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<!DOCTYPE html>
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	<title>The Typeclassopedia</title>

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<section class="slide" style="font-size: 100%">
	<h1>
    <div>The Typeclassopedia</div>
  	<div style="font-size: 50%">John Kodumal, Atlassian</div>
  	<div style="font-size: 50%">jkodumal@atlassian</div>
	<div style="font-size: 50%"><br/></div>
	<div style="font-size: 25%">Use left and right arrows to navigate.</div>
	<div style="font-size: 25%">Japanese translation by Shingo Omura is <a href="index-ja.html">here<a>.</div>
  </h1>
</section>


<section class="slide">
	<h2>Typeclasses: Open-world interfaces</h2>
	<div class="slide">
		<p>Typeclasses are akin to Java interfaces, but more flexible.</p>
	</div>
	<div class="slide">
		<p>We declare them in scala with traits:</p>
<pre class="scala">
trait Show[A] {
	def shows(a: A): String
}
</pre>	
		<p>Note the subtle difference compared to the mix-in "interface" style:
<pre class="scala">
trait Show {
	def shows(): String
}	
</pre>
	</div>		
	<div class="slide">
		<p>We also need a way to add a type to the class:</p>
<pre class="scala">
implicit val IntShow = new Show[Int] {
	def shows(a: Int) = a.toString
}
</pre>
</section>

<section class="slide">
	<h2>Using a typeclass</h2>
	<div class="slide">
		<p>We use scala implicits:</p>
<pre>
def shows[A](a : A)(implicit shower: Show[A]) = shower.shows(a)
</pre>
	</div>
	<div class="slide">
		<p>Or alternatively, using <em>context bounds</em>:</p>
<pre class="scala">
def shows[A : Show](a : A) = implicitly[Show[A]].shows(a)
</pre>
		<p>(implicitly is part of predef: <code>def implicitly[A](implicit a : A) = a</code>)</p>
	</div>
	<div class="slide">
		<p>Scalaz pimps a set of common typeclasses for us, so we can just do:</p>
<pre class="scala">
3.shows // must have a "Show[Int]" instance in scope, or will fail to type check
</pre>
	</div>	
</section>

<section class="slide">
	<h2>The Open-World Assumption</h2>
	<p>What have we gained?</p>
	<ul>
		<div class="slide">
			<li>We can declare instances <em>outside</em> of the types themselves</li>
			<ul>
				<li> <code>Int</code> knows nothing about <code>Show</code>
			</ul>
		</div>
		<li class="slide">This is the <em>open world assumption</em></li>
		<div class="slide">
			<li>In Scala, we can <em>override</em> the typeclass instance by putting a new one in scope:
<pre class="scala">
def unaryShows(x : Int) : String = { 
	implicit val AltIntShow = new Show[Int] {
		def shows(i : Int) = (1 to i) map(_ => "|") mkString
	}
	shows(x)
}
println shows(5)      // prints '5'
println unaryShows(5) // prints '|||||'	
</pre>
		</div>
	</ul>
</section>

<section class="slide">
	<h2>Typeclasses <code>/==</code> Subtype polymorphism</h2>
	<p>What we <em>can't</em> do with Scala's typeclasses (<a href="https://gist.github.com/2026129">not easily at least</a>):
<pre class="java">
interface Show {
	String show();
}

class Foo extends Show {
	public String show() {
		return "a foo";
	}
}

class Bar extends Show {
	public String show() {
		return "a bar";
	}
}

List[Show] showList = new ArrayList(new Foo(), new Bar(), new Foo());

for (String s : showList) {
	System.out.println(s.show);
}
</pre> 
</section>

<section class="slide">
	<h2>The Typeclassopedia</h2>
	<img src="images/typeclassopedia.png" style="float: right; width: 100%; margin-left: 20px;">
	<p>A set of interrelated typeclasses that have proven handy for structuring functional code:
	<ul>
		<li class="slide"><em>Functional design patterns</em></li>
		<li class="slide">There's mostly* nothing magical about these typeclasses</li>
		<li class="slide">In scala, they're provided by a library called scalaz (pronounced scala-zed)</li>
		<li class="slide">These examples are based on <a href="https://github.com/scalaz/scalaz/">scalaz 7.0.0</a></li>
		<li class="slide">This is how the hierarchy is defined in Haskell--- it isn't perfect</li>
	</ul>
	<p class="slide">We'll cover functors, applicative functors, monads (and monad transformers), foldable, and traversable</p>
	<p class="slide" style="font-size: 60%">*one exception: <code>for</code> comprehensions and monads</p>
</section>

