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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta name="generator" content="pandoc" />
<meta name="author" content="Mark Lentczner" />
<meta name="date" content="2011-10-14" />
<title>Haskell Amuse Bouche</title>
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<body>
<div class="slide titlepage">
<h1 class="title">Haskell Amuse-Bouche</h1>
<p class="author">
Mark Lentczner<br/><code>mzero﹫google·com</code><br/>
</p>
<p class="date">2011-10-14</p>
</div>
<div class="section slide level1" id="haskell-is-scary">
<h1>Haskell is Scary</h1>
<ul>
<li>Oh noes! Where's my state?</li>
<li>Hey, I don't want my program to be lazy!</li>
<li>Yo, PHP don't need no templates or combinators...</li>
<li>Uhm, I thought dynamic languages were better?</li>
<li>MONADS!</li>
</ul>
</div>
<div class="section slide level1" id="haskell-is-scary-cool">
<h1>Haskell is Scary Cool</h1>
<ul>
<li>Functional</li>
<li>Lazy</li>
<li>Higher order functions</li>
<li>Type inference</li>
<li><em>...shhhh: Monads</em></li>
</ul>
</div>
<div class="section slide level1" id="why-i-got-hooked">
<h1>Why I got hooked</h1>
<ul>
<li>It is a new way to thinking about programming.</li>
<li>It twists the brain in delightful ways.</li>
<li>It is very expressive, yet concise and clear.</li>
<li>It is beautiful.</li>
</ul>
</div>
<div class="section slide level1" id="warning">
<h1>Warning</h1>
<ul>
<li><p>I have a lot of code to show you</p></li>
<li><p>It's gonna look like crazy-moon language</p></li>
<li><p>Be brave</p></li>
</ul>
</div>
<div class="section slide level1" id="wanna-play-along">
<h1>Wanna play along?</h1>
<p>The slides and code are here:</p>
<p><code>https://github.com/mzero/haskell-amuse-bouche</code></p>
</div>
<div class="section slide level1" id="something-familiar">
<h1>Something Familiar</h1>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="kw">cat</span> poem <span class="kw">|</span> <span class="kw">sort</span></code></pre>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="kw">cat</span> poem <span class="kw">|</span> <span class="kw">rev</span> <span class="kw">|</span> <span class="kw">head</span></code></pre>
<pre class="sourceCode bash"><code class="sourceCode bash"><span class="kw">cat</span> poem <span class="kw">|</span> <span class="kw">tr</span> a-z A-Z <span class="kw">|</span> <span class="kw">sed</span> -e <span class="st">'s/$/!!!/'</span></code></pre>
</div>
<div class="section slide level1" id="what-do-they-all-do">
<h1>What do they all do?</h1>
<ul>
<li><p>Take input</p></li>
<li><p>Process the input.</p></li>
<li><p>Produce output as soon as they're able.</p></li>
<li><p>Don't modify any state.</p></li>
<li><p>In short, they are functional, pure, and lazy.</p></li>
</ul>
</div>
<div class="section slide level1" id="lets-write-that-in-haskell">
<h1>Let's write that in Haskell</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell">main <span class="fu">=</span> <span class="fu">readFile</span> <span class="st">"poem"</span> <span class="fu">>>=</span> <span class="fu">putStr</span> <span class="fu">.</span> process
process t <span class="fu">=</span> <span class="fu">unlines</span> (<span class="fu">sort</span> (<span class="fu">lines</span> t))</code></pre>
<p>Put that in a file named <code>Part1.hs</code> and then at the shell:</p>
<pre class="sourceCode bash"><code class="sourceCode bash">runhaskell Part1.hs</code></pre>
</div>
<div class="section slide level1" id="run-it">
<h1>Run it</h1>
<p>Original poem:</p>
<pre><code>occasional clouds
one gets a rest
from moon-viewing</code></pre>
<p>Program output:</p>
<pre><code>from moon-viewing
occasional clouds
one gets a rest</code></pre>
</div>
<div class="section slide level1" id="ignoring-the-main-behind-the-curtain...">
<h1>Ignoring the <code>main</code> behind the curtain...</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell">process t <span class="fu">=</span> <span class="fu">unlines</span> (<span class="fu">sort</span> (<span class="fu">lines</span> t))</code></pre>
<p>Remember <strong>f(g(x)) = (f⋅g)(x)</strong> from high school algebra?</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">process' t <span class="fu">=</span> (<span class="fu">unlines</span> <span class="fu">.</span> <span class="fu">sort</span> <span class="fu">.</span> <span class="fu">lines</span>) t</code></pre>
<p>And algebraic simplificiation works here too:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">process'' <span class="fu">=</span> <span class="fu">unlines</span> <span class="fu">.</span> <span class="fu">sort</span> <span class="fu">.</span> <span class="fu">lines</span></code></pre>
