oppi.li
advent of code solutions
aoc.oppi.li
haskell
aoc
2025/06: add type annotations
Signed-off-by: oppiliappan <me@oppi.li>
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out/2.4-day-6.html
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out/2.4-day-6.html
···
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the start of the list.</p>
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<div class="code">
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<div class="sourceCode" id="cb1"><pre
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-
class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>solve (<span class="st">"+"</span> <span class="op">:</span> rest) <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> <span class="fu">read</span> <span class="op">$</span> rest</span>
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<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>solve (<span class="st">"*"</span> <span class="op">:</span> rest) <span class="ot">=</span> <span class="fu">product</span> <span class="op">.</span> <span class="fu">map</span> <span class="fu">read</span> <span class="op">$</span> rest</span></code></pre></div>
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="ot">solve ::</span> [<span class="dt">String</span>] <span class="ot">-></span> <span class="dt">Int</span></span>
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<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>solve (<span class="st">"+"</span> <span class="op">:</span> rest) <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> <span class="fu">read</span> <span class="op">$</span> rest</span>
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<span id="cb1-3"><a href="#cb1-3" aria-hidden="true" tabindex="-1"></a>solve (<span class="st">"*"</span> <span class="op">:</span> rest) <span class="ot">=</span> <span class="fu">product</span> <span class="op">.</span> <span class="fu">map</span> <span class="fu">read</span> <span class="op">$</span> rest</span></code></pre></div>
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</div>
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</div>
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<div class="row">
···
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rotates a list forwards to move the last element to the first.</p>
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<div class="code">
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<div class="sourceCode" id="cb2"><pre
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a>rot <span class="ot">=</span> liftA2 (<span class="op">:</span>) <span class="fu">last</span> <span class="fu">init</span></span></code></pre></div>
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="ot">rot ::</span> [a] <span class="ot">-></span> [a]</span>
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<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a>rot <span class="ot">=</span> liftA2 (<span class="op">:</span>) <span class="fu">last</span> <span class="fu">init</span></span></code></pre></div>
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</div>
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</div>
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<div class="row">
···
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<div class="sourceCode" id="cb4"><pre
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class="sourceCode haskell haskell-top"><code class="sourceCode haskell"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="kw">import</span> <span class="dt">Data.List</span> (transpose)</span></code></pre></div>
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<div class="sourceCode" id="cb5"><pre
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a></span>
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="ot">p1 ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Int</span></span>
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<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a>p1 <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> solve <span class="op">.</span> align <span class="op">.</span> rot <span class="op">.</span> <span class="fu">lines</span></span>
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<span id="cb5-3"><a href="#cb5-3" aria-hidden="true" tabindex="-1"></a> <span class="kw">where</span></span>
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<span id="cb5-4"><a href="#cb5-4" aria-hidden="true" tabindex="-1"></a> align <span class="ot">=</span> transpose <span class="op">.</span> <span class="fu">map</span> <span class="fu">words</span></span></code></pre></div>
···
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<div class="sourceCode" id="cb10"><pre
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class="sourceCode haskell haskell-top"><code class="sourceCode haskell"><span id="cb10-1"><a href="#cb10-1" aria-hidden="true" tabindex="-1"></a><span class="kw">import</span> <span class="dt">Data.List.Split</span> (splitWhen)</span></code></pre></div>
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<div class="sourceCode" id="cb11"><pre
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a>p2 <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> solve <span class="op">.</span> align <span class="op">.</span> rot <span class="op">.</span> <span class="fu">lines</span></span>
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<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a> <span class="kw">where</span></span>
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<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> align <span class="ot">=</span></span>
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<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">zipWith</span> (<span class="op">:</span>)</span>
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<span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a> <span class="op"><$></span> (<span class="fu">words</span> <span class="op">.</span> <span class="fu">head</span>)</span>
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<span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a> <span class="op"><*></span> (splitWhen (<span class="fu">all</span> (<span class="ch">' '</span> <span class="op">==</span>)) <span class="op">.</span> transpose <span class="op">.</span> <span class="fu">tail</span>)</span></code></pre></div>
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb11-1"><a href="#cb11-1" aria-hidden="true" tabindex="-1"></a><span class="ot">p2 ::</span> <span class="dt">String</span> <span class="ot">-></span> <span class="dt">Int</span></span>
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<span id="cb11-2"><a href="#cb11-2" aria-hidden="true" tabindex="-1"></a>p2 <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> solve <span class="op">.</span> align <span class="op">.</span> rot <span class="op">.</span> <span class="fu">lines</span></span>
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<span id="cb11-3"><a href="#cb11-3" aria-hidden="true" tabindex="-1"></a> <span class="kw">where</span></span>
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<span id="cb11-4"><a href="#cb11-4" aria-hidden="true" tabindex="-1"></a> align <span class="ot">=</span></span>
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<span id="cb11-5"><a href="#cb11-5" aria-hidden="true" tabindex="-1"></a> <span class="fu">zipWith</span> (<span class="op">:</span>)</span>
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<span id="cb11-6"><a href="#cb11-6" aria-hidden="true" tabindex="-1"></a> <span class="op"><$></span> (<span class="fu">words</span> <span class="op">.</span> <span class="fu">head</span>)</span>
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<span id="cb11-7"><a href="#cb11-7" aria-hidden="true" tabindex="-1"></a> <span class="op"><*></span> (splitWhen (<span class="fu">all</span> (<span class="ch">' '</span> <span class="op">==</span>)) <span class="op">.</span> transpose <span class="op">.</span> <span class="fu">tail</span>)</span></code></pre></div>
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</div>
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</div>
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<div class="row">
···
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</div>
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</div>
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<pre><code>splitWhen (all isSpace) . transpose . tail</code></pre>
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<pre><code>splitWhen (all (' ' ==)) . transpose . tail</code></pre>
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<div class="row">
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<p>…is used to transpose numbers! Note what happens when we
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transpose the lines containing numbers, we get:</p>
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src/2025/06.lhs
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src/2025/06.lhs
···
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This part needs us to consider each operation on a column. `Data.List.transpose` fortunately makes this very simple. If we are able to massage our input into the form `["+", "1", "2", "3"]`, we can use this simple `solve` function to produce the result. The only motivation to use this format is the ease of pattern-matching over the start of the list.
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```haskell
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solve :: [String] -> Int
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solve ("+" : rest) = sum . map read $ rest
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solve ("*" : rest) = product . map read $ rest
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```
···
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Thus, to solve the first part, we can break up our input using `lines`. `rot` is a simple function that rotates a list forwards to move the last element to the first.
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```haskell
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rot :: [a] -> [a]
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rot = liftA2 (:) last init
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```
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···
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```
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```haskell
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-
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p1 :: String -> Int
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p1 = sum . map solve . align . rot . lines
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where
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align = transpose . map words
···
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```
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```haskell
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p2 :: String -> Int
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p2 = sum . map solve . align . rot . lines
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where
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align =
···
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The second part ...
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-
splitWhen (all isSpace) . transpose . tail
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splitWhen (all (' ' ==)) . transpose . tail
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...is used to transpose numbers! Note what happens when we transpose the lines containing numbers, we get:
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