move initial Maybe discussion up a bit, added slide on Type classes

Part2d.hs

@@ -1,5 +1,10 @@
 module Part2d where
 
+pickMessage :: Maybe Int -> String
+pickMessage (Just n) = "Pick a number, like " ++ show n ++ "."
+pickMessage Nothing = "Pick any number you like."
+
+
 findAfterStar :: String -> Maybe Char
 findAfterStar (c:d:r) =
   if c == '*' then Just d

slides.html

@@ -220,8 +220,19 @@ <h1 id="actually-not-even-that-much">Actually, not even that much</h1>
 <pre class="sourceCode"><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><br /><span class="kw">infixr</span> <span class="dv">5</span> <span class="fu">:</span><br /><br />empty <span class="fu">=</span> []<br />oneWord <span class="fu">=</span> [<span class="st">&quot;apple&quot;</span>]                 <span class="co">-- syntatic sugar</span><br />twoWords <span class="fu">=</span> [<span class="st">&quot;banana&quot;</span>, <span class="st">&quot;cantaloupe&quot;</span>] <span class="co">-- two teaspoons full</span><br /><br />mystery1 <span class="fu">=</span> <span class="st">&quot;pear&quot;</span> <span class="fu">:</span> empty<br />mystery2 <span class="fu">=</span> <span class="st">&quot;peach&quot;</span> <span class="fu">:</span> oneWord<br />mystery3 <span class="fu">=</span> <span class="st">&quot;pineapple&quot;</span> <span class="fu">:</span> mystery3<br />mystery4 <span class="fu">=</span> [<span class="dv">42</span>, <span class="st">&quot;apple&quot;</span>] <span class="co">-- sweet, but still won't compile</span><br /><br /><span class="ot">dropOne </span><span class="ot">::</span> [a] <span class="ot">-&gt;</span> [a]<br />dropOne (first<span class="fu">:</span>rest) <span class="fu">=</span> rest<br />dropOne [] <span class="fu">=</span> []<br /><br /><span class="ot">justOne </span><span class="ot">::</span> [a] <span class="ot">-&gt;</span> [a] <span class="co">-- don't confuse these &quot;[a]&quot;s</span><br />justOne (a<span class="fu">:</span>_) <span class="fu">=</span> [a]   <span class="co">-- with this &quot;[a]&quot;</span><br />justOne [] <span class="fu">=</span> []</code></pre>
 </div>
 <div class="slide">
-<h1 id="one-more-standard-thing">One more standard thing:</h1>
+<h1 id="two-more-standard-things">Two more standard things:</h1>
 <pre class="sourceCode"><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"><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="slide">
+<h1 id="a-bad-function">A bad function</h1>
+<pre class="sourceCode"><code class="sourceCode haskell"><span class="ot">firstOne </span><span class="ot">::</span> [a] <span class="ot">-&gt;</span> a<br />firstOne (a<span class="fu">:</span>_) <span class="fu">=</span> a<br />firstOne [] <span class="fu">=</span> <span class="fu">error</span> <span class="st">&quot;O Noes!&quot;</span></code></pre>
+</div>
+<div class="slide">
+<h1 id="maybe-a-better-way">Maybe a better way</h1>
+<pre class="sourceCode"><code class="sourceCode haskell"><span class="ot">firstOne' </span><span class="ot">::</span> [a] <span class="ot">-&gt;</span> <span class="dt">Maybe</span> a<br />firstOne' (a<span class="fu">:</span>_) <span class="fu">=</span> <span class="kw">Just</span> a<br />firstOne' [] <span class="fu">=</span> <span class="kw">Nothing</span></code></pre>
+<p>Use it like this:</p>
+<pre class="sourceCode"><code class="sourceCode haskell"><span class="ot">pickMessage </span><span class="ot">::</span> <span class="dt">Maybe</span> <span class="dt">Int</span> <span class="ot">-&gt;</span> <span class="dt">String</span><br />pickMessage (<span class="kw">Just</span> n) <span class="fu">=</span> <span class="st">&quot;Pick a number, like &quot;</span> <span class="fu">++</span> <span class="fu">show</span> n <span class="fu">++</span> <span class="st">&quot;.&quot;</span><br />pickMessage <span class="kw">Nothing</span> <span class="fu">=</span> <span class="st">&quot;Pick any number you like.&quot;</span></code></pre>
 </div>
 <div class="slide">
 <h1 id="now-lets-write-some-real-code">Now, let's write some real code</h1>
@@ -249,21 +260,13 @@ <h1 id="the-type-that-blew-my-mind">The type that blew my mind</h1>
 <pre class="sourceCode"><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="slide">
