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<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Introduction to Haskell - Lecture 5</title>
<meta name="description" content="A course on the world's fastest growing functional programming language">
<meta name="author" content="Nishant Shukla">
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<!-- Slides begin here! -->
<div class="slides">
<section>
<section>
<h1>Introduction To Haskell</h1>
<p>Lecture 5</p>
<p>
<br>
</p>
<p>Make Your Own Data Type</p>
</section>
<section>
<h3>Using These Slides</h3>
<h4>Every slide has a secret note.</h4>
<small>
<ul>
<li>On <b>Chrome</b>: press <code>F12</code>, then click <b>Console</b></li>
<li>On <b>IE</b>: press <code>F12</code>, then click <b>Console</b></li>
<li>On <b>Firefox</b>: <code>Ctrl+Shift+k</code></li>
</ul>
</small>
<br>
<br>
<p><h3>Shortcut Keys:</h3></p>
<center>
<table width="80%">
<tr>
<td><code>↓</code>, <code>PgDn</code>, <code>n</code>, <code>j</code></td>
<td>next slide</td>
</tr>
<tr>
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<td>prev slide</td>
</tr>
<tr>
<td><code>Esc</code></td>
<td>enables <code>ctrl+f</code> globally</td>
</tr>
</table>
</center>
<aside class="notes">Hi there! This is a secret lecture note. Every slide has a little blurb of text like this!</aside>
</section>
<script>
function clearAll() {
document.getElementById('convertA').style.display = 'none';
document.getElementById('convertB').style.display = 'none';
document.getElementById('convertC').style.display = 'none';
document.getElementById('convertD').style.display = 'none';
}
function showOne(convert) {
clearAll();
document.getElementById(convert).style.display = 'block';
}
</script>
<section>
<h3>Review of Homework 4</h3>
<small><a href="lec04.html#/0/19" target="_blank">Write a function to convert units</a></small>
<table style="width:100%">
<tr>
<td><center><a onclick="showOne('convertA');">A</a></center></td>
<td><center><a onclick="showOne('convertB');">B</a></center></td>
<td><center><a onclick="showOne('convertC');">C</a></center></td>
<td><center><a onclick="showOne('convertD');">D</a></center></td>
</tr>
</table>
<pre id='convertA'><code class='haskell'>
-- Designed by Matthew R. Bongiovi
-- using guards
convert :: (Double, [Char]) -> (Double, [Char])
convert (x,l)
| (l == "m") = (1.09361 * x,"yd")
| (l == "L") = (0.264172 * x,"gal")
| (l == "kg") = (2.20462 * x,"lb")
| (l == "yd") = (x / 1.09361, "m")
| (l == "gal") = (x / 0.264172,"L")
| (l == "lb") = (x / 2.20462,"kg")
| otherwise = error "Invalid input"
</code></pre>
<pre id='convertB' style="display:none;"><code class='haskell'>
-- Designed by James C. Sun
-- using pattern matching
convert :: (Double, [Char]) -> (Double, [Char])
convert (n, "m") = (n*1.09361, "yd")
convert (n, "L") = (n*0.264172, "gal")
convert (n, "kg") = (n*2.20462, "lb")
convert (n, "yd") = (n/1.09361, "m")
convert (n, "gal") = (n/0.264172, "L")
convert (n, "lb") = (n/2.20462, "kg")
convert _ = error "conversion not defined"
</code></pre>
<pre id='convertC' style="display:none;"><code class='haskell'>
-- Designed by Anthony A. Teate
-- the "I'll do it my own way!" solution
convert :: (Double, [Char]) -> (Double, [Char])
convert(v,s)=head[(a,b)|(a,b)<-(head[[(v*y,z),(v/y,x)]|(x,y,z)<-[("m",1.09361,"yd"),("L",0.264172,"gal"),("kg",2.20462,"lb")],x==s||z==s]),b/=s]
</code></pre>
<pre id='convertD' style="display:none;"><code class='haskell'>
-- Designed by François Beausoleil
-- the "right way"
module Homework where
data Unit = Meter
| Yard
| Litre
| Gallon
| Kilogram
| Pound
deriving (Ord, Eq)
instance Show Unit where
show Meter = "m"
show Yard = "yd"
show Litre = "l"
show Gallon = "gal"
show Kilogram = "kg"
show Pound = "lb"
factor :: Unit -> Double
factor Meter = 1.09361
factor Litre = 0.264172
factor Kilogram = 2.20462
factor Yard = 1 / (factor Meter)
factor Gallon = 1 / (factor Litre)
factor Pound = 1 / (factor Kilogram)
inverse :: Unit -> Unit
inverse Meter = Yard
inverse Yard = Meter
inverse Litre = Gallon
inverse Gallon = Litre
inverse Kilogram = Pound
inverse Pound = Kilogram
convert :: (Double, Unit) -> (Double, Unit)
