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+<h2>Function composition in Haskell</h2>
+<p>In Haskell, the dot operator <code>(.)</code>, written infix like <code>f . g</code>, is <em>function composition</em>.
+For example, suppose you have two functions:</p>
+<pre><code class="language-haskell">addone :: Int -&gt; Int
+addone n = n + 1
+
+timestwo :: Int -&gt; Int
+timestwo n = n * 2
+</code></pre>
+<p>then you can <em>compose</em> those two functions like <code>timestwo . addone</code>:</p>
+<pre><code class="language-haskell">-- computes 2 * (n + 1), which is 2 * n + 2
+both :: Int -&gt; Int
+both = timestwo . addone
+</code></pre>
+<p>And we can try it out in <code>ghci</code>:</p>
+<pre><code>Prelude&gt; both 10
+22
+Prelude&gt; timestwo (addone 10)
+22
+Prelude&gt; 2 * (10 + 1)
+22
+</code></pre>
+<p>This works well, like we expect.
+These functions are intentionally very simple; the point of this post is to explain how function composition works, not how to write complex Haskell functions.</p>
+<p>Now suppose we have an additional, two-argument function to play with:</p>
+<pre><code class="language-haskell">plus :: Int -&gt; Int -&gt; Int
+plus a b = a + b
+</code></pre>
+<p>If we give <code>plus</code> some arguments, we can apply <code>timestwo</code> to the result:</p>
+<pre><code>Prelude&gt; timestwo (plus 3 4)             -- 2 * (3 + 4) = 14
+14
+</code></pre>
+<p>Getting inspiration from the first example with <code>timestwo</code> and <code>addone</code>, we might want to try writing a function like <code>both</code> that composes <code>timestwo</code> and <code>plus</code> using <code>(.)</code>.</p>
+<pre><code class="language-haskell">both2 = timestwo . plus
+</code></pre>
+<p>But the compiler won't accept this:</p>
+<pre><code>    • Couldn't match type ‘Int -&gt; Int’ with ‘Int’
+      Expected type: Int -&gt; Int
+        Actual type: Int -&gt; Int -&gt; Int
+    • Probable cause: ‘plus’ is applied to too few arguments
+      In the second argument of ‘(.)’, namely ‘plus’
+      In the expression: timestwo . plus
+      In an equation for ‘both2’: both2 = timestwo . plus
+   |
+11 | both2 = timestwo . plus
+   |                    ^^^^
+</code></pre>
+<p>Why is this case different than the first example with <code>both</code>?
+The difference is that <code>plus</code> takes two arguments, not one.
+As always in Haskell, let's take a look at the types to see what's going on here.</p>
+<h3>Types</h3>
+<p>What are the types of the functions we're using?</p>
+<pre><code class="language-haskell">addone :: Int -&gt; Int
+timestwo :: Int -&gt; Int
+plus :: Int -&gt; Int -&gt; Int
+(.) :: (b -&gt; c) -&gt; (a -&gt; b) -&gt; (a -&gt; c)
+</code></pre>
+<p>Remember that you can look up the type of standard library functions online: e.g. <a href="https://hackage.haskell.org/package/base-4.14.1.0/docs/Prelude.html#v:.">here for <code>(.)</code></a>.</p>
+<p>Function composition, <code>(.)</code>, takes a function from type <code>b</code> to type <code>c</code> and a function from type <code>a</code> to type <code>b</code>, and returns a function from type <code>a</code> to type <code>c</code>.
+Note that we can also write the type of <code>(.)</code> slightly differently, without the final pair of parentheses:</p>
+<pre><code class="language-haskell">(.) :: (b -&gt; c) -&gt; (a -&gt; b) -&gt; (a -&gt; c)
+(.) :: (b -&gt; c) -&gt; (a -&gt; b) -&gt; a -&gt; c
+</code></pre>
+<p>These both mean the exact same thing.</p>
+<p>When at first, we wrote <code>timestwo . addone</code> as the definition of <code>both</code>, we were really applying <code>(.)</code> to two arguments: <code>timestwo</code> and <code>addone</code>.
+Indeed, we could also have written this:</p>
+<pre><code class="language-haskell">both :: Int -&gt; Int
+both = (.) timestwo addone
+</code></pre>
+<p>which means exactly the same thing as <code>both = timestwo . addone</code>.</p>
+<p>Let's do some of the compiler's work here and figure out how the types match up.
+<small>(If you want to learn more about how you can do type inference by hand, one guide is <a href="http://www.vex.net/~trebla/haskell/type-inference.html">this one</a>.)</small></p>
+<ul>
+<li>The first argument to <code>(.)</code> is <code>timestwo</code> of type <code>Int -&gt; Int</code>.
+From the type signature of <code>(.)</code> we read that the first argument to <code>(.)</code> <em>should</em> have type <code>b -&gt; c</code> for certain <code>b</code> and <code>c</code>; here we get that <code>b</code> and <code>c</code> are both <code>Int</code>.</li>
+<li>The second argument to <code>(.)</code> is <code>addone</code>, again of type <code>Int -&gt; Int</code>.
+This means that <code>a -&gt; b</code> (which, from the previous point, we know to be <code>a -&gt; Int</code>) is really <code>Int -&gt; Int</code>, so <code>a</code> is also <code>Int</code>.</li>
+</ul>
+<p>We don't apply any more arguments to <code>(.)</code>, so the return type is <code>a -&gt; c</code>; since we know that both <code>a</code> and <code>c</code> are <code>Int</code>, this is <code>Int -&gt; Int</code>, so <code>both</code> indeed has the right type signature.</p>
+<p>Now let's try to do the same thing with our second attempt in <code>both2</code>, namely <code>timestwo . plus</code>, which is the same as <code>(.) timestwo plus</code>.
