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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge,chrome=1">
<meta name="viewport" content="width=1024, user-scalable=no">
<title>Functional Programming</title>
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<style type="text/css">
.deck-container .slide code {
color: #c00; font-weight: bold;
}
.deck-container .slide h2 code {
color: #888;
}
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opacity: 1;
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<script src="modernizr.custom.js"></script>
</head>
<body class="deck-container">
<section class="slide">
<h1>Functional Programming</h1>
<h2>Scott Sauyet</h2>
</section>
<section class="slide">
<h2>An Overview of Functional Techniques<br/>
(for non FP'ers) using Javascript</h2>
<h3>Agenda</h3>
<ul>
<li>What is Functional Programming?</li>
<li>How Functional Programming Differs From Other Paradigms</li>
<li>Functional Programming in Javascript</li>
<li>Advantages and Disadvantages of Functional Style</li>
<li><del>Interaction of Functional and Object-Oriented Styles</del></li>
</ul>
</section>
<section class="slide">
<h2>What is Functional Programming?</h2>
<div class="slide">
<h3>No Single Definition</h3>
<p>There is no clear, widely-accepted definition of Functional Programming. It is a collection of
related featues which cohere well into a very useful style of programming.</p>
<ul>
<li>Some of these features are readily available in Javascript.</li>
<li>Others can be used if care is taken.</li>
<li>And some are essentially impossible at the language level.</li>
</ul>
</div>
</section>
<section class="slide">
<h2>Chief difference between OOP and FP</h2>
<p>Reg Braithwaite has a <a href="http://raganwald.com/2013/04/08/functional-vs-OOP.html">good description</a>
of the central difference between these two paradigms.</p>
<p class="slide">OO focuses on the <em>differences</em> in the data,
while FP concentrates on <em>consistent data structures</em>.</p>
<div class="slide">
<div style="float:left;width:45%;margin-right:3%;clear:both;">
<h3>Object-Oriented</h3>
<ul>
<li>Data and the operations upon it are tightly coupled</li>
<li>Objects hide their implementation of operations from other objects via their interfaces</li>
<li>The central model for abstraction is the data itself</li>
<li>The central activity is composing new objects and extending existing objects by adding new methods
to them</li>
</ul>
</div>
<div class="slide" style="float:left;width:45%;margin-right:3%;">
<h3>Functional</h3>
<ul>
<li>Data is only loosely coupled to functions</li>
<li>Functions hide their implementation, and the language’s abstractions speak to functions and the way
they are combined or expressed</li>
<li>The central model for abstraction is the function, not the data structure.</li>
<li>The central activity is writing new functions</li>
</ul>
</div>
</div>
</section>
<section class="slide">
<h2>What does this mean in the business environment?</h2>
<p>Is one of these techniques the clear-cut winner in the business world? Is it functional or object-oriented?</p>
<div class="slide">
<h3>Are you sure?</h3>
</div>
<div class="slide">
<br/>
<h3>How much business logic is written like this?</h3>
<div>
<div class="code" mode="plsql" style="display: none;">SELECT orders.order_id, orders.order_date, suppliers.supplier_name
FROM suppliers
RIGHT OUTER JOIN orders
ON suppliers.supplier_id = orders.supplier_id
WHERE orders.order_status = 'INCOMPLETE'
ORDER BY orders.order_date DESC;</div>
</div>
<div class="slide">
<br/>
<p>SQL is not a functional language <em>per se</em> but it shares the important characteristic of being
<strong>declarative</strong>.</p>
</div>
</div>
</section>
<section class="slide">
<h2>Declarative vs Imperative Programming</h2>
<ul>