</div>
<div class="section slide level1" id="we-could-code-some-other-common-ones">
<h1>We could code some other common ones:</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell">sortLines <span class="fu">=</span> <span class="fu">unlines</span> <span class="fu">.</span> <span class="fu">sort</span> <span class="fu">.</span> <span class="fu">lines</span>
reverseLines <span class="fu">=</span> <span class="fu">unlines</span> <span class="fu">.</span> <span class="fu">reverse</span> <span class="fu">.</span> <span class="fu">lines</span>
firstTwoLines <span class="fu">=</span> <span class="fu">unlines</span> <span class="fu">.</span> <span class="fu">take</span> <span class="dv">2</span> <span class="fu">.</span> <span class="fu">lines</span></code></pre>
<p>Anyone spot a pattern?</p>
</div>
<div class="section slide level1" id="we-can-factor-it-out">
<h1>We can factor it out!</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell">byLines f <span class="fu">=</span> <span class="fu">unlines</span> <span class="fu">.</span> f <span class="fu">.</span> <span class="fu">lines</span>
sortLines' <span class="fu">=</span> byLines <span class="fu">sort</span>
reverseLines' <span class="fu">=</span> byLines <span class="fu">reverse</span>
firstTwoLines' <span class="fu">=</span> byLines (<span class="fu">take</span> <span class="dv">2</span>)</code></pre>
</div>
<div class="section slide level1" id="what-if-we-want-to-modify-the-lines">
<h1>What if we want to modify the lines?</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell">indent s <span class="fu">=</span> <span class="st">" "</span> <span class="fu">++</span> s</code></pre>
<p>and then, obviously:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">indentLines <span class="fu">=</span> byLines indent</code></pre>
</div>
<div class="section slide level1" id="boom">
<h1>BOOM!</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell">indentLines <span class="fu">=</span> byLines indent</code></pre>
<p>doesn't compile:</p>
<pre><code> Couldn't match expected type `[Char]' with actual type `Char'
Expected type: [String] -> [String]
Actual type: String -> String
In the first argument of `byLines', namely `indent'
In the expression: byLines indent</code></pre>
</div>
<div class="section slide level1" id="how-did-it-work-before">
<h1>How did it work before?</h1>
<p>In this code</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">sortLines' <span class="fu">=</span> byLines <span class="fu">sort</span>
reverseLines' <span class="fu">=</span> byLines <span class="fu">reverse</span>
firstTwoLines' <span class="fu">=</span> byLines (<span class="fu">take</span> <span class="dv">2</span>)</code></pre>
<p>We apply <code>byLines</code> to arguments with these types</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="fu">sort</span><span class="ot"> ::</span> [<span class="dt">String</span>] <span class="ot">-></span> [<span class="dt">String</span>]
<span class="fu">reverse</span><span class="ot"> ::</span> [<span class="dt">String</span>] <span class="ot">-></span> [<span class="dt">String</span>]
(<span class="fu">take</span> <span class="dv">2</span>)<span class="ot"> ::</span> [<span class="dt">String</span>] <span class="ot">-></span> [<span class="dt">String</span>]</code></pre>
<p><em>(Don't look too hard at that last one.)</em></p>
<p><em>(This slide isn't in the video.)</em></p>
</div>
<div class="section slide level1" id="what-do-we-have-now">
<h1>What do we have now?</h1>
<p>Type type of <code>indent</code> is</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">indent ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
indent s <span class="fu">=</span> <span class="st">" "</span> <span class="fu">++</span> s</code></pre>
<p>and that isn't compatible with <code>byLines</code></p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">indentLines <span class="fu">=</span> byLines indent</code></pre>
<p><em>(This slide isn't in the video.)</em></p>
</div>
<div class="section slide level1" id="map-to-the-rescue">
<h1><code>map</code> to the rescue:</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="fu">map</span><span class="ot"> ::</span> (a <span class="ot">-></span> b) <span class="ot">-></span> [a] <span class="ot">-></span> [b]</code></pre>
<p>as in:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="fu">map</span> <span class="fu">reverse</span> [<span class="st">"red"</span>, <span class="st">"yellow"</span>, <span class="st">"blue"</span>]
