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The fourth dimension A practical introduction to Functional Reactive Programming by sergi mansilla | @sergimansilla

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@sergimansilla github.com/sergi

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Before

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Shameless linkbaiting

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Tame your async code with this one weird trick! A practical introduction to Functional Reactive Programming by sergi mansilla | @sergimansilla

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- Linkbaiting - Time

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Human beings have hard-wired time in their brain

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Developers have hard-wired time in their brain (in a slightly different way)

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1 CPU Cycle 1 s Level 1 cache access 3 s Level 2 cache access 9 s Level 3 cache access 43 s Main memory access (DRAM, from CPU) 6 min Solid-state disk I/O (flash memory) 2-6 days Rotational disk I/O 1-12 months Internet: San Francisco to Australia 19 years

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JS developers have hard-wired async in their brain (in a messed up way)

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Callbacks Promises Generators Events

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We use events to deal with asynchronous tasks

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } });

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Why are we still micromanaging code?

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } });

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } });

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } });

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } });

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } });

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2 && isAPressed) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } }); ! var isAPressed = false; document.addEventListener('keydown', e => { isAPressed = e.keyCode === 65; }, false); ! document.addEventListener('keyup', e => { isAPressed = false; }, false);

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We still code the how instead of the what

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Programming should be more about the what

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Declarative

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Declarative

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document.addEventListener('keyup', e => { return isAPressed = false; }, false); Void callbacks

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2 && isAPressed) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } }); ! var isAPressed = false; document.addEventListener('keydown', e => { isAPressed = e.keyCode === 65; }, false); ! document.addEventListener('keyup', e => { isAPressed = false; }, false);

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Climbers

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State is dangerous

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Event limbo

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } });

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Awkward error handling Memory leaks Complex (state)

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Isn’t that the problem promises try to solve?

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var y = f(x); var z = g(y); fAsync(x).then(...); gAsync(x).then(...); res = stocks .filter(q => q.symbol == 'FB') .map(q => q.quote) ! res.forEach(x => ... res = stocks //async retrieval .filter(q => q.symbol == 'FB') .map(q => q.quote) ! res.subscribe(x => ... Sync Sync Promises Reactive

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Click! … … Click! Click!

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Click! , , Click! Click! [ ]

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[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] .filter(n => n % 2) .map(n => 'item ' + n) .forEach(n => console.log(n)) ! // "item 1" // "item 3" // "item 5" // "item 7" // "item 9"

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F R P

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Final Resting Place

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Fantasy Role Playing

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Functional Reactive Programming

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Deal with values that change over time

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Alternatives: - Bacon.js - Elm Reactive Extensions for JavaScript

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RxJS helps us compose asynchronous programs in a declarative way

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var clicks = 0; document.addEventListener('click', function register(e) { if (clicks < 10) { if (e.clientX > innerWidth / 2) { console.log(e.clientX, e.clientY); clicks += 1; } } else { document.removeEventListener('click', register); } }); Filter Limit to 10 Print the coordinates

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fromEvent(document, 'click') .filter(c => c.clientX > innerWidth / 2 }) .take(10) .subscribe(c => console.log(c.clientX, c.clientY) })

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fromEvent(document, 'click') .filter(c => c.clientX > innerWidth / 2 }) .take(10) .subscribe(c => console.log(c.clientX, c.clientY) }) Create Observable

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fromEvent(document, 'click') .filter(c => c.clientX > innerWidth / 2 }) .take(10) .subscribe(c => console.log(c.clientX, c.clientY) }) Create filtered Observable
 from the first one

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fromEvent(document, 'click') .filter(c => c.clientX > innerWidth / 2 }) .take(10) .subscribe(c => console.log(c.clientX, c.clientY) }) Create final Observable
 taking only first 10 results

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fromEvent(document, 'click') .filter(c => c.clientX > innerWidth / 2 }) .take(10) .subscribe(c => console.log(c.clientX, c.clientY) }) Actually kick off computation

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Observable Observer pattern Iterator pattern

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Rx.Observer - OnNext() - OnError() - OnComplete()

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// Creates an observable sequence of 5 integers var source = Rx.Observable.range(1, 5) ! // Prints out each item var subscription = source.subscribe( x => { console.log('onNext: ' + x) }, e => { console.log('onError: ' + e.message) }, () => { console.log('onCompleted') }) ! // => onNext: 1 // => onNext: 2 // => onNext: 3 // => onNext: 4 // => onNext: 5 // => onCompleted

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var mousemove = fromEvent(document, 'mousemove'); ! var mouseCoords = mousemove.map(e => ({ left: e.clientX, top: e.clientY })) ! var mouseSide = mousemove.map(e => (e.clientX > window.innerWidth / 2 ? 'right' : 'left')) ! mouseCoords.subscribe(pos => coords.innerHTML = pos.top + 'px ' + pos.left + 'px') mouseSide.subscribe(s => side.innerHTML = s); Composability