<li><strong>Imperative:</strong> Describe <em>how</em> to do something.</li>
<li><strong>Declarative:</strong> Describe <em>what</em> you want done.</li>
</ul>
<div class="slide">
<h3>Imperative style</h3>
<div>
<textarea class="code" mode="javascript" style="display: none;" runnable="true">var sumOfSquares = function(list) {
var result = 0;
for (var i = 0; i < list.length; i++) {
result += square(list[i]);
}
return result;
};
console.log(sumOfSquares([2, 3, 5]));</textarea>
</div>
</div>
<br/>
<div class="slide">
<h3>Functional style</h3>
<div>
<textarea class="code" mode="javascript" style="display: none;" runnable="true">var sumOfSquares = pipe(map(square), sum);
console.log(sumOfSquares([2, 3, 5]));</textarea>
</div>
<br/>
<br/>
</div>
</section>
<section class="slide">
<h2>Functional Features in Javascript</h2>
<h3>Easily available in Javascript</h3>
<ul>
<li class="slide">First-class functions</li>
<li class="slide">Lambdas/Anonymous Functions with closures</li>
<li class="slide">Compact, even terse, functions</li>
</ul>
<h3 class="slide">Possible to accomplish in Javascript with some care</h3>
<ul>
<li class="slide">Mostly stateless processing</li>
<li class="slide">Side-effect-free function calls</li>
</ul>
</section>
<section class="slide">
<h2>Not available in current versions Javascript</h2>
<ul>
<li class="slide">Performant recursion through tail call optimization</li>
<li class="slide">Pattern matching (Haskell, Erlang)</li>
<li class="slide">Lazy Evaluation (Miranda, Haskell)</li>
<li class="slide">Homoiconicity (mostly LISP-like languages?)</li>
</ul>
<div class="slide">
<hr/>
<p><del>We will take a look a brief look at these functional programming features of other languages first,</del>
then turn our focus to those things we can actually accomplish in Javascript.</p>
</div>
</section>
<!--
<section class="slide">
<h2>Shared Example</h2>
<p>Following an example from <a href="http://paulbarry.com/articles/2009/08/30/tail-call-optimization">Paul Barry</a>
we will use the example of the chances of winning a lottery. This calculates the odds of choosing the correct
`n` numbers out of the `p` possibilities.</p>
<p>Here is an iterative version of the code:</p>
<div>
<textarea class="code" mode="javascript" style="display: none;" runnable="false">function odds(n, p) {
var acc = 1;
for(var i = 0; i < n; i++) {
acc *= (n - i) / (p - i);
}
return acc;
}
console.log(odds(3, 10)); //=> (3/10) * (2/9) * (1/8) => (1/120) => 0.008333...</textarea>
</div>
<br/>
</section>
<section class="slide">
<h2>Recursive Version</h2>
<p>This is similar, but written recursively:</p>
<div><textarea class="code" mode="javascript" style="display: none;" runnable="false">// Recursive version
function odds(n, p) {
return (n == 0) ? 1 : (n / p) * odds(n - 1, p - 1);
}
console.log(odds(3, 10)); //=> (3/10) * (2/9) * (1/8) => (1/120) => 0.008333...</textarea></div>
<div class="slide">
<br/>
<p>There are many reasons that functional programmers prefer recursion, but one very simple one is that
recursive functions are often much more elegant than their iterative cousins. It's easier to reason
about them.</p>
</div>
<div class="slide">
<p>Unfortunately, they often don't perform as well. All the overhead of creating stack contexts for
function calls tends to add up. But certain kinds of recursive calls <em>can</em> be easily optimized.</p>
</div>
</section>
<section class="slide">
<h2>This code in tail-call formulation:</h2>
<div><textarea class="code" mode="javascript" style="display: none;" runnable="false">var odds = (function(){
var odds1 = function(n, p, acc) {
return (n == 0) ? acc : odds1(n - 1, p - 1, (n / p) * acc);
}
return function(n, p) {
return odds1(n, p, 1)
}
})();</textarea></div>
<div class="slide">
<br/>
<p>Note that the recursive call in <code>odds1</code> is the last statement in its branch of the function.