+Remember the types:</p>
+<pre><code class="language-haskell">timestwo :: Int -&gt; Int
+plus :: Int -&gt; Int -&gt; Int
+(.) :: (b -&gt; c) -&gt; (a -&gt; b) -&gt; (a -&gt; c)
+</code></pre>
+<ul>
+<li>
+<p>First argument to <code>(.)</code>: given is <code>timestwo :: Int -&gt; Int</code>, which should match <code>b -&gt; c</code>.
+So: <code>b</code> and <code>c</code> are both <code>Int</code>.</p>
+</li>
+<li>
+<p>Second argument to <code>(.)</code>: given is <code>plus :: Int -&gt; Int -&gt; Int</code>, which should match <code>a -&gt; b</code>; since we already know that <code>b</code> is <code>Int</code>, this is <code>a -&gt; Int</code>.
+So <code>Int -&gt; Int -&gt; Int</code>, which is the same as <code>Int -&gt; (Int -&gt; Int)</code>, should match <code>a -&gt; Int</code> for some <code>a</code>:</p>
+<pre><code>Int -&gt; (Int -&gt; Int)
+ a  -&gt;     Int
+</code></pre>
+<p>But this can never be true!
+We can make <code>a</code> equal to <code>Int</code>, sure, but <code>Int -&gt; Int</code> will never match <code>Int</code>.
+Remember the compiler error we got earlier:</p>
+<pre><code>• Couldn't match type ‘Int -&gt; Int’ with ‘Int’
+  Expected type: ...
+</code></pre>
+<p>Suspiciously similar, isn't it?</p>
+</li>
+</ul>
+<h3>Seen it coming</h3>
+<p>Could we have seen this coming?
+Of course.
+Remember that the following Haskell types mean exactly the same:</p>
+<pre><code class="language-haskell">a -&gt; b -&gt; c -&gt; d -&gt; e
+a -&gt; (b -&gt; (c -&gt; (d -&gt; e)))
+</code></pre>
+<p>This can be explained (or remembered) in two ways:</p>
+<ol>
+<li><code>-&gt;</code> is right-associative: its parentheses collect to the right.
+This is exactly like you may have learned that subtraction is left-associative (<code>1 - 2 - 3 = (1 - 2) - 3</code>) and exponentiation is right-associative (<code>1 ^ 2 ^ 3 = 1 ^ (2 ^ 3)</code> = 1<sup>2<sup>3</sup></sup>).</li>
+<li>Multi-argument functions in Haskell aren't multi-argument functions; instead, they're functions that take one argument and return a function taking the rest.</li>
+</ol>
+<p>The <em>second</em> interpretation is the one that you should use, because it is by far the most useful one to really understand what is going on in Haskell with functions -- and since Haskell is a functional programming language, (almost) everything is functions, so understanding them is important!</p>
+<p>Now look again at the type of <code>(.)</code> (all three mean the same):</p>
+<pre><code class="language-haskell">(.) :: (b -&gt; c) -&gt; (a -&gt; b) -&gt; a -&gt; c
+(.) :: (b -&gt; c) -&gt; (a -&gt; b) -&gt; (a -&gt; c)
+(.) :: (b -&gt; c) -&gt; ((a -&gt; b) -&gt; (a -&gt; c))
+</code></pre>
+<p><small>(Why can't we remove the parentheses around the <code>b -&gt; c</code> and <code>a -&gt; c</code> in there? That is because it the type would then suddenly mean something different. Try to work that out for yourself!)</small></p>
+<p>We can read this as: <code>(.)</code> takes a function <code>g :: b -&gt; c</code>, a function <code>f :: a -&gt; b</code> and a value <code>x :: a</code>.
+It applies <code>f</code> to <code>x</code>, applies <code>g</code> to the result, and returns the result of that.
+Indeed, <code>(.)</code> can be implemented like this:</p>
+<pre><code class="language-haskell">(.) :: (b -&gt; c) -&gt; (a -&gt; b) -&gt; a -&gt; c
+(.) g f x = g (f x)
+</code></pre>
+<p>Finally, now look again at our failed attempt to write <code>both2</code> as <code>timestwo . plus</code>.
+<code>plus</code> really takes only a single argument (an <code>Int</code>eger), and it returns a <em>function</em> that takes a second integer and returns the sum of those two integers.
+So the value <code>f x</code> in the definition of <code>(.)</code> is a function in the case of <code>both2</code>!
+And <code>timestwo</code> takes an <code>Int</code> as input, not a function.
+So this is not going to work, as we've seen in a different way above.</p>
+<h3>Conclusion</h3>
+<p>We figured out why <code>timestwo . plus</code> doesn't work in two different ways: by following what the compiler does, and by reasoning on a somewhat more abstract level about what <code>(.)</code> does.
+Both arrive at the same conclusion, which is always good: if you have two different ways of deriving something and they give the same result, then the chances are already smaller that <em>both</em> are wrong.</p>
+<p>Is there a way to <em>fix</em> the error, and write the composition of <code>timestwo</code> and <code>plus</code> somehow?
+Yes; there are multiple ways, in fact.
+In this case, the best one is probably to just write it out:</p>
+<pre><code class="language-haskell">both2 a b = timestwo (plus a b)
+</code></pre>
+<p>Arguably, this makes it the clearest what's going on.</p>
+<p>If you really want a <a href="https://wiki.haskell.org/Pointfree"><em>pointfree</em></a> (also jokingly called &quot;pointless&quot;) version that doesn't mention any variable names, try one of the following:</p>
+<pre><code class="language-haskell">both2 = (timestwo .) . plus    -- same as: (\f -&gt; timestwo . f) . plus
+both2 = curry (timestwo . uncurry plus)
+</code></pre>
+<p>But that's only for fun.
+Don't actually do that.</p>