to languages where function calls and variable accesses are syntactically distinct.

throwaway2837193• 12 years ago

Not true. Consider the following snippet -

    a = 10
    b = function() { return expensive_computation(a) }

Now, even if you can call b without parens, it still has to redo the expensive calculation every time you access it (because a might have changed... even if it hasn't).

The "destiny" operator is more like

    a = 10
    b = function() {
            persistent cached_value = null
            if is_null(cached_value) or has_changed(a) {
                cached_value = expensive_computation(a)
            }
            return cached_value
        }

but instead of that, you get to write (in this totally fictional programming language)

    a = 10
    b <- expensive_computation(a)

and have the compiler take care of the caching and updating for you.

Sharlin2• 12 years ago

Plus the real power of reactivity is that it is push, not pull. The changes are propagated without ever needing to poll for the new value manually - for instance, you could display the value of b in a UI and have it change whenever the value of a changes.

tinco2• 12 years ago

A proper function will always return the same value if you give it the same input, so there's no need for the computer to always re-run the expensive computation.

dragonwriter• 12 years ago

Now, even if you can call b without parens, it still has to redo the expensive calculation every time you access it (because a might have changed... even if it hasn't).

That's true in most programming languages, but not in languages that distinguish side-effect free code. (Such a language implementation might still rerun a pure function because it doesn't include a way of "remembering" that a function has been called with particular arguments before, but that's an implementation detail.)

zak_mc_kracken• 12 years ago

That's called memoization and you usually want to leave this decision up to the caller and not the callee since only the caller knows if they are fine with receiving the same value as calculated previously or whether they want a new one each time.

ef44• 12 years ago

You're reinforcing my point. You're assuming imperative semantics.

Wherever "destiny" would be appropriate, you necessarily have a referentially transparent function. Wherever you have a referentially transparent function, a smart compiler or runtime can avoid recalculating anyway, without bothering the programmer with the distinction between "function" and "destiny".

ggurgone• 12 years ago

Netflix uses reactive programming massively https://www.youtube.com/watch?v=XRYN2xt11Ek

charlespwd• 12 years ago

Interesting talk. Thanks for sharing!

tel2• 12 years ago

The linked Stack Overflow answer by Conal Elliott does a much better job explaining FRP. It has less to do with "linking" as via this destiny operator and more to do with manipulating values of "entire histories and futures" all at once. In other words, the heart of FRP is not in the operators, but the nouns. And not in an OO sense tied to ambient mutation, but instead as concrete, immutable values.

throwaway2837193• 12 years ago

I knew it was by Conal before I got to the end, as soon as I saw the phrase "denotational semantics" ;)

I should really learn what that means one day...

chriswarbo2• 12 years ago

Denotational semantics: code is a meaningful artefact.

This fits well with the functional paradigm, where values are immutable and therefore 'timeless'.

In contrast, imperative code, if it has any actual semantics at all, tends to be operational:

Operational semantics: the "meaning" of code is how it affects the behaviour of the machine that's interpreting it.

The problem with operational semantics is exactly that of imperative code: in order to understand what some code will do, you need to know everything which 'happened before' (ie. the current state of the machine).

amouat• 12 years ago

To save others clicking through the article again, the SO question is here: http://stackoverflow.com/questions/1028250/what-is-functiona...

factorialboy• 12 years ago

Reactive = Responding to (often asynchronous) events

chriswarbo2• 12 years ago

The words "responding" and "asynchronous" seem to imply a particular execution model, in particular one which seems to contradict the original definition of Function Reactive Programming

http://conal.net/papers/icfp97/

Perhaps a more neutral description would be:

Reactive = Values are parameterised by the current time

kazinator3• 12 years ago

This gives me an great idea. What if we put these variables into a grid displayed on the screen? Then you will be able to enter some values into some of the cells, and the dependent ones update themselves automatically. The elements of the grid could be regarded as array elements indexed by coordinate, and use relative or absolute addressing to refer to each other's coordinates, making it easy to replicate dependency rules across rows and columns, to generate entire tables which update themselves.

throwaway2837193• 12 years ago

Sounds excellent.

meowface• 12 years ago

Clever.

seanmcdirmid2• 12 years ago

Scaling would be quite difficult. I'm sure you could hack maybe a Turing machine out of this paradigm, but it would be ugly and mostly an esoteric exercise.

pjc50• 12 years ago

The spreadsheet, the one "graphical programming language" to make it into widespread use by nonexperts.