If this is true for all recursive calls, then the function is <code>tail-recursive</code>, and the
compiler can replace the entire set of nested calls with simple <code>JUMP</code> operations.</p>
</div>
<div class="slide">
<p>Such optimizations are required by many functional languages.</p>
</div>
<div class="slide">
<p>This is slated to become required in the next version of the specification for Javascript, but it won't
be consistently available client-side for some time.</p>
</div>
</section>
<section class="slide">
<h2>Pattern Matching</h2>
<p>Some languages make use of an interesting techniqe to define functions: pattern matching. Rather than use
<code>if-blocks</code> in the body of the function, parameter-matching is used to choose which of a collection
of related functions should be called. Here we can see it in Erlang.</p>
<div class="slide">
<h3>Simple recursive version</h3>
<div><textarea class="code" mode="erlang" style="display: none;">odds(0, _) -> 1;
odds(N, P) -> (N / P) * odds(N - 1, P - 1).</textarea></div>
</div>
<div class="slide">
<br/>
<h3>Tail-call version</h3>
<div><textarea class="code" mode="erlang" style="display: none;">-export([odds/2]).
odds(N, P) -> odds(N, P, 1).
odds(0, _, Acc) -> Acc;
odds(N, P, Acc) -> odds(N - 1, P - 1, (N / P) * Acc).</textarea></div>
</div>
</section>
<section class="slide">
<h2>Lazy Evaluation</h2>
<p>Many of us are most familiar with languages that evaluate their expressions as soon as they're encountered. But
there are some that wait until the last possible instant. This can have some real benefits.</p>
<div class="slide">
<p>We'll switch examples here, as there's not much call to do lazy evaluation for lottery odds. We will follow a
<a href="http://stackoverflow.com/a/284180/1243641">Haskell example</a> from Chris Eidhof</p>
<div><textarea class="code" mode="haskell" style="display: none;">quickSort [] = []
quickSort (x:xs) = quickSort (filter (< x) xs)
++ [x]
++ quickSort (filter (>= x) xs)
minimum ls = head (quickSort ls)</textarea></div>
<br/>
<p>Note that the right half of quicksort is not calculated in finding the minimum. (Also note the pattern matching here too!)</p>
</div>
<p class="slide">One <a href="http://stackoverflow.com/a/2165370/1243641">very interesting comparison</a> is that
lazy evaluation is to the CPU what garbage collection is to memory. The garbage collector allows you to pretend
that you have infinite memory; lazy evaluation allows you to pretend that you have infinite processing power.</p>
<p class="slide">But there are many reasons to like lazy evaluation. It allows you to operate on theoretically
infinite data structures, calculating only those parts you need. And it allows you to define your own
efficient control structures inside the language instead of only at the level of the language syntax.</p>
<br/>
</section>
<section class="slide">
<h2>Homoiconicity</h2>
<div class="slide">
<p>No groaning. At one time you had to learn what <em>polymorphism</em> meant, too!</p>
</div>
<div class="slide">
<h3><em>homoiconicity</em></h3>
<ul>
<li><em>homo</em>: from the Greek <em>homo</em>, same</li>
<li><em>icon</em>: from the Greek <em>eikon</em>, likeness, image, portrait</li>
<li><em>icity</em>: from the Greek <em>icit</em>, extra letters to make words more pretentious</li>
</ul>
</div>
</section>
<section class="slide">
<h2>Homoiconicity in LISP-like languages</h2>
<p>Homoiconicity has to do with the fact that in some languages, programs are written in a format easily
interpreted also as a data structure. In Scheme, the following is just a quoted list of three items,
<code>define</code>, <code>(square x)</code> (itself a list), and <code>(* x x)</code> (another list)</p>
<div>
<div class="code" mode="scheme" style="display: none;">'(define (square x) (* x x))</div>
</div>
<div class="slide">
<br/>
<p>But the following, which looks almost identical, is a function definition:</p>
<div>
<div class="code" mode="scheme" style="display: none;">(define (square x) (* x x))</div>
</div>
</div>
<div class="slide">
<br/>
<p>LISP-like languages can use this feature to build very powerful domain-specific languages. Many would
claim that this is the key feature that has allowed LISP to become the oldest language still in widespread
use.</p>
</div>
</section>
-->
<section class="slide">
<h2>Functional Programming in Javascript</h2>
<p>For the remainder of this talk, we will use the <a href="https://github.com/ramda/ramda">Ramda</a> library that
<a href="https://github.com/buzzdecafe">Michael Hurley</a> and I have been developing. But there are a number
of interesting alternatives available:</p>
<ul>
<li class="slide"><a href="https://github.com/raganwald/allong.es">allong.es</a> is a fairly new functional
combinators and decorators library</li>
<li class="slide"><a href="https://github.com/fogus/lemonad">Lemonad</a> is a general-purpose functional
programming library</li>
<li class="slide"><a href="https://github.com/dtao/lazy.js">Lazy</a> and
<a href="https://github.com/goatslacker/lz">Lz</a> are libraries for lazy list processing</li>
<li class="slide"><a href="http://underscorejs.org/">Underscore</a> and its more performant clone,
<a href="http://lodash.com/">Lo-Dash</a>, are general utility libraries with a number of functional
capabilities</li>
<li class="slide"><a href="https://github.com/eliperelman/fn.js/">fn.js</a> is a very recent general-purpose
functional programming library, similar in ideas to Ramda (though of course not as good.
