Functional Reactive Programming in Clojurescript
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The document discusses functional reactive programming (FRP) and how it can be used to handle asynchronous workflows and time-varying values. It introduces reactive extensions (Rx) as an implementation of FRP and shows examples of using Rx to turn server results into an observable event stream. This allows processing the stream without explicitly managing state, including accessing the previous and current results with no local variables by zipping a stream with itself while skipping one element. Code examples are provided to demonstrate polling an API continuously to update displayed results reactively as the questions or results change over time.
![But that’s hardly useful when
we have multiple values
(def vals [a b c])
Friday, 17 May 13](https://image.slidesharecdn.com/frp-slides-130519002640-phpapp02/85/Functional-Reactive-Programming-in-Clojurescript-5-320.jpg)
![What about a list of - as of yet
unknown - values?
[a,b,c]? ? ?
Friday, 17 May 13](https://image.slidesharecdn.com/frp-slides-130519002640-phpapp02/85/Functional-Reactive-Programming-in-Clojurescript-11-320.jpg)
![Rx 101
(-> (.fromArray js/Rx.Observable
(clj->js [10 20 30]))
(.map #(* % 2))
(.reduce +)
(.subscribe #(.log js/console %)))
;; 120
Friday, 17 May 13](https://image.slidesharecdn.com/frp-slides-130519002640-phpapp02/85/Functional-Reactive-Programming-in-Clojurescript-22-320.jpg)
![Rx 101
(defn project-range [n]
(.returnValue js/Rx.Observable (range n)))
(-> (.fromArray js/Rx.Observable
(clj->js [1 2 3]))
(.selectMany project-range)
(.subscribe #(.log js/console (clj->js %))))
;; [0]
;; [0 1]
;; [0 1 2]
Friday, 17 May 13](https://image.slidesharecdn.com/frp-slides-130519002640-phpapp02/85/Functional-Reactive-Programming-in-Clojurescript-23-320.jpg)
![(def results-connectable
(let [obs (-> js/Rx.Observable
(.interval 2000)
(.selectMany results-observable)
(.publish)
(.refCount))
obs-1 (.skip obs 1)]
(.zip obs obs-1 (fn [prev curr]
{:prev prev
:curr curr}))))
Turn server results into an event stream
{
The core idea
Clone stream, skip one
Zip them together
{
Friday, 17 May 13](https://image.slidesharecdn.com/frp-slides-130519002640-phpapp02/85/Functional-Reactive-Programming-in-Clojurescript-37-320.jpg)
Functional Reactive Programming in Clojurescript
- 1. Taming asynchronous workflows with Functional Reactive Programming LambdaJam - Brisbane, 2013 Leonardo Borges @leonardo_borges www.leonardoborges.com www.thoughtworks.com Friday, 17 May 13
- 2. Leonardo Borges @leonardo_borges www.leonardoborges.com www.thoughtworks.com • Thoughtworker • Functional Programming enthusiast • Clojure Evangelist • Founder & Organiser of the Sydney Clojure User Group (clj-syd) • World traveller • Fan of Murray’s Beers :) about:me Friday, 17 May 13
- 3. Functional programmers like programming with values: a, b, c... and pure functions: f, g, h... Friday, 17 May 13
- 4. We get new values by applying functions to it (f a) ;;=> b Friday, 17 May 13
- 5. But that’s hardly useful when we have multiple values (def vals [a b c]) Friday, 17 May 13
- 6. So we use Higher Order Functions (map f vals) Friday, 17 May 13
- 7. And compose them as we see fit (-> vals (filter f) (map g) (reduce h)) Friday, 17 May 13
- 8. But what if the value isn’t known...yet? a? Friday, 17 May 13
- 9. We make promises ;; thread#1 (def a (promise)) ;; ...later in the program (f @a) ;;<= blocks thread ;; thread#2 (deliver a 10) ;; now thread#1 continues Friday, 17 May 13
- 10. Not great if we want to ‘react’ to a new value Friday, 17 May 13
- 11. What about a list of - as of yet unknown - values? [a,b,c]? ? ? Friday, 17 May 13