<section class="slide">
	<h2>Introducing Functor</h2>
	<div class="slide">
	<p>A functor is simply something you can map over. It defines a single method:</p>
<pre class="scala">
trait Functor[F[_]] {
	def map[A,B](fa: F[A])(f: A => B) : F[B]
}
</pre>
	</div>
	<div class="slide">
		<p>An instance for <code>Option</code>:</p>
<pre class="scala">
implicit val OptionFunctor = new Functor[Option] {
	def map[A, B](fa: Option[A])(f: A => B) : Option[B] = 
		fa match {
			case Some(a) => Some(f(a))
			case None => None
		}
}
</pre>
	</div>
	<div class="slide">
		<p>In Haskell, the arguments are reversed:</p>
<pre class="haskell">
class Functor f where  
    fmap :: (a -> b) -> f a -> f b	
</pre>
	</div>
	<div class="slide">
		<p>In the Haskell version, we see how <code>f</code> is <em>lifted</em> into the functor context.</p>
	</div>
</section>

<section class="slide">
	<h2>Fun with Functions as Functors</h2>
	<p>"Thing you can map over" leads to obvious examples:</p>
	<ul>
		<li class="slide">Lists</li>
		<li class="slide">Trees</li>
		<li class="slide">Maps</li>
	</ul>
	<div class="slide">
		<p>Let's look at something less obvious: Functions!</p>
<pre>
implicit def FunctionFunctor[R] = new Functor[({type l[a] = R=>a})#l] {
	def map[A, B](fa: R => A)(f: A => B) : R => B = (x => f(fa(x)))
}
</pre>
	</div>
	<h6 class="slide">Pop quiz: I defined map in a slightly funky way. What simple concept is this expressing?</h6>
	<div class="slide">
		<p>Again, the Haskell version makes this much clearer. Start with <code>fmap</code>:</p>
<pre class="haskell">
    fmap :: (a -> b) -> f a -> f b	
</pre>
	</div>
	<div class="slide">
		<p>Substitute <code>r -> a</code> for <code>f a</code></p>
<pre class="haskell">
    fmap :: (a -> b) -> (r -> a) -> (r -> b)	
</pre>
	</div>
	<p class="slide">Aside: This is where the "box" analogy starts falling apart, and we start throwing around vague terms like "computational context"...</p>
</section>

<section class="slide">
	<h2>Functor Laws</h2>
	<p>There are some additional constraints on Functors that make them "behave appropriately":</p>
	<ul>
		<div class="slide">
			<li><code>map(fa)(id) === fa</code></li> (Mapping over the identity function produces the original functor)
		</div>
		<div class="slide">	
			<li><code>map(fa)(f compose g) === map(map(fa)(g))(f)</code></li> (Mapping over a composed function is the same as mapping over each function)
		</div>
	</ul>
	<p class="slide">These laws prevent ridiculous definitions of <code>map</code>, and make it possible to predict how functions behave.</p>
	<p class="slide">Note that these laws are <strong>not</strong> enforced by the type system.
</section>

<section class="slide">
	<h2>Aside: Parametricity</h2>
	<blockquote class="slide">
		<p>With great weakness comes great power...</p>
		<cite>Ben "Pierce" Parker</cite>
	</blockquote>
	<blockquote class="slide">
		<p>Mo' power, mo' problems...</p>
		<cite>Biggie Smalls</cite>
	</blockquote>
	<div class="slide">
<pre class="scala">
def fun[A](a : A) : A = ...
</pre>
	<p>What is <code>fun</code>?</p>
	</div>
	<div class="slide">
<pre class="scala">
val x : List[Int] = ...
def foo[F[_] : Functor](fs : F[Int]) = ...
</pre>
	<p>What is <code>foo(x).length?</code><p>
	</div>
</section>

<section class="slide">
	<h2>Introducing Applicative Functor</h2>
	<p>You can get pretty far on <code>map</code> alone... but sometimes you need more.</p>
	<div class="slide">
		<p>Consider the following:</p>
<pre class="scala">
def parse(s : String) : Option[Int] = ...
</pre>
	</div>
	<div class="slide">
		<p>Let's try to add two parsed integers with <code>map</code>:</p>
<pre class="scala">
parse("3").map((x: Int) => (y: Int) =>  x + y) // ???
</pre>
	</div>
	<div class="slide">
		<p>We're left with an <code>Option[Int => Int]</code>... but what can we do with that?
	</div>
	<h5 class="slide">We'll need a more powerful <code>map</code>-like operation</h5>
</section>

