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<h1>Alan Pearce - Functional Programming in JavaScript</h1>
</div>

</div>
<div id="content" class="presentation">
<div id='title-slide' class='slide'>
<h1>Functional Programming in JavaScript</h1>
<h2></h2>
<h2>Alan Pearce</h2>
<h3><a href="mailto:alan@alanpearce.eu">alan@alanpearce.eu</a></h3>
<h4></h4>
</div>
<div id='table-of-contents' class='slide'>
<h1>Table of Contents</h1>
<div id="text-table-of-contents">
<ul>
<li>1. Why?</li>
<li>2. Concepts</li>
<li>3. Further concepts</li>
<li>4. First-class functions</li>
<li>5. Higher-order functions</li>
<li>6. Higher-order functions (cont.)</li>
<li>7. Pure functions</li>
<li>8. Recursion</li>
<li>9. Partial application</li>
<li>10. Partial application (cont.)</li>
<li>11. Currying</li>
<li>12. Easy Currying</li>
<li>13. Practical Currying</li>
<li>14. Functional composition</li>
<li>15. Functional composition (cont.)</li>
<li>16. pipe</li>
<li>17. Point-free programming</li>
<li>18. Further Resources</li>
</ul>
</div>
</div>

<div id="outline-container-org05e10f9" class="outline-1  slide">
<h1 id="org05e10f9"><span class="section-number-1">1</span> Why?</h1>
<div class="outline-text-1" id="text-1">
<p>
Imperative programming is concerned with <b>how</b>
</p>

<p>
Functional programming is concerned with <b>what</b>
</p>
</div>
</div>

<div id="outline-container-org14fa5d9" class="outline-1  slide">
<h1 id="org14fa5d9"><span class="section-number-1">2</span> Concepts</h1>
<div class="outline-text-1" id="text-2">
<ul class="org-ul">
<li>First-class Functions</li>
<li>Higher-order Functions</li>
<li>Recursion</li>
<li>Pure functions</li>
<li>Currying &amp; Partial Application</li>
</ul>
</div>
</div>

<div id="outline-container-orgd930436" class="outline-1  slide">
<h1 id="orgd930436"><span class="section-number-1">3</span> Further concepts</h1>
<div class="outline-text-1" id="text-3">
<ul class="org-ul">
<li>Lazy Evaluation</li>
<li>Types &amp; Data Structures</li>
<li>Category Theory</li>
</ul>
</div>
</div>

<div id="outline-container-orgbd39891" class="outline-1  slide">
<h1 id="orgbd39891"><span class="section-number-1">4</span> First-class functions</h1>
<div class="outline-text-1" id="text-4">
<ul class="org-ul">
<li>Are values</li>
<li>Have no restriction on their use</li>
<li>Enable the use of callback functions in JavaScript</li>
</ul>

<p>

</p>

<div class="org-src-container">
<pre><code class="src src-js">var fn = function () {
  return 2
}
</code></pre>
</div>
</div>
</div>


<div id="outline-container-org8d1ee8a" class="outline-1  slide">
<h1 id="org8d1ee8a"><span class="section-number-1">5</span> Higher-order functions</h1>
<div class="outline-text-1" id="text-5">
<p>
Functions that operate on other functions are higher-order functions
</p>

<p>

</p>

<div class="org-src-container">
<pre><code class="src src-js">var succ = function (x) {
  return x + 1
}

var arr = [1, 2, 3, 4]

arr.map(succ)
</code></pre>
</div>

<p>
Here, <code>Array.prototype.map</code> is the higher-order function
</p>
</div>
</div>

<div id="outline-container-org3bbf4ac" class="outline-1  slide">
<h1 id="org3bbf4ac"><span class="section-number-1">6</span> Higher-order functions (cont.)</h1>
<div class="outline-text-1" id="text-6">
<p>
Functions that return functions are also higher-order functions
</p>