convert (n, u) = (n * (factor u), inverse u)
data Measurement = Measurement Double Unit deriving (Eq)
instance Show Measurement where
show (Measurement n u) = (show n) ++ " " ++ (show u)
conv (Measurement n u) = measurement $ convert (n, u)
where measurement (n1, u1) = Measurement n1 u1
</code></pre>
<aside class="notes">Here's a good couple solutions you guys came up with.</aside>
</section>
<section>
<h3>Data Types</h3>
<br>
<ul>
<li><p><code>Bool</code> = False | True</p></li>
<br>
<li><p><code>Int</code> = -2<sup>31</sup> ... -1 | 0 | 1 ... 2<sup>31</sup>-1</p></li>
<br>
<li><p><code>[Char]</code> = [] | Char : [Char]</p></li>
<br>
<li><p><code>Double</code></p></li>
</ul>
<aside class="notes">You're been playing with these data types for a while now. You can also create your own.</aside>
</section>
<section>
<h3>Making Data Types</h3>
<ul>
<li>for well structured code</li>
<br>
<li>for better readability</li>
<br>
<li>for improving type safety</li>
</ul>
<aside class="notes">The homework done last week was not very type safe. As a developer, you had to remember which units were valid (m, L, kg, etc). When you make your own Data types, code becomes maintainable.</aside>
</section>
<section>
<h3><code class="haskell">data</code> keyword</h3>
<p>In a new file <code class="haskell">MyData.hs</code></p>
<p>create a Data type called <code class="haskell">MetricUnits</code></p>
<pre><code class="haskell">
data MetricUnit = Meter | Liter | KiloGram
</code></pre>
<p><i>MetricUnit</i> is called the <b>type constructor</b></p>
<p>Everything after the equal sign is called the <b>value constructor</b></p>
<br>
<p><small>Notice the capitalization!</small></p>
<aside class="notes">Cool. You just made a new Data type. Let's run this code though GHCi and see what happens.</aside>
</section>
<section>
<h3>What Exactly is a Type?</h3>
<p>Let's look at <i>Meter</i>, <i>Liter</i>, and <i>KiloGram</i></p>
<pre><code class="haskell">
Prelude> :type True
Bool :: True
Prelude> :load MyData.hs
Prelude> :type Meter
Meter :: MetricUnit
Prelude> :type Liter
Liter :: MetricUnit
Prelude> :type KiloGram
KiloGram :: MetricUnit
</code></pre>
<p><b>Meter</b> <i>has the type</i> <b>MetricUnit</b></p>
<aside class="notes">Creating a MetricUnit data type is better than just using a String representation of units like we had before.</aside>
</section>
<section data-state="soothe">
<h3>Remember <i>Show</i>?</h3>
<small><code class="haskell">Show</code> is a Typeclass that enables data to be shown as a String</small>
<div class="fragment roll-in">
<pre><code class="haskell">
Prelude> True
True
Prelude> Meter
No instance for (Show MetricUnit) arising from a use of `print'
Possible fix: add an instance declaration for (Show MetricUnit)
</code></pre>
<p>Why did we get this error?</p>
</div>
<aside class="notes">Our data type needs to derive the Show typeclass in order to be printed to the console.</aside>
</section>
<section>
<h3><code class="haskell">deriving (Show)</code></h3>
<ul>
<li><p>GHCi is trying to call the print function</p></li>
<li><p><code class="haskell">print</code> only works for things that derive Show</p></li>
<li><p>but our MetricUnit doesn't derive Show</p></li>
</ul>
<br>
<br>
<div class="fragment roll-in">
<pre><code class="haskell">
data MetricUnit = Meter | Liter | KiloGram deriving (Show)
</code></pre>
<small><i>F T F Y</i></small>
</div>
<aside class="notes">Now things that are of type MetricUnit can be called by the print or show functions.</aside>
</section>
<section>
<h3>Type Safety</h3>
<ul>
<li><p>Haskell programs are type safe.</p></li>
<li><p>We can't use our MetricUnit type unless a function accepts it</p></li>
</ul>
<br>
<br>
<p>So let's make our own function</p>
<aside class="notes">It's like you're solving a jig-saw puzzle. You code will compile only if all types match.</aside>
</section>
<section data-state="soothe">
<h3>Function that uses MetricUnit</h3>
<p>Write a simple function that takes in a MetricUnit, and returns its symbol</p>
<ul>
<li>Meter → "m"</li>
<li>Liter → "L"</li>
<li>KiloGram → "kg"</li>
</ul>
<pre id="a"><code class="haskell" contenteditable>
symbol :: MetricUnit -> String