<section class="slide">
	<h2>Applicative Functors</h2>
	<div class="slide">
		<p>Let's add the power we need to <code>Functor</code>:
<pre class="scala">
trait Applicative[F[_]] extends Functor {
	def <*>[A, B](fa: F[A])(f : F[A => B]) : F[B]
	// ... there's more to Applicative, to be shown shortly
}
</pre>
	</div>
	<div class="slide">
		<p>Which makes our previous example look something like:</p>
<pre class="scala">
(parse("3")).<*>(parse("Nope").map(x => (y : Int) => x + y))
</pre>
		<p style="font-size: 60%">(Yuck)</p>
	</div>
</section>

<section class="slide">
	<h2>Applicative Syntax</h2>
	<div>
		<p>In Haskell, things look a little more obvious: </p>
<pre class="haskell">
(+) <$> parse("3") <*> parse("Nope") 
</pre>	
	</div>
	<div class="slide">
		<p>Generalizing a bit, we go from calling a pure function: </p>
<pre class="haskell">
f x y z
</pre>
	</div>
	<div class="slide">
		<p>To calling a function on "effectful" arguments: </p>
<pre class="haskell">
f <$> x <*> y <*> z
</pre>
	</div>
	<div class="slide">
		<p>Scalaz's <code>ApplicativeBuilder</code> improves the situation somewhat:</p>
<pre class="scala">
(parse("3") |@| parse("Nope"))(_ + _)
</pre>
	</div>	
</section>

<section class="slide">
	<p style="font-size: 2500%">|@|</p>
</section>

<section class="slide">
	<img src="images/macaulay.jpg" style="float: right; width: 100%;">
	<h3>(The Macaulay Culkin Function)</h3>
</section>

<section class="slide">
	<h2>More on Applicatives</h2>
	<div class="slide">
		<p>What if some of the arguments aren't <code>Option</code>?
<pre class="scala">
	(parse("3") |@| 4 /* uh oh, no macaulay for my culkin */) (_ + _)
</pre>
	</div>	
	<div class="slide">
		<p>One small addition to our type class:</p>
<pre class="scala">
trait Applicative[F[_]] extends Functor {
	def <*>[A, B](fa: F[A])(f : F[A => B]) : F[B]
	<strong>def point[A](a : A): F[A]</strong>
}
</pre>
	<h6 class="slide">Exercise: Define the applicative instance for <code>Option</code></h6>
	</div>
	<div class="slide">
	<p>With <code>point</code>, we can express the above example as:</p>
<pre class="scala">
	(parse("3") |@| 4.point[Option]) (_ + _)
</pre>
	</div>
</section>

<!--
<section class="slide">
	<p style="font-size: 2250%"><|*|></p>
</section>

<section class="slide">
	<img src="images/mr-incredible.jpg" style="float: right; width: 100%;">
	<h3>(Asterisks Man, Arms Akimbo)</h3>
</section>
-->

<section class="slide">
	<h2>A Tutorial on Monads</h2>
	<div class="slide">
		<img src="images/ackbar.jpeg" style="width=100%">
	</div>
	<div class="slide">
		<a href="http://byorgey.wordpress.com/2009/01/12/abstraction-intuition-and-the-monad-tutorial-fallacy/"><h4>Yes, it's a trap</h4></a>
	</div>
</section>

<section class="slide">
	<h2>Introducing Monads</h2>
	<p>As before, we'll introduce a new task and see what goes wrong...</p>
	<div class="slide">
		<p>Take our <code>parse</code> example again:</p>
<pre class="scala">
def parse(s : String) : Option[Int] = ...
</pre>
	</div>
	<div class="slide">
		<p>What if our input <code>String</code> was an optional query parameter?
<pre class="scala">
val x : Option[String] = params.get("x")
</pre>
		<p>We've got an <code>Option[String]</code> and a function <code>String => Option[Int]</code></p>
	</div>
	<div class="slide">
		<p>When all you have is a <code>map</code> hammer...</p>
<pre class="scala">
x.map(parse) // Option[Option[Int]] ??
</pre>
	</div>	
</section>

<section class="slide">
	<h2>Monads</h2>
	<div class="slide">
		<p>Let's add the power we need to <code>Applicative</code>:
<pre class="scala">
trait Monad[F[_]] extends Applicative {
	def >>=[A, B](fa: F[A])(f : A => F[B]) : F[B]
}
</pre>
	<p>This is pronounced <code>bind</code></p>
	</div>
	<div class="slide">
		<p>It makes our previous example look something like:</p>
<pre class="scala">
params.get("x") >>= parse // Option[Int]
</pre>
	</div>
	<div class="slide">
		<p>A more complete web-based calculator example:</p>
<pre class="scala">
params.get("x") >>= parse >>= (x => (params.get("y") >>= parse) map (_ + x) )
</pre>
<p style="font-size: 60%">(Yuck)</p>
	</div>
</section>