[<span class="st">"der"</span>,<span class="st">"wolley"</span>,<span class="st">"eulb"</span>]
<span class="fu">map</span> <span class="fu">sort</span> [<span class="st">"red"</span>, <span class="st">"yellow"</span>, <span class="st">"blue"</span>]
[<span class="st">"der"</span>,<span class="st">"ellowy"</span>,<span class="st">"belu"</span>]</code></pre>
<p>compare:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="fu">reverse</span> [<span class="st">"red"</span>, <span class="st">"yellow"</span>, <span class="st">"blue"</span>]
[<span class="st">"blue"</span>,<span class="st">"yellow"</span>,<span class="st">"red"</span>]
<span class="fu">sort</span> [<span class="st">"red"</span>, <span class="st">"yellow"</span>, <span class="st">"blue"</span>]
[<span class="st">"blue"</span>,<span class="st">"red"</span>,<span class="st">"yellow"</span>]</code></pre>
</div>
<div class="section slide level1" id="so-then">
<h1>So then:</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">indentEachLine ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
indentEachLine <span class="fu">=</span> byLines (<span class="fu">map</span> indent)
<span class="ot">eachLine ::</span> (<span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>) <span class="ot">-></span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
eachLine f <span class="fu">=</span> <span class="fu">unlines</span> <span class="fu">.</span> <span class="fu">map</span> f <span class="fu">.</span> <span class="fu">lines</span>
<span class="ot">indentEachLine' ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
indentEachLine' <span class="fu">=</span> eachLine indent</code></pre>
<p>and we get:</p>
<pre><code> occasional clouds
one gets a rest
from moon-viewing</code></pre>
</div>
<div class="section slide level1" id="but-wait-wheres-the-2nd-argument">
<h1>But wait, where's the 2nd argument?</h1>
<p>How can we write:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">eachLine f <span class="fu">=</span> <span class="fu">unlines</span> <span class="fu">.</span> <span class="fu">map</span> f <span class="fu">.</span> <span class="fu">lines</span></code></pre>
<p>Think of <code>map</code>'s type this way:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="fu">map</span><span class="ot"> ::</span> (a <span class="ot">-></span> b) <span class="ot">-></span> ([a] <span class="ot">-></span> [b])</code></pre>
<p>It takes a function and transforms (lifts) it into a function over lists</p>
</div>
<div class="section slide level1" id="lets-yell">
<h1>Let's YELL!!!</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">yell ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
yell s <span class="fu">=</span> <span class="fu">map</span> <span class="fu">toUpper</span> s <span class="fu">++</span> <span class="st">"!!!"</span>
<span class="ot">yellEachLine ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
yellEachLine <span class="fu">=</span> eachLine yell</code></pre>
<p>gives</p>
<pre><code>OCCASIONAL CLOUDS!!!
ONE GETS A REST!!!
FROM MOON-VIEWING!!!</code></pre>
</div>
<div class="section slide level1" id="what-if-we-wanted-it-by-words">
<h1>What if we wanted it by words?</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">eachWord ::</span> (<span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>) <span class="ot">-></span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
eachWord f <span class="fu">=</span> <span class="fu">unwords</span> <span class="fu">.</span> <span class="fu">map</span> f <span class="fu">.</span> <span class="fu">words</span>
<span class="ot">yellEachWord ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
yellEachWord <span class="fu">=</span> eachWord yell</code></pre>
<p><em>d'oh!</em></p>
<pre><code>OCCASIONAL!!! CLOUDS!!! ONE!!! GETS!!! A!!! REST!!! FROM!!! MOON-VIEWING!!!</code></pre>
</div>
<div class="section slide level1" id="we-want-by-words-by-lines...">
<h1>We want by words, by lines...</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">eachWordOnEachLine ::</span> (<span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>) <span class="ot">-></span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
eachWordOnEachLine f <span class="fu">=</span> eachLine (eachWord f)
<span class="ot">yellEachWordOnEachLine ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">String</span>
yellEachWordOnEachLine <span class="fu">=</span> eachWordOnEachLine yell</code></pre>
<p>Ah, got it:</p>
<pre><code>OCCASIONAL!!! CLOUDS!!!
ONE!!! GETS!!! A!!! REST!!!