<p>

</p>

<div class="org-src-container">
<pre><code class="src src-js">function <span style="font-weight: bold;">adder</span> (n) {
  return function (x) {
    return n + x
  }
}

var add1 = adder(1)
</code></pre>
</div>

<p>
<code>adder</code> is a higher-order function
</p>
</div>
</div>

<div id="outline-container-orgfb6eee2" class="outline-1  slide">
<h1 id="orgfb6eee2"><span class="section-number-1">7</span> Pure functions</h1>
<div class="outline-text-1" id="text-7">
<p>
Functions without side-effects
</p>

<div class="org-src-container">
<pre><code class="src src-js">var succ = (x) =&gt; x + 1

console.log(succ(succ(1)))

<span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">// </span><span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">could be optimised away by a compiler, e.g.:</span>

console.log(3)
</code></pre>
</div>
</div>
</div>

<div id="outline-container-org16e9966" class="outline-1  slide">
<h1 id="org16e9966"><span class="section-number-1">8</span> Recursion</h1>
<div class="outline-text-1" id="text-8">
<p>
Functions that call themselves
</p>

<div class="org-src-container">
<pre><code class="src src-js">function <span style="font-weight: bold;">fibonacci</span> (n) {
  switch (n) {
    case 0:
    case 1:
      return 1
    default: 
      return fibonacci(n - 1) + fibonacci(n - 2)
  }
}
</code></pre>
</div>
</div>
</div>

<div id="outline-container-org1b7b0af" class="outline-1  slide">
<h1 id="org1b7b0af"><span class="section-number-1">9</span> Partial application</h1>
<div class="outline-text-1" id="text-9">
<p>
The infamous <code>Function.prototype.bind</code> in JavaScript
</p>

<div class="org-src-container">
<pre><code class="src src-js">function <span style="font-weight: bold;">add</span> (x, y) {
  return x + y
}

var add1 = add.bind(add, 1)

add1(3) <span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">// </span><span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">= 4</span>
</code></pre>
</div>
</div>
</div>

<div id="outline-container-org796e6d3" class="outline-1  slide">
<h1 id="org796e6d3"><span class="section-number-1">10</span> Partial application (cont.)</h1>
<div class="outline-text-1" id="text-10">
<p>
After ES6 introduced arrow functions, partial application has become
more popular
</p>

<div class="org-src-container">
<pre><code class="src src-js">var add = x =&gt; y =&gt; x + y
</code></pre>
</div>
</div>
</div>

<div id="outline-container-orgeb4d67b" class="outline-1  slide">
<h1 id="orgeb4d67b"><span class="section-number-1">11</span> Currying</h1>
<div class="outline-text-1" id="text-11">
<p>
Related to partial application, but more implicit and general
</p>

<p>
Translates <b><i>1</i> function of arity <i>n</i></b> to <b><i>n</i> functions of arity <i>1</i></b>
</p>

<div class="org-src-container">
<pre><code class="src src-js">function <span style="font-weight: bold;">volume</span> (w, d, h) {
  return w * d * h
}

var vol = curry(volume)
vol(10)(20)(30)
<span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">// </span><span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">is strictly equivalent to</span>
volume(10, 20, 30)
</code></pre>
</div>
</div>
</div>

<div id="outline-container-orgdfd8353" class="outline-1  slide">
<h1 id="orgdfd8353"><span class="section-number-1">12</span> Easy Currying</h1>
<div class="outline-text-1" id="text-12">
<p>
In order to make currying (and partial application) easier to use,
move the <b>most important</b> argument to a function to the end:
</p>

<div class="org-src-container">
<pre><code class="src src-js">var badMap = (arr, fn) =&gt; arr.map(fn)
var goodMap = (fn, arr) =&gt; arr.map(fn)
var curriedBadMap = curry(badmap)
var curriedGoodMap = curry(goodMap)

var goodDoubleArray = goodMap(x =&gt; x * 2)
var badDoubleArray = badMap(_, x =&gt; x * 2)
</code></pre>
</div>