<section class="slide">
	<h2>Syntactic Sugar to the Rescue</h2>
	<div class="slide">
		<p>The last example hints at a fairly typical pattern: </p>
<pre class="scala">
monadicX >>= (x => monadicY >>= (y => monadicZ map (z => x+y+z))) 
</pre>
		<p>We have nested <code>bind</code>'s with a final <code>map</code></p>
	</div>
	<div class="slide">
		<p>Reformatting it a bit: </p>
<pre class="scala">
monadicX >>= (x => 
monadicY >>= (y => 
monadicZ map (z => 
   x+y+z
))) 
</pre>
	</div>
	<div class="slide">
		<p>With Scala <code>for</code> comprehensions, this becomes:
<pre class="scala">
for {
	x <- monadicX
	y <- monadicY
	z <- monadicZ
} yield x + y + z
</pre>
	<p style="font-size:60%">(Much nicer!)</p>
	</div>

</section>

<section class="slide">
	<h2>One more motivating example</h2>
	<div class="slide">
	<p>Recall that there is a functor instance for functions:</p>
	<pre>
def map[A, B](fa: R => A)(f: A => B) : R => B = (x => f(fa(x)))
	</pre>
	</div>
	<div class="slide">
		<p>What about a monad instance?</p>	
		<pre>
implicit def FunctionMonad[R] = new Monad[({type l[a] = R=>a})#l] {
	def flatMap[A, B](fa: R => A)(f: A => R => B) : R => B = (x => f(fa(x))(x))
	...
}
		</pre>
	</div>
	<div class="slide">
		<p>This is commonly referred to as the <code>Reader</code> monad. Here's why:</p>
		<pre>
val f = for {
	t1 <- ((_:Int) + 2)
  t2 <- ((_:Int) - 1)
} yield t1 * t2
		</pre>
	<p>Think of <code>Reader</code> as a computation awaiting a <em>read-only environment</em> (the argument) to be supplied.</p>
	</div>	
</section>

<section class="slide">
	<h2>Monad as Embedded Language</h2>
	<div class="slide">
		<p>Here's a <a href="http://en.wikibooks.org/wiki/Haskell/Understanding_monads">stab</a> at providing an intuition for monads. Consider:</p>
<pre class="scala">
for {
	x <- monadicX
	y <- monadicY
} yield x + y
</pre>
		<p>Q: What does this code do?</p>
	</div>
	<div class="slide">
		<p>A: Depends on the monad.</p>
	</div>
	<div class="slide">
		<p>A monadic <code>for</code> comprehension is an embedded programming language with semantics defined by the monad:</p>
	</div>
	<div class="slide">
		<table style="width: 100%">
			<tr>
				<th align="left">Monad</th>
				<th align="left">Semantics</th>
			</tr>
			<tr class="slide">
				<td><code>Option</code></td>
				<td>Anonymous exceptions</td>
			</tr>
			<tr class="slide">
				<td><code>Reader</code></td>
				<td>Read-only environment</td>
			</tr>
			<tr class="slide">
				<td><code>Validation</code></td>
				<td>Descriptive exceptions</td>
			</tr>
			<tr class="slide">
				<td><code>List</code></td>
				<td>Nondeterministic computation</td>
			</tr>
		</table>
	</div>
</section>

<section class="slide">
	<h2>Languages with Multiple Effects</h2>
	<p>In the real world, we have to deal with multiple monads (effects):</p>
	<ul>
		<li class="slide">A data store takes a DB configuration (<code>Reader</code>), and the connection may fail (<code>Validation</code>)</li>
		<li class="slide">A computation may be asynchronous (<code>Future</code>), and may produce multiple results (<code>List</code>)</li>
		<li class="slide">A remote call may fail if the service is down (<code>Validation</code>), and we want to log responses (<code>Writer</code>)</li>
	</ul>
	<div class="slide">
	<p>Let's see why working with stacked monads gets ugly. We'll add a read-only environment to our calculator:</p>
<pre class="scala">
def lookup(s: String)(env: Map[String, Int]) = env.get(s) orElse parse(s)
</pre>
	</div>
	<div class="slide">
		<p>Now we'll add two numbers, using <code>Reader</code> to avoid repeating the <code>env</code> argument:</p>		
<pre class="scala">
for {
 xOpt <- lookup("3") _
 yOpt <- lookup("y") _
} yield (for {
          x <- xOpt
          y <- yOpt
         } yield x + y)
</pre>
	<p style="font-size: 60%">(Yuck)</p>
	</div>
</section>

<section class="slide">
	<img src="images/turtle.jpg" style="float: right; width: 100%;">
	<h3>...all the way down</h3>
</section>

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