<img src="img/smiley.png" alt="Smiley" style="display:inline; margin-bottom: 0; height: 24px; width: 24px;"/>)<a/></li>
<li class="slide"><a href="http://osteele.com/sources/javascript/functional/">Functional Javascript</a>,
although perhaps outdated today, was the very first successful functional programming library for
Javascript</li>
<li class="slide" style="margin-top: 1em;">And many, many others</li>
</ul>
</section>
<section class="slide">
<h2>Using Functional Techniques in Javacript</h2>
<p>Converting an imperative Task List application into a functiona version.</p>
<p>We start by fetching something like the following data from the server:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var data = {
result: "SUCCESS",
interfaceVersion: "1.0.3",
requested: "10/17/2013 15:31:20".
lastUpdated: "10/16/2013 10:52:39",
tasks: [
{id: 104, complete: false, priority: "high",
dueDate: "11/29/2013", member: "Scott",
title: "Do something", created: "9/22/2013"},
{id: 105, complete: false, priority: "medium",
dueDate: "11/22/2013", member: "Lena",
title: "Do something else", created: "9/22/2013"},
{id: 107, complete: true, priority: "high",
dueDate: "11/22/2013", member: "Mike",
title: "Fix the foo", created: "9/22/2013"},
{id: 108, complete: false, priority: "low",
dueDate: "11/15/2013", member: "Punam",
title: "Adjust the bar", created: "9/25/2013"},
{id: 110, complete: false, priority: "medium",
dueDate: "11/15/2013", member: "Scott",
title: "Rename everything", created: "10/2/2013"},
{id: 112, complete: true, priority: "high",
dueDate: "11/27/2013", member: "Lena",
title: "Alter all quuxes", created: "10/5/2013"}
// , ...
]
};</textarea></div>
<br/>
</section>
<section class="slide">
<h2>Our Goal</h2>
<div><textarea class="code" mode="javascript" style="display: none;">tasks: [
{id: 104, complete: false, priority: "high",
dueDate: "11/29/2013", member: "Scott",
title: "Do something", created: "9/22/2013"},
{id: 105, complete: false, priority: "medium",
dueDate: "11/22/2013", member: "Lena",
title: "Do something else", created: "9/22/2013"},
// , ...