FROM!!! MOON-VIEWING!!!</code></pre>
<p><em>(This slide is fixed from the one in the video.)</em></p>
</div>
<div class="section slide level1" id="what-bus-hit-us">
<h1>What bus hit us?</h1>
<p>Higher Order Functions</p>
</div>
<div class="section slide level1" id="section">
<h1></h1>
<p>(Pause to catch breath)</p>
</div>
<div class="section slide level1" id="section-1">
<h1></h1>
<p><em>Onward!</em></p>
</div>
<div class="section slide level1" id="structured-data">
<h1>Structured data</h1>
<p>By which I mean lists, of course...</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">data</span> <span class="dt">List</span> α <span class="fu">=</span> <span class="dt">EndOfList</span>
<span class="fu">|</span> <span class="dt">Link</span> α (<span class="dt">List</span> α)</code></pre>
<p>we can make some values of this type:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">empty <span class="fu">=</span> <span class="dt">EndOfList</span>
oneWord <span class="fu">=</span> <span class="dt">Link</span> <span class="st">"apple"</span> <span class="dt">EndOfList</span>
twoWords <span class="fu">=</span> <span class="dt">Link</span> <span class="st">"banana"</span> (<span class="dt">Link</span> <span class="st">"cantaloupe"</span> <span class="dt">EndOfList</span>)</code></pre>
</div>
<div class="section slide level1" id="pop-quiz">
<h1>Pop quiz</h1>
<p>Given these..</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">empty <span class="fu">=</span> <span class="dt">EndOfList</span>
oneWord <span class="fu">=</span> <span class="dt">Link</span> <span class="st">"apple"</span> <span class="dt">EndOfList</span>
twoWords <span class="fu">=</span> <span class="dt">Link</span> <span class="st">"banana"</span> (<span class="dt">Link</span> <span class="st">"cantaloupe"</span> <span class="dt">EndOfList</span>)</code></pre>
<p>What are these?</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell">mystery1 <span class="fu">=</span> <span class="dt">Link</span> <span class="st">"pear"</span> empty
mystery2 <span class="fu">=</span> <span class="dt">Link</span> <span class="st">"peach"</span> oneWord
mystery3 <span class="fu">=</span> <span class="dt">Link</span> <span class="st">"pineapple"</span> mystery3
mystery4 <span class="fu">=</span> <span class="dt">Link</span> <span class="dv">42</span> (<span class="dt">Link</span> <span class="st">"apple"</span> <span class="dt">EndOfList</span>)</code></pre>
</div>
<div class="section slide level1" id="some-functions-on-list">
<h1>Some functions on List</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">dropOne ::</span> <span class="dt">List</span> α <span class="ot">-></span> <span class="dt">List</span> α
dropOne (<span class="dt">Link</span> first rest) <span class="fu">=</span> rest
dropOne <span class="dt">EndOfList</span> <span class="fu">=</span> <span class="dt">EndOfList</span>
<span class="ot">justOne ::</span> <span class="dt">List</span> α <span class="ot">-></span> <span class="dt">List</span> α
justOne (<span class="dt">Link</span> a _) <span class="fu">=</span> <span class="dt">Link</span> a <span class="dt">EndOfList</span>
justOne <span class="dt">EndOfList</span> <span class="fu">=</span> <span class="dt">EndOfList</span></code></pre>
</div>
<div class="section slide level1" id="actually-we-dont-type-so-much">
<h1>Actually, we don't type so much</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">data</span> [] a <span class="fu">=</span> [] <span class="fu">|</span> a <span class="fu">:</span> [a] <span class="co">-- this is in the standard library</span>
<span class="kw">infixr</span> <span class="dv">5</span> <span class="fu">:</span>
empty <span class="fu">=</span> []
oneWord <span class="fu">=</span> <span class="st">"apple"</span> <span class="fu">:</span> []
twoWords <span class="fu">=</span> <span class="st">"banana"</span> <span class="fu">:</span> <span class="st">"cantaloupe"</span> <span class="fu">:</span> []
mystery1 <span class="fu">=</span> <span class="st">"pear"</span> <span class="fu">:</span> empty
mystery2 <span class="fu">=</span> <span class="st">"peach"</span> <span class="fu">:</span> oneWord
mystery3 <span class="fu">=</span> <span class="st">"pineapple"</span> <span class="fu">:</span> mystery3
mystery4 <span class="fu">=</span> <span class="dv">42</span> <span class="fu">:</span> <span class="st">"apple"</span> <span class="fu">:</span> []
