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Author: ploeh

_posts/2019-04-15-lazy-monoids.html

@@ -476,4 +476,41 @@ <h2 id="comments-header">
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         <div class="comment-date">2025-02-25 18:40 UTC</div>
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+	<div class="comment" id="f6a9ef03acc848849ea6c8bc54b235d9">
+		<div class="comment-author"><a href="/">Mark Seemann</a> <a href="#f6a9ef03acc848849ea6c8bc54b235d9">#</a></div>
+		<div class="comment-content">
+			<p>
+				Ian, thank you for writing. I agree that there's some hand-waving going on in that article. To make a long story short, that particular code base also includes this overload of <code>Apply</code>:
+			</p>
+			<p>
+				<pre><span style="color:blue;">public</span>&nbsp;<span style="color:blue;">static</span>&nbsp;<span style="color:#2b91af;">Lazy</span>&lt;<span style="color:#2b91af;">Func</span>&lt;<span style="color:#2b91af;">T2</span>,&nbsp;<span style="color:#2b91af;">TResult</span>&gt;&gt;&nbsp;<span style="color:#74531f;">Apply</span>&lt;<span style="color:#2b91af;">T1</span>,&nbsp;<span style="color:#2b91af;">T2</span>,&nbsp;<span style="color:#2b91af;">TResult</span>&gt;(
+&nbsp;&nbsp;&nbsp;&nbsp;<span style="color:blue;">this</span>&nbsp;<span style="color:#2b91af;">Func</span>&lt;<span style="color:#2b91af;">T1</span>,&nbsp;<span style="color:#2b91af;">T2</span>,&nbsp;<span style="color:#2b91af;">TResult</span>&gt;&nbsp;<span style="font-weight:bold;color:#1f377f;">selector</span>,
+&nbsp;&nbsp;&nbsp;&nbsp;<span style="color:#2b91af;">Lazy</span>&lt;<span style="color:#2b91af;">T1</span>&gt;&nbsp;<span style="font-weight:bold;color:#1f377f;">source</span>)
+{
+&nbsp;&nbsp;&nbsp;&nbsp;<span style="font-weight:bold;color:#8f08c4;">return</span>&nbsp;<span style="color:blue;">new</span>&nbsp;<span style="color:#2b91af;">Lazy</span>&lt;<span style="color:#2b91af;">Func</span>&lt;<span style="color:#2b91af;">T2</span>,&nbsp;<span style="color:#2b91af;">TResult</span>&gt;&gt;(
+&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;()&nbsp;=&gt;&nbsp;<span style="font-weight:bold;color:#1f377f;">y</span>&nbsp;=&gt;&nbsp;<span style="font-weight:bold;color:#1f377f;">selector</span>(<span style="font-weight:bold;color:#1f377f;">source</span>.Value,&nbsp;<span style="font-weight:bold;color:#1f377f;">y</span>));
+}</pre>
+			</p>
+			<p>
+				I keep learning during and after writing these articles, and it's possible that some of the connections you point out weren't entirely clear to me when I wrote. Or perhaps I was driven by a didactic motivation of presenting the concept in as clean a way as possible. Sometimes, I gloss over some details in order to get an overall point across. I honestly no longer remember exactly what motivated me to leave out that detail back then, but it was probably some combination of the above.
+			</p>
+			<p>
+				You are correct that currying is the key to making this simpler. <a href="/2018/10/15/an-applicative-password-list#0ded7ac93aad8ba7b1063dd49c2051f1">Tyson Williams pointed this out to me in 2018</a>.
+			</p>
+			<p>
+				FWIW, until recently I had a tendency to forget about <code>liftA2</code> and its cousins, so I developed a habit of writing Haskell expressions like <code>(+) &lt;$&gt; (Just 10) &lt;*&gt; (Just 32)</code>, as witnessed by, among others, <a href="/2022/01/17/enumerate-wordle-combinations-with-an-applicative-functor">this article</a>.
+			</p>
+			<p>
+				Is your final C# example to be preferred? I'm not sure. I haven't tried to actually write it out, so perhaps I'm missing something, but as far as I can tell, it's an extension method on <code>Func</code> that takes a single 'applicative value' as an argument. If so, isn't it more or less the above <code>Apply</code> overload with a different name?
+			</p>
+			<p>
+				As far as I can tell, that's not the idiomatic <code>Select</code> shape that most C# developers are used to, since it has its arguments swapped. You and I may realize that this is still isomorphic to the 'standard' <code>Select</code> signature, but I think that we've now moved beyond what most professional C# developers are willing to grapple with.
+			</p>
+			<p>
+				I don't mean that as an attack on your comments. I think that you are correct. Rather, during and after writing this article series on applicative functors, I've come to appreciate that however useful that abstraction is, it's not a good fit in C#.
+			</p>
+		</div>
+		<div class="comment-date">2025-03-04 08:04 UTC</div>
+	</div>
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