]</textarea></div>
<br/>
<p>Create a function accepting a `member` parameter that fetches the data, chooses her
incomplete tasks, returns the ids, priorities, titles, and due dates of those tasks, sorted by due date.</p>
<p>Since the fetch is asynchronous, we'll hook everything together with promises. Ignore error-checking for now.</p>
</section>
<section class="slide">
<h2>Sample Imperative Approach</h2>
<div><textarea class="code" mode="javascript" style="display: none;">var getIncompleteTaskSummariesForMember_imperative = function(memberName) {
return fetchData()
.then(function(data) {
return data.tasks;
})
.then(function(tasks) {
var results = [];
for (var i = 0, len = tasks.length; i < len; i++) {
if (tasks[i].member == memberName) {
results.push(tasks[i]);
}
}
return results;
})
.then(function(tasks) {
var results = [];
for (var i = 0, len = tasks.length; i < len; i++) {
if (!tasks[i].complete) {
results.push(tasks[i]);
}
}
return results;
})</textarea></div>
<br/>
<p>(continued on the next slide.)</p>
</section>
<section class="slide">
<h2>Sample Imperative Approach (continued)</h2>
<div><textarea class="code" mode="javascript" style="display: none;">
.then(function(tasks) {
var results = [], task;
for (var i = 0, len = tasks.length; i < len; i++) {
task = tasks[i];
results.push({
id: task.id,
dueDate: task.dueDate,
title: task.title,
priority: task.priority
})
}
return results;
})
.then(function(tasks) {
tasks.sort(function(first, second) {
return first.dueDate - second.dueDate;
});
return tasks;
});
};</textarea></div>
</section>
<section class="slide">
<h2>Similar Object-Oriented Approach</h2>
<div><textarea class="code" mode="javascript" style="display: none;"> // main method
var getIncompleteTaskSummariesForMember_objectOriented = function(memberName) {
return fetchData()
.then(function(data) {
var taskList = new TaskList(data.tasks);
taskList.chooseByMember(memberName);
taskList.chooseByCompletion(false);
var briefTaskList = taskList.getSummaries();
briefTaskList.sort(new TaskListSorter("dueDate"));
return briefTaskList.tasks;
});
};</textarea></div>
<br/>
<p>(continued on the next slide.)</p>
</section>
<section class="slide">
<h2>Object-Oriented Approach (continued)</h2>
<div><textarea class="code" mode="javascript" style="display: none;">var TaskList = (function() {
var TaskList = function(/*Task[]*/ tasks) {
this.tasks = tasks;
};
TaskList.prototype.chooseByMember = function(memberName) {
var results = [];
for (var i = 0, len = this.tasks.length; i < len; i++) {
if (this.tasks[i].member === memberName) {
results.push(this.tasks[i]);
}
}
this.tasks = results;
};
TaskList.prototype.chooseByCompletion = function(completion) {
var results = [];
for (var i = 0, len = this.tasks.length; i < len; i++) {
if (this.tasks[i].complete == completion) {
results.push(this.tasks[i]);
}
}
this.tasks = results;
};</textarea></div>
</section>
<section class="slide">
<h2>Object-Oriented Approach (continued)</h2>
<div><textarea class="code" mode="javascript" style="display: none;">TaskList.prototype.getSummaries = function() {
var results = [], task;
for (var i = 0, len = this.tasks.length; i < len; i++) {
task = this.tasks[i];
results.push({
id: task.id,
dueDate: task.dueDate,
title: task.title,
priority: task.priority
})
}
return new TaskList(results);
};
TaskList.prototype.sort = function(/*TaskListSorter*/ sorter) {
this.tasks.sort(sorter.getSortFunction());
};
return TaskList;
}());</textarea></div>
<br/>
<p>(continued on the next slide.)</p>
</section>
<section class="slide">
<h2>Object-Oriented Approach (continued)</h2>
<div><textarea class="code" mode="javascript" style="display: none;">var TaskListSorter = (function() {
var TaskListSorter = function(propName) {
this.propName = propName;
};
TaskListSorter.prototype.getSortFunction = function() {
var propName = this.propName;
return function(first, second) {
return first[propName] < second[propName] ? -1 :
first[propName] > second[propName] ? +1 : 0;
}
};
return TaskListSorter;
}());</textarea></div>
<br/>
<p>We could continue by defining <code>Task</code> and <code>MinimalTask</code>, but that's just overkill.</p>
<p class="slide">The difference between the imperative and the OO code is mostly organizational. The same code
runs in both; it's just arranged differently. (And it's full of `<code>this</code>` keywords.