<span class="ot">dropOne ::</span> [a] <span class="ot">-></span> [a]
dropOne (first<span class="fu">:</span>rest) <span class="fu">=</span> rest
dropOne [] <span class="fu">=</span> []
<span class="ot">justOne ::</span> [a] <span class="ot">-></span> [a]
justOne (a<span class="fu">:</span>_) <span class="fu">=</span> a<span class="fu">:</span>[]
justOne [] <span class="fu">=</span> []</code></pre>
</div>
<div class="section slide level1" id="actually-not-even-that-much">
<h1>Actually, not even that much</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">data</span> [] a <span class="fu">=</span> [] <span class="fu">|</span> a <span class="fu">:</span> [a] <span class="co">-- this is in the standard library</span>
<span class="kw">infixr</span> <span class="dv">5</span> <span class="fu">:</span>
empty <span class="fu">=</span> []
oneWord <span class="fu">=</span> [<span class="st">"apple"</span>] <span class="co">-- syntatic sugar</span>
twoWords <span class="fu">=</span> [<span class="st">"banana"</span>, <span class="st">"cantaloupe"</span>] <span class="co">-- two teaspoons full</span>
mystery1 <span class="fu">=</span> <span class="st">"pear"</span> <span class="fu">:</span> empty
mystery2 <span class="fu">=</span> <span class="st">"peach"</span> <span class="fu">:</span> oneWord
mystery3 <span class="fu">=</span> <span class="st">"pineapple"</span> <span class="fu">:</span> mystery3
mystery4 <span class="fu">=</span> [<span class="dv">42</span>, <span class="st">"apple"</span>] <span class="co">-- sweet, but still won't compile</span>
<span class="ot">dropOne ::</span> [a] <span class="ot">-></span> [a]
dropOne (first<span class="fu">:</span>rest) <span class="fu">=</span> rest
dropOne [] <span class="fu">=</span> []
<span class="ot">justOne ::</span> [a] <span class="ot">-></span> [a] <span class="co">-- don't confuse these "[a]"s</span>
justOne (a<span class="fu">:</span>_) <span class="fu">=</span> [a] <span class="co">-- with this "[a]"</span>
justOne [] <span class="fu">=</span> []</code></pre>
</div>
<div class="section slide level1" id="two-more-standard-things">
<h1>Two more standard things:</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">type</span> <span class="dt">String</span> <span class="fu">=</span> [<span class="dt">Char</span>]</code></pre>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">data</span> <span class="dt">Maybe</span> a <span class="fu">=</span> <span class="kw">Nothing</span> <span class="fu">|</span> <span class="kw">Just</span> a</code></pre>
</div>
<div class="section slide level1" id="use-it-like-this">
<h1>Use it like this</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">pickMessage ::</span> <span class="dt">Maybe</span> <span class="dt">Int</span> <span class="ot">-></span> <span class="dt">String</span>
pickMessage (<span class="kw">Just</span> n) <span class="fu">=</span> <span class="st">"Pick a number, like "</span> <span class="fu">++</span> <span class="fu">show</span> n <span class="fu">++</span> <span class="st">"."</span>
pickMessage <span class="kw">Nothing</span> <span class="fu">=</span> <span class="st">"Pick any number you like."</span></code></pre>
</div>
<div class="section slide level1" id="the-awkward-and-the-bad">
<h1>The awkward and the bad</h1>
<p>This is awkward:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">justOne ::</span> [a] <span class="ot">-></span> [a]
justOne (a<span class="fu">:</span>_) <span class="fu">=</span> [a]
justOne [] <span class="fu">=</span> []</code></pre>
<p>This is bad:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">firstOne ::</span> [a] <span class="ot">-></span> a
firstOne (a<span class="fu">:</span>_) <span class="fu">=</span> a
firstOne [] <span class="fu">=</span> <span class="fu">error</span> <span class="st">"O Noes!"</span></code></pre>
</div>
<div class="section slide level1" id="maybe-theres-a-better-way">
<h1><code>Maybe</code>, there's a better way</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">firstOne' ::</span> [a] <span class="ot">-></span> <span class="dt">Maybe</span> a
firstOne' (a<span class="fu">:</span>_) <span class="fu">=</span> <span class="kw">Just</span> a
firstOne' [] <span class="fu">=</span> <span class="kw">Nothing</span></code></pre>
</div>
<div class="section slide level1" id="now-lets-write-some-real-code">
<h1>Now, let's write some real code</h1>