</code></pre>
<pre id="b" style="display:none;"><code class="haskell">
symbol :: MetricUnit -> String
symbol Meter = "m"
symbol Liter = "L"
symbol KiloGram = "kg"
</code></pre>
<span style="font-size:12px;">
<a onclick="toggleElements('b', 'a');">(clear answer)</a>
<a onclick="toggleElements('a', 'b');">(show answer)</a>
</span>
<aside class="notes">You can instead just define the show function from the Show typeclass, but we won't cover that yet.</aside>
</section>
<section data-state="soothe">
<h3>Same Function With Guards</h3>
<p>Rewrite <code class="haskell">symbol</code> using guards</p>
<pre id="c"><code class="haskell" contenteditable>
symbol :: MetricUnit -> String
</code></pre>
<pre id="d" style="display:none;"><code class="haskell">
symbol :: MetricUnit -> String
symbol x
| x==Meter = "m"
| x==Liter = "L"
| x==KiloGram = "kg"
</code></pre>
<span style="font-size:12px;">
<a onclick="toggleElements('d', 'c');">(clear answer)</a>
<a onclick="toggleElements('c', 'd');">(show answer)</a>
</span>
<p>Try compiling it. What happens?</p>
<aside class="notes">You'll get an error message. You are using the (==) function which can only be used between two things that can be compared for equivalency. In other words, if you use the (==) function, the arguments must derive the Eq typeclass.</aside>
</section>
<section>
<h3><code class="haskell">deriving (Eq)</code></h3>
<p>GHCi is yelling at us because we used '=='</p>
<pre><code class="haskell">
No instance for (Eq MetricUnit) arising from a use of `=='
Possible fix: add an instance declaration for (Eq MetricUnit)
In the expression: x == Meter
</code></pre>
<br>
<p>So MetricUnit should derive Eq in this case</p>
<pre><code class="haskell">
data MetricUnit = Meter | Liter | KiloGram deriving (Show, Eq)
</code></pre>
<aside class="notes">Play close attention to the syntax. To derive multiple typeclasses, you separate them by commas.</aside>
</section>
<section>
<h2>Metric to Imperial</h2>
<p>Let's write a <code class="haskell">convert</code> function with our types.</p>
<p>We will convert from Metric to Imperial.</p>
<p>Just like last week's homework</p>
<br>
<p class="fragment roll-in">But this time it will be elegant!</p>
<aside class="notes">We're about to write a convert function using our own Data types.</aside>
</section>
<section>
<p>1. Define <i>MetricUnit</i> and <i>ImperialUnit</i></p>
<pre><code class="haskell" contenteditable>
data MetricUnit = Meter
| Liter
| KiloGram
deriving (Show, Eq)
data ImperialUnit = Yard
| Gallon
| Pound
deriving (Show)
</code></pre>
<br>
<aside class="notes">First create your Data type</aside>
</section>
<section>
<p>2. Create a Measurement Data Type</p>
<br>
<p>Look closely. This syntax is new!</p>
<pre><code class="haskell">
data Measurement = MetricMeasurement Double MetricUnit
| ImperialMeasurement Double ImperialUnit
deriving (Show)
</code></pre>
<br>
<small><code class="haskell">(MetricMeasurement 3.1 Liter) :: Measurement</code></small>
<small><code class="haskell">(ImperialMeasurement 200 Pound) :: Measurement</code></small>
<aside class="notes">A MetricMeasurement requires a value (Double) and a unit (MetricUnit). Likewise, an ImperialMeasurement requires a value (Double) and a unit (ImperialUnit).</aside>
</section>
<section>
<p>3. Create the <code class="haskell">convert</code> function</p>
<pre><code class="haskell">
convert :: Measurement -> Measurement
</code></pre>
<br>
<p>Implement it with pattern matching and guards</p>
<pre class="fragment roll-in"><code class="haskell">
convert (MetricMeasurement x u)
| u==Meter = ImperialMeasurement (1.0936*x) Yard
| u==Liter = ImperialMeasurement (0.2642*x) Gallon
| u==KiloGram = ImperialMeasurement (2.2046*x) Pound
</code></pre>
<pre class="fragment roll-in"><code class="haskell">
convert (ImperialMeasurement x u)
| u==Yard = MetricMeasurement (0.9144*x) Meter
| u==Gallon = MetricMeasurement (3.7854*x) Liter
| u==Pound = MetricMeasurement (0.4536*x) KiloGram
</code></pre>
<small class="fragment roll-in"> <a href="https://gist.github.com/BinRoot/4771288" target="_blank">Full code listing here</a> </small>