<img src="img/smiley.png" alt="Smiley" style="display:inline; margin-bottom: 0; height: 24px; width: 24px;"/>)</p>
<p class="slide">This means that we can focus on the simpler imperative code.</p>
</section>
<section class="slide">
<h2>Converting to Functional Code</h2>
<p>We will now convert this code into concise, readable, functional code, one block at a time, explaining some of
the basic building blocks of functional programming as we go. First up is this little function:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(function(data) {
return data.tasks;
})</textarea></div>
<br/>
<div class="slide">
<h3>Functional Version</h3>
<div><textarea class="code" mode="javascript" style="display: none;">.then(get('tasks'))</textarea></div>
</div>
<div class="slide">
<br/>
<p>So the obvious question, then, is, what is the <code>get</code> function?</p>
</div>
</section>
<section class="slide">
<h2>The <code>get</code> Function</h2>
<p>This is the definition of the <code>get</code> function in the Ramda library:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var get = curry(function(prop, obj) {return obj[prop];});</textarea></div>
<br/>
<div class="slide">
<p>Ignoring the <code>curry</code> wrapper, this is pretty simple. <code>get</code> is a function which accepts
a property name and an object, and returns the property of the object with that name.</p>
</div>
<div class="slide">
<br/>
<p>Our <code>then</code> call needs a function. So <code>curry</code> must be doing something interesting with
this function, which should return an object propery, to instead returning a new function. We need to
take a detour to discuss <code>curry</code>.</p>
</div>
</section>
<section class="slide">
<h2>Curry</h2>
<img src="img/curry.png"/>
<div class="slide">
<br/>
<h3>Very sorry, no delicious spicy food here.</h3>
</div>
</section>
<section class="slide">
<h2>Currying Functions</h2>
<p>Currying is the process of converting functions that take multiple arguments into ones that, when supplied
fewer arguments, return new functions that accept the remaining ones.</p>
<div class="slide">
<div><textarea class="code" mode="javascript" style="display: none;" runnable="true">var add = curry(function(a, b) {return a + b;});
var addFortyTwo = add(42);
console.log(addFortyTwo(10)); //=> 52
console.log(addFortyTwo(7)); // 49</textarea></div>
</div>
</section>
<section class="slide">
<h2>Currying multiple arguments</h2>
<p>Different version of currying work slightly differently. In Ramda, you keep getting back functions until you finally
pass the required number of parameters.</p>
<div><textarea class="code" mode="javascript" style="display: none;" runnable="true">var formatName = curry(function(first, middle, last) {
return first + " " + middle + " " + last;
});
var f = formatName("James"); // returns a function
var g = f("Earl"); // returns a function
g("Jones"); //=> "James Earl Jones"
var h = formatName("James", "Earl"); // returns a function
h("Jones"); //=> "James Earl Jones"
// Note that g and h are equivalent functions
formatName("James", "Earl", "Jones"); //=> "James Earl Jones"</textarea></div>
<div>
<br/>
<p>Some insist that this is not truly currying, but should be called <code>partial application</code>. In any
case, it serves the same role as currying does in a more strongly typed language.</p>
</div>
</section>
<section class="slide">
<h2>Back to <code>get</code></h2>
<p>Remember the definition of <code>get</code>:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var get = curry(function(prop, obj) {return obj[prop];});</textarea></div>
<div class="slide">
<br/>
<p>Now that we understand <code>curry</code>, we can see this is equivalent to <sup><code>*</code></sup>:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var get = function(prop) {
return function(obj) {
return obj[prop];
};
};</textarea></div>
</div>
<div class="slide">
<br/>
<p>And that means that our new <code>get('tasks')</code> is equivalent to</p>
<div><textarea class="code" mode="javascript" style="display: none;">function(obj) {
return obj['tasks'];
}</textarea></div>
</div>
<br/>
<div class="slide"><sup><code>*</code></sup> Technically not quite equivalent. Anyone see why?</div>
</section>
<section class="slide">
<h2>Filtering</h2>
<p>So far, we've been able to replace this block:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(function(data) {
return data.tasks;
})</textarea></div>
<br/>
<p>with this one:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(get('tasks'))</textarea></div>