<p>Find the first character after a star:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">findAfterStar ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Char</span>
findAfterStar (c<span class="fu">:</span>d<span class="fu">:</span>r) <span class="fu">=</span>
<span class="kw">if</span> c <span class="fu">==</span> <span class="ch">'*'</span> <span class="kw">then</span> <span class="kw">Just</span> d
<span class="kw">else</span> findAfterStar (d<span class="fu">:</span>r)
findAfterStar _ <span class="fu">=</span> <span class="kw">Nothing</span></code></pre>
</div>
<div class="section slide level1" id="make-it-a-little-bit-more-generic">
<h1>Make it a little bit more generic</h1>
<p>Find the first character after some other character:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">findAfterChar ::</span> <span class="dt">Char</span> <span class="ot">-></span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Char</span>
findAfterChar m (c<span class="fu">:</span>d<span class="fu">:</span>r) <span class="fu">=</span>
<span class="kw">if</span> c <span class="fu">==</span> m <span class="kw">then</span> <span class="kw">Just</span> d
<span class="kw">else</span> findAfterChar m (d<span class="fu">:</span>r)
findAfterChar _ _ <span class="fu">=</span> <span class="kw">Nothing</span></code></pre>
</div>
<div class="section slide level1" id="more-generic-still">
<h1>More generic still</h1>
<p>Find the first thing after some other thing:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">findAfterElem ::</span> <span class="kw">Eq</span> a <span class="ot">=></span> a <span class="ot">-></span> [a] <span class="ot">-></span> <span class="dt">Maybe</span> a
findAfterElem m (c<span class="fu">:</span>d<span class="fu">:</span>r) <span class="fu">=</span>
<span class="kw">if</span> c <span class="fu">==</span> m <span class="kw">then</span> <span class="kw">Just</span> d
<span class="kw">else</span> findAfterElem m (d<span class="fu">:</span>r)
findAfterElem _ _ <span class="fu">=</span> <span class="kw">Nothing</span></code></pre>
</div>
<div class="section slide level1" id="section-2">
<h1></h1>
<p>(Pause to catch breath)</p>
</div>
<div class="section slide level1" id="section-3">
<h1></h1>
<p><em>Onward!</em></p>
</div>
<div class="section slide level1" id="the-type-that-blew-my-mind">
<h1>The type that blew my mind</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="kw">data</span> <span class="dt">Maybe</span> a <span class="fu">=</span> <span class="kw">Nothing</span> <span class="fu">|</span> <span class="kw">Just</span> a</code></pre>
</div>
<div class="section slide level1" id="maybe-quite-useful">
<h1><code>Maybe</code> quite useful:</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">elemIndex ::</span> a <span class="ot">-></span> [a] <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Int</span>
<span class="fu">lookup</span><span class="ot"> ::</span> k <span class="ot">-></span> <span class="dt">Map</span> k a <span class="ot">-></span> <span class="dt">Maybe</span> a
<span class="ot">stripPrefix ::</span> <span class="dt">Text</span> <span class="ot">-></span> <span class="dt">Text</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Text</span>
<span class="ot">port ::</span> <span class="dt">URIAuthority</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Int</span></code></pre>
</div>
<div class="section slide level1" id="power-lifting-fmap">
<h1>Power lifting: <code>fmap</code></h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">addAWeek ::</span> <span class="dt">Day</span> <span class="ot">-></span> <span class="dt">Day</span>
addAWeek d <span class="fu">=</span> addDays <span class="dv">7</span> d
<span class="ot">interestingDates ::</span> [<span class="dt">Day</span>]
interestingDates <span class="fu">=</span> <span class="fu">...</span>
<span class="ot">anInterestingDate ::</span> <span class="dt">Maybe</span> <span class="dt">Day</span>
anInterestingDate <span class="fu">=</span> firstOne' interestingDates
<span class="ot">aWeekLater ::</span> <span class="dt">Maybe</span> <span class="dt">Day</span>
aWeekLater <span class="fu">=</span> <span class="fu">fmap</span> addAWeek anInterestingDate</code></pre>
<p><em>(See the source for some intersting dates.)</em></p>
</div>
<div class="section slide level1" id="thinking-like-a-haskeller">