- 12. Or better yet, a value that changes over time? 0 37.5 75 112.5 150 10s 20s 30s 40s 50s 60 Value Time Friday, 17 May 13
- 13. Does this sound familiar? Friday, 17 May 13
- 14. Spreadsheets: a poor man’s reactive programming model Values Function Friday, 17 May 13
- 15. Spreadsheets: a poor man’s reactive programming model As we change a value Our function cell reacts to the change Friday, 17 May 13
- 16. ‘Changing a value’ is an event Several events over time form an event stream Friday, 17 May 13
- 17. “Functional Reactive Programming is about effectively processing event streams without explicitly managing state” - me Friday, 17 May 13
- 18. “FRP is about handling time- varying values like they were regular values.” - Haskell wiki Friday, 17 May 13
- 19. We’ll use Reactive Extensions (Rx) - but there are many implementations Friday, 17 May 13
- 20. In Rx, event streams are called Observable sequences Friday, 17 May 13
- 21. Rx 101 (-> (.returnValue js/Rx.Observable 42) (.map #(* % 2)) (.subscribe #(.log js/console %))) ;; 84 Friday, 17 May 13
- 22. Rx 101 (-> (.fromArray js/Rx.Observable (clj->js [10 20 30])) (.map #(* % 2)) (.reduce +) (.subscribe #(.log js/console %))) ;; 120 Friday, 17 May 13
- 23. Rx 101 (defn project-range [n] (.returnValue js/Rx.Observable (range n))) (-> (.fromArray js/Rx.Observable (clj->js [1 2 3])) (.selectMany project-range) (.subscribe #(.log js/console (clj->js %)))) ;; [0] ;; [0 1] ;; [0 1 2] Friday, 17 May 13
- 24. Observables are Monads Friday, 17 May 13
- 25. The Monad Type Class class Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b Friday, 17 May 13
- 26. Monad functions: return return :: a -> m a returnValue :: a -> Observable a Friday, 17 May 13
- 27. (>>=) :: m a -> (a -> m b) -> m b selectMany :: Observable a -> (a -> Observable b) -> Observable b Monad functions: >>= (bind) Friday, 17 May 13
- 28. Demo: Simple polling app Friday, 17 May 13
- 29. Server exposes poll questions and results e.g.: {:id 7 :question "Which is the best music style?" :results {:a 10 :b 47 :c 17}} Friday, 17 May 13
- 30. What we want • Render results • Continuously poll server every 2 secs • If current question is the same as the previous one update results; • Otherwise: • Stop polling; • Display countdown message; • Render new question and results; • Restart polling; Friday, 17 May 13
- 31. The core idea Friday, 17 May 13
- 32. Turn server results into an event stream 112334 Friday, 17 May 13
- 33. Duplicate stream, skipping one 112334 123345 skip 1 Friday, 17 May 13
- 34. Zip them together 112334 1 2 zip 2 3 3 3 3 4 4 5 1 123345 1 Friday, 17 May 13
- 35. Now we have access to both the previous and current results, with no local variables Friday, 17 May 13
- 36. Show me the code! https://github.com/leonardoborges/frp-code Friday, 17 May 13
- 37. (def results-connectable (let [obs (-> js/Rx.Observable (.interval 2000) (.selectMany results-observable) (.publish) (.refCount)) obs-1 (.skip obs 1)] (.zip obs obs-1 (fn [prev curr] {:prev prev :curr curr})))) Turn server results into an event stream { The core idea Clone stream, skip one Zip them together { Friday, 17 May 13
- 38. “FRP is about handling time- varying values like they were regular values.” - Haskell wiki Friday, 17 May 13
- 39. Questions? Leonardo Borges @leonardo_borges www.leonardoborges.com www.thoughtworks.com Friday, 17 May 13
- 40. References Code - https://github.com/leonardoborges/frp-code RxJS - https://github.com/Reactive-Extensions/RxJS RxJava - https://github.com/Netflix/RxJava Other FRP implementations: Reactive-banana - http://www.haskell.org/haskellwiki/Reactive-banana Javelin (Clojurescript) - https://github.com/tailrecursion/javelin Bacon.js - https://github.com/raimohanska/bacon.js Friday, 17 May 13