<div class="slide">
<br/>
<p>The next block to replace looks like this:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(function(tasks) {
var results = [];
for (var i = 0, len = tasks.length; i < len; i++) {
if (tasks[i].member == memberName) {
results.push(tasks[i]);
}
}
return results;
})</textarea></div>
</div>
<div class="slide">
<br/>
<p>We're running a filter on the input list, keeping those that have the correct
<code>member</code>. Let's do this functionally.</p>
</div>
</section>
<section class="slide">
<h2>Functional Filtering</h2>
<p><code>filter</code> is a very simple function. You pass it a predicate function, which is used to determine if
an individual item should be included, and a list of items. It returns you a new list of those that list
items that made it. </p>
<p>Remembering that the <code>then</code> block will pass the list of tasks to us, we really want to call filter
with a predicate, getting back a curried function that will accept a list.</p>
<div class = slide>
<p>Here's a first pass:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(filter(function(task) {
return task.member == memberName;
}))</textarea></div>
<br/>
<p>(Remember that <code>memberName</code> was a parameter to our original function.)</p>
</div>
</section>
<section class="slide">
<h2>Focused Code</h2>
<p>So, for one thing, we've reduced the weight of our custom code:</p>
<div class="slide">
<h3>Original code</h3>
<div><textarea class="code" mode="javascript" style="display: none;">.then(function(tasks) {
var results = [];
for (var i = 0, len = tasks.length; i < len; i++) {
if (tasks[i].member == memberName) {
results.push(tasks[i]);
}
}
return results;
})</textarea></div>
</div>
<div class="slide">
<br/>
<h3>Functional version</h3>
<div><textarea class="code" mode="javascript" style="display: none;">.then(filter(function(task) {
return task.member == memberName;
}))</textarea></div>
</div>
<div class="slide">
<br/>
<p>But we've done something more important too: We've moved the focus from iteration and updating the state
of a local collection to the real point of this block: choosing the tasks with the proper
<code>member</code> property.</p>
<p>An important feature of functional programming is that it makes it easier to focus on the logical issues.</p>
</div>
</section>
<section class="slide">
<h2>Rejecting elements</h2>
<p>The next block is similar, except that instead of using <code>filter</code>, we will use <code>reject</code>,
which behaves exactly the same except that it chooses those members of the list that <strong>don't</strong>
match the predicate. We replace this code:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(function(tasks) {
var results = [];
for (var i = 0, len = tasks.length; i < len; i++) {
if (!tasks[i].complete) {
results.push(tasks[i]);
}
}
return results;
})</textarea></div>
<br/>
<p>with this:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(reject(function(task) {
return task.complete === true;
)))</textarea></div>
</section>
<section class="slide">
<h2>Refactoring... already</h2>
<p>A reasonable question would be why with didn't do this instead, which would work equally well:</p>
<div><textarea class="code" mode="javascript" style="display: none;">.then(filter(function(task) {
return task.complete !== true;
))</textarea></div>
<div class="slide">
<br/>
<p>Because these two blocks are so similar, we have an early chance to refactor our code:</p>
<div><textarea class="code" mode="javascript" style="display: none;">function(task) {
return task.member == memberName;
}</textarea></div>
<br/>
<div><textarea class="code" mode="javascript" style="display: none;">function(task) {
return task.complete === true;
)</textarea></div>
</div>
<br/>
<div class="slide">
<p>Both of these functions accept an object and return a boolean that describes whether a particular
property of the object has a given value. Perhaps a good name for a function that generates such
functions would be <code>propEq</code>. Let's implement that.</p>
</div>
</section>
<section class="slide">
<h2>Implementing <code>propEq</code></h2>
<h3>Simplest Approach</h3>
<div><textarea class="code" mode="javascript" style="display: none;">var propEq = function(prop, val) {
return function(obj) {
return obj[prop] === val;
};
}</textarea></div>
<div class="slide">
<br/>
<p>This works, and we could leave it there, but we're going to take another detour into a popular style of
functional programming known as <code>points-free</code> coding.</p>
</div>
<div class="slide">