<h1>Thinking like a Haskeller</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">addAWeek ::</span> <span class="dt">Day</span> <span class="ot">-></span> <span class="dt">Day</span>
addAWeek d <span class="fu">=</span> addDays <span class="dv">7</span> d
<span class="ot">maybeAddAWeek ::</span> <span class="dt">Maybe</span> <span class="dt">Day</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Day</span>
maybeAddAWeek <span class="fu">=</span> <span class="fu">fmap</span> addAWeek
<span class="ot">aWeekLater' ::</span> <span class="dt">Maybe</span> <span class="dt">Day</span>
aWeekLater' <span class="fu">=</span> maybeAddAWeek anInterestingDate</code></pre>
</div>
<div class="section slide level1" id="power-alternatives">
<h1>Power alternatives: <code><|></code></h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">pickShow ::</span> <span class="dt">Person</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">String</span>
pickShow p <span class="fu">=</span>
favoriteShow (name p)
<span class="fu"><|></span> showWithName (name p)
<span class="fu"><|></span> showForYear (year p)</code></pre>
<p>Given:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">favoriteShow ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">String</span>
<span class="ot">showWithName ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">String</span>
<span class="ot">showForYear ::</span> <span class="dt">Int</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">String</span></code></pre>
<p>Like short circuit due to lazy evaluation</p>
</div>
<div class="section slide level1" id="power-injection">
<h1>Power injection: <code>>>=</code></h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell">getHeader <span class="st">"Date"</span> message <span class="fu">>>=</span> parseDate <span class="fu">>>=</span> mailboxForDate</code></pre>
<p>Given:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">getHeader ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">MimeMessage</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">String</span>
<span class="ot">parseDate ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Date</span>
<span class="ot">mailboxForDate ::</span> <span class="dt">Date</span> <span class="ot">-></span> <span class="dt">Maybe</span> <span class="dt">Mailbox</span></code></pre>
<p><code>>>=</code> is actually pronounced "bind"</p>
</div>
<div class="section slide level1" id="more-generic">
<h1>More generic</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="fu">fmap</span><span class="ot"> ::</span> <span class="kw">Functor</span> f <span class="ot">=></span> (a <span class="ot">-></span> b) <span class="ot">-></span> f a <span class="ot">-></span> f b
<span class="ot">(<|>) ::</span> <span class="dt">Alternative</span> f <span class="ot">=></span> f a <span class="ot">-></span> f a <span class="ot">-></span> f a
<span class="ot">(>>=) ::</span> <span class="kw">Monad</span> m <span class="ot">=></span> m a <span class="ot">-></span> (a <span class="ot">-></span> m b) <span class="ot">-></span> m b</code></pre>
<p>Type classes and instances:</p>
<pre><code>Functor Maybe, [], (Either a), IO
Alternative Maybe, []
Monad Maybe, [], (Either a), IO</code></pre>
</div>
<div class="section slide level1" id="section-4">
<h1></h1>
<p>(Time for just one more?)</p>
</div>
<div class="section slide level1" id="section-5">
<h1></h1>
<p><em>Go!</em></p>
</div>
<div class="section slide level1" id="types-you-dont-type">
<h1>Types you don't type</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">runLengthEncode ::</span> <span class="kw">Eq</span> a <span class="ot">=></span> [a] <span class="ot">-></span> [(a, <span class="dt">Int</span>)]
runLengthEncode [] <span class="fu">=</span> []
runLengthEncode (x<span class="fu">:</span>xs) <span class="fu">=</span> nextGroup x <span class="dv">1</span> xs
<span class="kw">where</span>
nextGroup e n [] <span class="fu">=</span> [(e, n)]
nextGroup e n (y<span class="fu">:</span>ys)
<span class="fu">|</span> e <span class="fu">==</span> y <span class="fu">=</span> nextGroup e (n <span class="fu">+</span> <span class="dv">1</span>) ys
<span class="fu">|</span> <span class="fu">otherwise</span> <span class="fu">=</span> (e, n) <span class="fu">:</span> nextGroup y <span class="dv">1</span> ys</code></pre>
</div>
<div class="section slide level1" id="lets-try-that-in-c">
<h1>Let's try that in C++</h1>