VLM2• 12 years ago

And in practice "use relative or absolute addressing" is resolved via random number generation. Any later problems resulting from confusing the two can just be fixed by filing tickets with IT.

Also used as the corporate standard database management system and as what boils down to a mail merge client, and although this appears to outsiders as bad standup comedy, it is unfortunately merely factual.

Possibly one of the quickest ways to measure the dilbertian level of rot of an organization is to propose a small database and ask what tool they'd use to hold that information. Mysql, OK. Postgresql, eh, OK. Oracle, ummm. Excel, find the exit door and start running. Microsoft Word, Cthulhu take us to end the agony. I've seen all of the above. And survived. Barely. Perhaps not with my sanity intact.

edited: spreadsheets fit into the design pattern of jokes where you got a problem to fix, so you try to implement a spreadsheet to fix it, now you got two problems to fix. This is probably funnier in its original regex and perl and web framework variants.

kyllo2• 12 years ago

What a novel idea!

Seriously though, Angular gives you the ability to do this in the browser, here's an example of how to implement an Excel-like spreadsheet using Angular: http://thomasstreet.com/blog/legacy/spreadsheet.html

sehr• 12 years ago

It's not really unique to Angular is it?

taylodl• 12 years ago

That's what Lotus Improv did, twenty years ago. It was too radical for users at the time and so sales were pretty much nonexistent.

https://en.wikipedia.org/wiki/Lotus_Improv

kazinator3• 12 years ago

That's also what VisiCalc did; released in 1981 on Apple II machines:

https://en.wikipedia.org/wiki/VisiCalc

RyanMcGreal• 12 years ago

Wait, we're still using semicolon line terminators in 2051?

Sigh.

willismichael• 12 years ago

...and mutable values by default.

VLM2• 12 years ago

... and single character ASCII non-UTF-8 variable names.

... and no types. So if a and b are strings in a "mostly string-ish language" then the value of b is "101" halfway thru and "111" at the end, right?

(I'd cut them a break if they used a single expressive and meaningful kanji even ifs only one glyph, although I'm struggling to think of a kanji that implies type and a compiler smart enough to understand the type embedded in the kanji... although it could happen)

kybernetikos• 12 years ago

Pretty sure that's a mistake in the article. We're supposed to be in 2051 looking back at a sample of code written in 2021.

michaelsbradley• 12 years ago

I enjoyed the post, but I think the proposed scope of reactive programming is a bit limited. What the author describes is, I think, more related to the concerns of "bidirectional programming"[1] and lenses[2]. Granted, I think those concerns do fit broadly under the umbrella of reactive programming.

Below is Prof. David Harel's (the inventor of statecharts, formalized as UML State Machines) general characterization[3] of a reactive system; reactive programming would then be any programming methodology, language or environment which aims to service those characteristics.

* It continuously interacts with its environment, using inputs and outputs that are either continuous in time or discrete. The inputs and outputs are often asynchronous, meaning that they may arrive or change values unpredictably at any point in time.

* It must be able to respond to interrupts, that is, high-priority events, even when it is busy doing something else.

* Its operation and reaction to inputs often reflects stringent time requirements.

* It has many possible operational scenarios, depending on the current mode of operation and the current values of its data as well as its past behavior.

* It is very often based on interacting processes that operate in parallel.

[1] http://www.cs.cornell.edu/~jnfoster/papers/jnfoster-disserta...

[2] http://www.cis.upenn.edu/~bcpierce/papers/lenses-etapsslides...

[3] http://www.wisdom.weizmann.ac.il/~harel/reactive_systems.htm...

cpr• 12 years ago

https://gist.github.com/staltz/868e7e9bc2a7b8c1f754 is an excellent introduction to reactive programming.

What is reactive programming?

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