-<h1 id="a-bad-function">A bad function</h1>
-<pre class="sourceCode"><code class="sourceCode haskell"><span class="ot">firstOne </span><span class="ot">::</span> [a] <span class="ot">-&gt;</span> a<br />firstOne (a<span class="fu">:</span>_) <span class="fu">=</span> a<br />firstOne [] <span class="fu">=</span> <span class="fu">error</span> <span class="st">&quot;O Noes!&quot;</span></code></pre>
-</div>
-<div class="slide">
-<h1 id="maybe-a-better-way">Maybe a better way</h1>
-<pre class="sourceCode"><code class="sourceCode haskell"><span class="ot">firstOne' </span><span class="ot">::</span> [a] <span class="ot">-&gt;</span> <span class="dt">Maybe</span> a<br />firstOne' (a<span class="fu">:</span>_) <span class="fu">=</span> <span class="kw">Just</span> a<br />firstOne' [] <span class="fu">=</span> <span class="kw">Nothing</span></code></pre>
-</div>
-<div class="slide">
-<h1 id="useful">Useful!</h1>
+<h1 id="maybe-quite-useful"><code>Maybe</code> quite useful:</h1>
 <pre class="sourceCode"><code class="sourceCode haskell"><span class="ot">elemIndex </span><span class="ot">::</span> a <span class="ot">-&gt;</span> [a] <span class="ot">-&gt;</span> <span class="dt">Maybe</span> <span class="dt">Int</span><br /><br /><span class="fu">lookup</span><span class="ot"> </span><span class="ot">::</span> k <span class="ot">-&gt;</span> <span class="dt">Map</span> k a <span class="ot">-&gt;</span> <span class="dt">Maybe</span> a<br /><br /><span class="ot">stripPrefix </span><span class="ot">::</span> <span class="dt">Text</span> <span class="ot">-&gt;</span> <span class="dt">Text</span> <span class="ot">-&gt;</span> <span class="dt">Maybe</span> <span class="dt">Text</span><br /><br /><span class="ot">port </span><span class="ot">::</span> <span class="dt">URIAuthority</span> <span class="ot">-&gt;</span> <span class="dt">Maybe</span> <span class="dt">Int</span></code></pre>
 </div>
 <div class="slide">
 <h1 id="power-lifting-fmap">Power lifting: <code>fmap</code></h1>
 <pre class="sourceCode"><code class="sourceCode haskell"><span class="ot">addAWeek </span><span class="ot">::</span> <span class="dt">Day</span> <span class="ot">-&gt;</span> <span class="dt">Day</span><br />addAWeek d <span class="fu">=</span> addDays <span class="dv">7</span> d<br /><br /><span class="ot">interestingDates </span><span class="ot">::</span> [<span class="dt">Day</span>]<br />interestingDates <span class="fu">=</span> <span class="fu">...</span><br /><br /><span class="ot">anInterestingDate </span><span class="ot">::</span> <span class="dt">Maybe</span> <span class="dt">Day</span><br />anInterestingDate <span class="fu">=</span> firstOne' interestingDates<br /><br /><span class="ot">aWeekLater </span><span class="ot">::</span> <span class="dt">Maybe</span> <span class="dt">Day</span><br />aWeekLater <span class="fu">=</span> <span class="fu">fmap</span> addAWeek anInterestingDate</code></pre>
-<p>See the source for some intersting dates.</p>
+<p><em>(See the source for some intersting dates.)</em></p>
 </div>
 <div class="slide">
 <h1 id="thinking-like-a-haskeller">Thinking like a Haskeller</h1>
@@ -284,6 +287,17 @@ <h1 id="power-injection">Power injection: <code>&gt;&gt;=</code></h1>
 <p><code>&gt;&gt;=</code> is actually pronounced &quot;bind&quot;</p>
 </div>
 <div class="slide">
+<h1 id="more-generic">More generic</h1>
+<pre class="sourceCode"><code class="sourceCode haskell"><span class="fu">fmap</span><span class="ot"> </span><span class="ot">::</span> <span class="kw">Functor</span> f <span class="ot">=&gt;</span> (a <span class="ot">-&gt;</span> b) <span class="ot">-&gt;</span> f a <span class="ot">-&gt;</span> f b<br /><br /><span class="ot">(&lt;|&gt;) </span><span class="ot">::</span> <span class="dt">Alternative</span> f <span class="ot">=&gt;</span> f a <span class="ot">-&gt;</span> f a <span class="ot">-&gt;</span> f a<br /><br /><span class="ot">(&gt;&gt;=) </span><span class="ot">::</span> <span class="kw">Monad</span> m <span class="ot">=&gt;</span> m a <span class="ot">-&gt;</span> (a <span class="ot">-&gt;</span> m b) <span class="ot">-&gt;</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="slide">
 <p>(Time for just one more?)</p>
 </div>
 <div class="slide">