<p>
The bad version requires the curry function to support a magic
placeholder argument and doesn't look as clean.
</p>
</div>
</div>

<div id="outline-container-org30accd5" class="outline-1  slide">
<h1 id="org30accd5"><span class="section-number-1">13</span> Practical Currying</h1>
<div class="outline-text-1" id="text-13">
<p>
Currying is not automatic in JavaScript, as in other languages
</p>

<p>
External tools aren't (so far) able to statically analyse curried
functions
</p>

<p>
Solution: Don't expose curried functions
Instead, write functions as if currying were automatic
</p>

<p>
If consumers want to curry, they can.  If they don't, their editor or
language server will show them the arguments
</p>
</div>
</div>

<div id="outline-container-org5f352dc" class="outline-1  slide">
<h1 id="org5f352dc"><span class="section-number-1">14</span> Functional composition</h1>
<div class="outline-text-1" id="text-14">
<p>
Creating functions from other functions
</p>

<p>
Usually provided by <code>compose</code> (right-to-left) and <code>pipe</code> (left-to-right)
</p>

<p>
A very simple definition of <code>compose</code> for only two functions would look like this
</p>

<div class="org-src-container">
<pre><code class="src src-js">function <span style="font-weight: bold;">compose</span> (f, g) {
  return function (...args) {
    return f(g(...args))
  }
}
</code></pre>
</div>
</div>
</div>

<div id="outline-container-org0c2c4f9" class="outline-1  slide">
<h1 id="org0c2c4f9"><span class="section-number-1">15</span> Functional composition (cont.)</h1>
<div class="outline-text-1" id="text-15">
<div class="org-src-container">
<pre><code class="src src-js">var plusOne = x =&gt; x + 1
var timesTwo = x =&gt; x * 2

var plusOneTimesTwo = compose(timesTwo, plusOne)
var timesTwoPlusOne = compose(plusOne, timesTwo)

nextDoubled(3) <span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">// </span><span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">= (3 + 1) * 2 = 8</span>
doubledPlusOne(3) <span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">// </span><span style="color: #b8b8b8; background-color: #f8f8f8; font-style: italic;">= (3 * 2) + 1 = 7</span>
</code></pre>
</div>
</div>
</div>

<div id="outline-container-orgddc61f6" class="outline-1  slide">
<h1 id="orgddc61f6"><span class="section-number-1">16</span> pipe</h1>
<div class="outline-text-1" id="text-16">
<p>
What about <code>pipe</code>?
</p>

<p>
<code>pipe</code> does the same thing, but runs the functions the other way around
</p>

<p>
<code>pipe(f, g)</code> is the same as <code>compose(g, f)</code>
</p>
</div>
</div>

<div id="outline-container-org7426c58" class="outline-1  slide">
<h1 id="org7426c58"><span class="section-number-1">17</span> Point-free programming</h1>
<div class="outline-text-1" id="text-17">
<p>
With currying and higher-order functions, we (often) don't need to declare function arguments
</p>

<div class="org-src-container">
<pre><code class="src src-js">var modulo = a =&gt; b =&gt; b % a
var eq = a =&gt; b =&gt; a === b

var isEven = x =&gt; eq(0)(modulo(2)(x))
var isEvenPointFree = compose(eq(0), modulo(2))
</code></pre>
</div>
</div>
</div>

<div id="outline-container-org8868cfb" class="outline-1  slide">
<h1 id="org8868cfb"><span class="section-number-1">18</span> Further Resources</h1>
<div class="outline-text-1" id="text-18">
<ul class="org-ul">
<li><a href="https://drboolean.gitbooks.io/mostly-adequate-guide/content/">Mostly adequate guide to FP (in javascript)</a></li>
<li><a href="http://ramdajs.com/">Ramda</a>, a general-purpose FP library</li>
<li><a href="https://sanctuary.js.org/">Sanctuary</a>, a JavaScript library for Haskellers</li>
</ul>
</div>
</div>


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