Reactive functional programming

rxjs

core ideas

from http://reactivex.io/rxjs/manual/overview.html#subscribing-to-observables

Observables are like functions with zero arguments, but generalize those to allow multiple values.

observer.next(42) is like return 42

 var foo = Rx.Observable.create(function (observer) {
   console.log('Hello');
   observer.next(42);
 });

 foo.subscribe(function (x) {
   console.log(x);
 });

Subscribing to an Observable is analogous to calling a Function.

 console.log('before');
 foo.subscribe(function (x) {
   console.log(x);
 });
 console.log('after');

Observables are able to deliver values either synchronously or asynchronously.

internally:

   function map(fn) {
     var source=this
     result = Rx.Observable.create(function subscribe(observer){
       source.subscribe(
         function(x) {observer.next(fn(x));},
         function(err){ oberver.error(err));},
         function() {observer.complete();}
       )
     })
   }
   Rx.Observable.prototype.map = map

operators

Rx.Observable.of(4,5,6,7,8) 
(45678|)

Rx.Observable.interval(500).take(4),  a kind of filter, also: first, takeLast, last, skip, skipLast
---0---1---2---3|

[4,5,6,7].do(x=>console.log('dog is '+x)) //doesn't touch the Observable, passes it through

 Rx.Observable.interval(500).take(4)
---0---1---2---3|   foo       
(8,9,10,11|)        more
foo.concat(more) 
---0---1---2---3(8,9,10,11|)

(a|)   foo
----0----1----2  bar
bar.startWith(foo)

other combination: 'or' style

---0---1---2---4
-a-b-c-d-e-f-
  merge
-a-(0b)-c-(1d)-e-(2f)---4

'and' style

---0---1---2---4
-5-6-7---8--- 
foo.combineLatest(bar, (x,y)=>x+y)
---5,6,7-8-9-10--12

---H---e---l---l
--0-1---0-1-0-- 
foo.withLatestFrom(bar, (c,n)=>n==1?..toUpperCase():c.toLowerCase())
---h---E---L--l

zip first+first, second+second

 ----0----2---4----5|
 -5-6-7-8
 foo.zip(bar, (x+y)=>x+y)
 ----5----8---1----13

 o mo better...
  (hekk|)
  ---0---1---2---3
  foo.zip(bar, (x+y)=>x)
  ---h---e---k---k

transformation

 accumulates horizantally
   scan
     ----h---e---d---f--
     scan(acc,x,'d')
     d--(dh)-(dhe)-(dhef)

     to add up clicks
     ---x----x--x
     clicks.map((x)=>1)
     ---1----1--1-
     .scan((accc,x)=>acc+x,0)
     0--1----2--3

   buffer
     ----h----e----l---l----o    (foo)
     foo.bufferCount(2)   
     ---------he-------ll---[o]  
     or
     foo.bufferTime(900ms)
     -------h-------el-------[o]

     -----------1---1---------1 (bar)
     foo.buffer(bar)
     -----------he--l---------lo

   delay
     0---1---2---3---4  
     foo.delay(tommorrow)
     -------------0---1---2---3---4

     --0--1--2--3--4|
     var result = foo.delayWhen(x =>
       Rx.Observable.interval(x*x*100).take(1)
     );
 rate limiting (filtering)operators
   debounce //drops things if they happen too fast
     0---1---2---3---4  
     debounceTime(1000)-------//waits for silence
     -----------------------4
     throttleTime(1000) //emits then silence
     0------2------3------4

   distinct
     --a--b--a--c--b|
     foo.distinct()
     --a--b-----c---  
   distinctUntilChanged
     --a--b--a--a--b|
     --a--b--a-----b|
 error handling
   catch - replace error with observable
     --a--b--c--d--2|     (foo)
     map(toUpperCase)
     --A--B--C--D--#      (bar)
     catch(# => -Z|)
     --A--B--C--D--Z|
     var result = bar.catch(error => Rx.Observable.of('Z')); 

     repeat
       --a--b--c--d|     (foo)
       map(toUpperCase)
       --A--B--C--D|      (bar)
        repeat(3)
       --A--B--C--D--A--B--C--D--A--B--C--D|   
   retry on error resubscribe
     --a--b--c--d--2|     (foo)
     map(toUpperCase)
     --A--B--C--D--#      (bar)
     retry(2))
     --A--B--C--D------A--B--C--D------A--B--C--D--#
   retryWhen
     var result = bar.retryWhen(errorObs => errorObs.delay(3000))
     --A--B--C--D-----------A--B--C--D-----------A--B--C--D-----------
 

netflix reactive extensions

• github library reactive extensions
• netflix staff training exercises
• https://medium.com/javascript-scene/the-two-pillars-of-javascript-pt-2-functional-programming-a63aa53a41a4#.7v5td6gdw