slides.md

@@ -393,12 +393,40 @@ justOne (a:_) = [a]   -- with this "[a]"
 justOne [] = []
 ~~~~
 
-# One more standard thing:
+# Two more standard things:
 
 ~~~~ {.haskell}
 type String = [Char]
 ~~~~
 
+~~~~ {.haskell}
+data Maybe a = Nothing | Just a
+~~~~
+
+# A bad function
+
+~~~~ {.haskell}
+firstOne :: [a] -> a
+firstOne (a:_) = a
+firstOne [] = error "O Noes!"
+~~~~
+
+# Maybe a better way
+
+~~~~ {.haskell}
+firstOne' :: [a] -> Maybe a
+firstOne' (a:_) = Just a
+firstOne' [] = Nothing
+~~~~
+
+Use it like this:
+
+~~~~ {.haskell}
+pickMessage :: Maybe Int -> String
+pickMessage (Just n) = "Pick a number, like " ++ show n ++ "."
+pickMessage Nothing = "Pick any number you like."
+~~~~
+
 # Now, let's write some real code
 
 Find the first character after a star:
@@ -444,30 +472,13 @@ findAfterElem _ _ = Nothing
 *Onward!*
 
 
-
 # The type that blew my mind
 
 ~~~~ {.haskell}
 data Maybe a = Nothing | Just a
 ~~~~
 
-# A bad function
-
-~~~~ {.haskell}
-firstOne :: [a] -> a
-firstOne (a:_) = a
-firstOne [] = error "O Noes!"
-~~~~
-
-# Maybe a better way
-
-~~~~ {.haskell}
-firstOne' :: [a] -> Maybe a
-firstOne' (a:_) = Just a
-firstOne' [] = Nothing
-~~~~
-
-# Useful!
+# `Maybe` quite useful:
 
 ~~~~ {.haskell}
 elemIndex :: a -> [a] -> Maybe Int
@@ -495,7 +506,7 @@ aWeekLater :: Maybe Day
 aWeekLater = fmap addAWeek anInterestingDate
 ~~~~
 
-See the source for some intersting dates.
+_(See the source for some intersting dates.)_
 
 # Thinking like a Haskeller
 
@@ -550,6 +561,26 @@ mailboxForDate :: Date -> Maybe Mailbox
 
 `>>=` is actually pronounced "bind"
 
+# More generic
+
+~~~~ {.haskell}
+fmap :: Functor f => (a -> b) -> f a -> f b
+
+(<|>) :: Alternative f => f a -> f a -> f a
+
+(>>=) :: Monad m => m a -> (a -> m b) -> m b
+~~~~
+
+Type classes and instances:
+
+~~~~
+Functor       Maybe, [], (Either a), IO
+
+Alternative   Maybe, []
+
+Monad         Maybe, [], (Either a), IO
+~~~~
+
 ----
 
 (Time for just one more?)
@@ -558,6 +589,8 @@ mailboxForDate :: Date -> Maybe Mailbox
 
 *Go!*
 
+
+
 # Types you don't type
 
 ~~~~ {.haskell}