<aside class="notes">This function is very maintainable. It properly accepts a Measurement as an input, converts it, and returns a Measurement.</aside>
</section>
<section>
<p>4. Compile and Test</p>
<pre><code class="haskell">
Prelude> let m = MetricMeasurement 2 Meter
Prelude> convert m
ImperialMeasurement 2.18722 Yard
Prelude> convert (convert m)
MetricMeasurement 2 Meter
</pre></code>
<img class="fragment roll-in" src="L05_files/awwyea.jpg">
<aside class="notes">More than likely, you code will work if it compiles. A lot of brain-work comes before the code compiles, but when it does, you're probably done.</aside>
</section>
<section>
<h2>Record Syntax</h2>
<p>Remember, Haskell is lazy, and so are we.</p>
<p>Creating getter functions is monotonous!</p>
<pre><code class="haskell">
data Point = Point Double Double
xval :: Point -> Double
xval (Point x _) = x
yval :: Point -> Double
yval (Point _ y) = y
</code></pre>
<br>
<div class="fragment roll-in">
<p>Haskell provides a shortcut called record syntax</p>
<pre><code class="haskell">
data Point = Point { xval::Double, yval::Double }
</code></pre>
</div>
<aside class="notes">Record syntax is a little syntatic sugar to make code look readable.</aside>
</section>
<section>
<h3>Record Syntax Looks Nice</h3>
<p>It makes code more readable</p>
<pre><code class="haskell">
let a = Point 2 3
let b = Point {xval = 2, yval = 3}
</code></pre>
<small><p>There's more to write, but anyone can easily figure out what's going on.</p></small>
<small><p>Especially for large data types!</p></small>
<aside class="notes">It's good practice.</aside>
</section>
<section>
<h2>Type Parameters</h2>
<p>Oh yea, even types can have arguments.</p>
<pre><code class="haskell">
data ThreeThings a = ThreeThings a a a deriving (Show)
</code></pre>
<br>
<small><code class="haskell">ThreeThings</code> is bundle of three identical types</small>
<pre><code class="haskell">
Prelude> ThreeThings 1 2 3
ThreeThings 1 2 3
Prelude> ThreeThings "hello" "there" "lad"
ThreeThings "hello" "there" "lad"
</code></pre>
<aside class="notes">ThreeThings is a Data type that stores three things of the same type.</aside>
</section>
<section>
<h2>Modules</h2>
<p>We can export useful code to a module</p>
<p>so that other projects can reuse them.</p>
<br>
<small>Let's make a module out of our <code>convert</code> code.</small>
<p>Step 1</p>
<pre><code class="haskell">
module MyData
(MetricUnit(..),
ImperialUnit(..),
Measurement(..),
convert)
where
</code></pre>
<div class="fragment roll-in">
<p>No step 2. That was easy!</p>
<small>Note: your file name should be the same as your module name!</small>
</div>
<aside class="notes">Now you can use the convert function from anywhere.</aside>
</section>
<section>
<h2>Importing Code</h2>
<p>Use <code class="haskell">convert</code> from anywhere!</p>
<pre class="fragment roll-in"><code class="haskell">
-- OtherFile.hs
import MyData
</code></pre>
<pre class="fragment roll-in"><code class="haskell">
-- display a measurement in Metric
reportMeasurement :: Measurement -> String
</code></pre>
<pre class="fragment roll-in"><code class="haskell">
-- if metric, print out the measurement
reportMeasurement (MyData.MetricMeasurement x u)
= (show x) ++ " " ++ (show u)
</code></pre>
<pre class="fragment roll-in"><code class="haskell">
-- otherwise if imperial, convert the measurement and try again
reportMeasurement m
= reportMeasurement (convert m)
</code></pre>
<aside class="notes">You can keep making useful Modules and import them in your future projects.</aside>
</section>
<section>
<h1>Homework</h1>
<h2>Trees in Haskell</h2>
<ol>
<li>Fill out <a target="_blank" href="https://docs.google.com/forms/d/1Jc0H2JCWLuVFN97iLXqqZHTFpznOKv8_C7WDm6FnzYg/viewform">this week's form!</a></li>
<li>Create a binary tree data type of Int nodes</li>
<pre><code class="haskell">data Tree = ... </code></pre>
<li>Write a function to sum all nodes in the tree</li>
<pre><code class="haskell">add :: Tree -> Int</code></pre>
<center><a target="_blank" href="http://en.wikipedia.org/wiki/Binary_tree"><img src="L05_files/tree.png"></img></a></center>
</ol>
</section>
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