<pre class="sourceCode cpp"><code class="sourceCode cpp"><span class="kw">template</span><<span class="kw">typename</span> T>
list<pair<T,<span class="dt">int</span>> > runLengthEncode(<span class="dt">const</span> list<T>& as) {
list<pair<T, <span class="dt">int</span>> > runs;
<span class="kw">if</span> (!as.empty()) {
<span class="kw">typename</span> list<T>::const_iterator it = as.begin();
T elem = *it;
<span class="dt">int</span> count = <span class="dv">0</span>;
<span class="kw">for</span> (; it != as.end(); it++) {
<span class="kw">if</span> (elem != *it) {
runs.push_back(make_pair(elem, count));
elem = *it;
count = <span class="dv">0</span>;
}
count += <span class="dv">1</span>;
}
runs.push_back(make_pair(elem, count));
}
<span class="kw">return</span> runs;
}</code></pre>
</div>
<div class="section slide level1" id="quick-check">
<h1>Quick Check</h1>
<p>Just write some properties that should hold:</p>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="ot">rlePropLengthPreserved ::</span> [<span class="dt">Int</span>] <span class="ot">-></span> <span class="dt">Bool</span>
rlePropLengthPreserved <span class="kw">as</span> <span class="fu">=</span> <span class="fu">length</span> <span class="kw">as</span> <span class="fu">==</span> (<span class="fu">sum</span> <span class="fu">$</span> <span class="fu">map</span> <span class="fu">snd</span> <span class="fu">$</span> runLengthEncode <span class="kw">as</span>)
<span class="ot">rlePropDupesCollapsed ::</span> <span class="dt">Int</span> <span class="ot">-></span> <span class="dt">Bool</span>
rlePropDupesCollapsed n
<span class="fu">|</span> m <span class="fu">==</span> <span class="dv">0</span> <span class="fu">=</span> runLengthEncode <span class="st">""</span> <span class="fu">==</span> []
<span class="fu">|</span> <span class="fu">otherwise</span> <span class="fu">=</span> runLengthEncode (<span class="fu">replicate</span> m <span class="ch">'x'</span>) <span class="fu">==</span> [(<span class="ch">'x'</span>, m)]
<span class="kw">where</span> m <span class="fu">=</span> n <span class="ot">`mod`</span> <span class="dv">100</span>
<span class="ot">rlePropRoundTrip ::</span> [<span class="dt">Int</span>] <span class="ot">-></span> <span class="dt">Bool</span>
rlePropRoundTrip ns <span class="fu">=</span> runLengthEncode xs <span class="fu">==</span> is
<span class="kw">where</span> is <span class="fu">=</span> <span class="fu">zip</span> [<span class="ch">'a'</span><span class="fu">..</span>] <span class="fu">$</span> <span class="fu">map</span> (\n <span class="ot">-></span> n <span class="ot">`mod`</span> <span class="dv">100</span> <span class="fu">+</span> <span class="dv">1</span>) ns
xs <span class="fu">=</span> <span class="fu">concatMap</span> (\(i,n) <span class="ot">-></span> <span class="fu">replicate</span> n i) is</code></pre>
</div>
<div class="section slide level1" id="quick-check-em">
<h1>Quick Check 'em:</h1>
<pre class="sourceCode haskell"><code class="sourceCode haskell"><span class="fu">></span> quickCheck rlePropRoundTrip
<span class="fu">+++</span> <span class="dt">OK</span>, passed <span class="dv">100</span> tests<span class="fu">.</span>
<span class="fu">></span> quickCheck rlePropDupesCollapsed
<span class="fu">+++</span> <span class="dt">OK</span>, passed <span class="dv">100</span> tests<span class="fu">.</span>
<span class="fu">></span> quickCheck rlePropRoundTrip
<span class="fu">+++</span> <span class="dt">OK</span>, passed <span class="dv">100</span> tests<span class="fu">.</span></code></pre>
</div>
<div class="section slide level1" id="section-6">
<h1></h1>
<p>Whew</p>
</div>
<div class="section slide level1" id="oh-and-some-more-things">
<h1>Oh, and some more things:</h1>
<ul>
<li>ghci</li>
<li>cabal & hackage</li>
<li>haddock</li>
<li>Hoogle</li>
<li><code>#haskell</code> on <code>irc.freenode.org</code></li>
</ul>
</div>
<div class="section slide level1" id="want-a-bigger-helping">
<h1>Want a bigger helping?</h1>
<ul>
<li>Learn You a Haskell for Great Good!</li>
</ul>
<p>http://learnyouahaskell.com/</p>
<ul>
<li>Real World Haskell</li>
</ul>
<p>http://book.realworldhaskell.org/</p>
<ul>
<li>Haskell.org</li>
</ul>
</div>
<div class="section slide level1" id="thanks">
<h1>Thanks</h1>
<p>Mark Lentczner</p>
<p><code>mark﹫glyphic·com</code> <|> <code>mzero﹫google·com</code></p>
<p><code>mzero</code> on IRC</p>
</div>
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