<p>The name has nothing to do with <code>'.'</code> characters. It derives from mathematics and has something
to do with homomorphisms on topological spaces.</p>
<br/>
</div>
<div class="slide">
<p>Don't worry…</p>
</div>
<div class="slide">
<p>This won't be on the test.</p>
</div>
</section>
<section class="slide">
<h2>Points-free definitions</h2>
<p>With the functions <code>add</code> (which adds two numbers) and <code>reduce</code> (which runs the supplied
function against an accumulator and each element of the list, feeding the result of each call into the next one
and returning the final result), we can easily define a <code>sum</code> function like this:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var sum = function(list) {
return reduce(add, 0, list);
};</textarea></div>
<br/>
<div class="slide">
<br/>
<p>Because of the automatic currying, though, the following is entirely equivalent:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var sum = reduce(add, 0);</textarea></div>
</div>
<div class="slide">
<br/>
<p>This is the points-free style, defining functions without ever making direct reference to their
arguments.</p>
</div>
<div class="slide">
<p>There are plenty of <!--<del>arguments in favor of it</del>, plenty of <del>points to support it</del>, -->
reasons to like it, but the most important one might just be the simplicity. There is a great deal to be
said for elegant, readable code.</p>
</div>
</section>
<section class="slide">
<h2>A points-free version of <code>propEq</code></h2>
<p>Can we redefine the following in a points-free style?</p>
<div><textarea class="code" mode="javascript" style="display: none;">var propEq = function(prop, val) {
return function(obj) {
return obj[prop] === val;
};
}</textarea></div>
<div class="slide">
<br/>
<p>Here's a version that is closer to points-free, removing the direct reference to <code>obj</code>:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var propEq = function(prop, val) {
return compose(eq(val), get(prop));
}</textarea></div>
<div class="slide">
<br/>
<p>Huh? What? <code>compose</code>? <code>eq</code>?</p>
</div>
<div class="slide">
<code>eq</code> is easy: like all good functions of multiple parameters, it's curried, and it simply reports
whether its two arguments are equal. So <code>eq(val)</code> is a function which reports whether its
parameter has the same value as does <code>val</code>. But now we need to discuss <code>compose</code>.
</div>
</div>
</section>
<section class="slide">
<h2>Functional composition</h2>
<p>I have <a href="http://scott.sauyet.com/Javascript/Talk/Compose/">another short talk</a> dedicated entirely to
techniques of functional composition. This is a very brief overview:</p>
<p>In mathematics <code>f ∘ g</code> (pronounced "f composed with g") is the function that given <code>x</code>,
returns <code>f(g(x))</code>.</p>
<p>So if we follow the mathematical model <code>compose(add1, square)(x)</code> should equal <code>add1(square(x))</code>.</p>
<br/>
<div class="slide">
<h3>Simplest Implementation</h3>
<div><textarea class="code" mode="javascript" style="display: none;">var compose = function(f, g) {
return function(x) {
return f(g(x));
};
};</textarea></div>
<br/>
<p>Note that Ramda also defines <code>pipe</code>, which does much the same thing, but runs the functions
in the opposite order. So <code>pipe(add1, square)(x)</code> equals <code>square(add1(x))</code>. Both
styles have their uses.</p>
</div>
</section>
<section class="slide">
<h2>Back to <code>propEq</code></h2>
<p>So now this definition makes sense:</p>
<div><textarea class="code" mode="javascript" style="display: none;">var propEq = function(prop, val) {
return pipe(get(prop), eq(val));
}</textarea></div>
<br/>
<p>Note the switch from <code>compose</code> to <code>pipe</code>.</p>
<div class="slide">
<p>This gives us a further way to clean it up, and make it entirely points-free, using a useful feature of Ramda we
haven't seen implemented in other libraries, which we call <code>use-over</code>. Used like
<div><textarea class="code" mode="javascript" style="display: none;">use(func).over(transformer1, ... transformerN)</textarea></div><br/>
this returns a function which accepts N parameters,
feeds them to the respective transformers, and then calls <code>func</code> using the results of all these.
This gives us the final version of propEq:</p>
</div>
<div class="slide">
<div><textarea class="code" mode="javascript" style="display: none;">var propEq = use(pipe).over(get, eq);</textarea></div>
</div>
</section>