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var emsc = Rx.DOM.jsonpRequest({ url: ‘http://seismicportal.eu/?callback=cbfunc...', jsonpCallback: 'cbfunc' }).retry(); ! var usgs = Rx.DOM.jsonpRequest({ url: QUAKE_URL, jsonpCallback: 'eqfeed_callback' }); ! var quakes = Rx.Observable.interval(5000) .flatMap(() => emsc.merge(usgs)) .flatMap(dataset => Rx.Observable.fromArray(dataset.features)) .distinct(quake => quake.code || quake.unid) .subscribe(quake => { var coords = quake.geometry.coordinates; L.circle([coords[1], coords[0]], quake.size).addTo(map); }); Concurrency

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// Search Wikipedia for a given term function searchWikipedia(term) { var cleanTerm = global.encodeURIComponent(term); var url = ‘/search?q=‘ + cleanTerm + '&cb=JSONPCallback'; return Rx.Observable.getJSONPRequest(url); } ! var input = document.querySelector('#searchtext'), results = document.querySelector('#results'); ! // Get all distinct key up events from the input and var keyup = fromEvent(input, 'keyup') .map(e => e.target.value) .where(text => text.length > 2) // Longer than 2 chars .throttle(200) // Pause for 200ms .distinctUntilChanged(); // Only if the value has changed

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var searcher = keyup .map(text => searchWikipedia(text)) // Search wikipedia .switchLatest() // Ensure no out of order results .where(data => (data.length === 2)); // Where we have data ! searcher.subscribe(data => { // Append the results (data[1]) }, error => { // Handle any errors });

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// Search Wikipedia for a given term function searchWikipedia(term) { var cleanTerm = global.encodeURIComponent(term); var url = ‘/search?q=‘ + cleanTerm + '&cb=JSONPCallback'; return Rx.Observable.getJSONPRequest(url); } ! var input = document.querySelector(‘#searchtext'); var results = document.querySelector('#results'); ! // Get all distinct key up events from the input and var keyup = fromEvent(input, 'keyup') .map(e => e.target.value) .where(text => text.length > 2) // Longer than 2 chars .throttle(200) // Pause for 200ms .distinctUntilChanged(); // Only if the value has changed

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// Search Wikipedia for a given term function searchWikipedia(term) { return fromArray(['JavaScript', 'JavaServer Pages', 'JavaSoft', 'JavaScript library', 'JavaScript Object Notation', 'JavaScript engine', 'JavaScriptCore']); } ! var input = document.querySelector('#searchtext'), results = document.querySelector('#results'); ! // Get all distinct key up events from the input and var keyup = fromEvent(input, 'keyup') .map(e => e.target.value) .where(text => text.length > 2) // Longer than 2 chars .throttle(200) // Pause for 200ms .distinctUntilChanged(); // Only if the value has changed

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fromArray fromCallback fromEvent fromEventPattern fromIterable fromNodeCallback fromPromise

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var mouseup = fromEvent(dragTarget, 'mouseup'); var mousemove = fromEvent(document, 'mousemove'); var mousedown = fromEvent(dragTarget, 'mousedown'); ! var mousedrag = mousedown.flatMap(md => { var startX = md.clientX + window.scrollX, startY = md.clientY + window.scrollY, startLeft = parseInt(md.target.style.left, 10) || 0, startTop = parseInt(md.target.style.top, 10) || 0; ! // Calculate delta with mousemove until mouseup return mousemove.map(mm => { mm.preventDefault(); ! return { left: startLeft + mm.clientX - startX, top: startTop + mm.clientY - startY }; }).takeUntil(mouseup); }); ! subscription = mousedrag.subscribe(pos => { dragTarget.style.top = pos.top + 'px'; dragTarget.style.left = pos.left + 'px'; });

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[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] .filter(n => n % 2) .map(n => 'item ' + n) .forEach(n => console.log(n)) ! // "item 1" // "item 3" // "item 5" // "item 7" // "item 9"

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[1, 2, 3, 4, 5, 6, 7, 8, 9, 10] .filter(n => n % 2) .map(n => 'item ' + n) .forEach(n => console.log(n)) ! // "item 1" // "item 3" // "item 5" // "item 7" // "item 9" loop loop loop

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fromArray([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) .filter(n => n % 2) .map(n => n * 100) .map(n => 'item ' + n) .subscribe(n => console.log(n))

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First Class async Composability Concurrency Encapsulation Michael Hasselhof sings limbo

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I am writing a book about this! @sergimansilla

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Thanks! @sergimansilla