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POScalaz: request for use case for Cokleisli composition
 

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  1. POScalaz: request for use case for Cokleisli composition
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    <p>This question isn't meant as flame-bait! As it might be apparent, I've been looking at <a href="http://github.com/scalaz/scalaz" rel="nofollow noreferrer">Scalaz</a> recently. I'm trying to understand <em>why</em> I need some of the functionality that the library provides. Here's something:</p> <pre><code>import scalaz._ import Scalaz._ type NEL[A] = NonEmptyList[A] val NEL = NonEmptyList </code></pre> <p>I put some println statements in my functions to see what was going on (<em>aside: what would I have done if I was trying to avoid side effects like that?</em>). My functions are:</p> <pre><code>val f: NEL[Int] =&gt; String = (l: NEL[Int]) =&gt; {println("f: " + l); l.toString |+| "X" } val g: NEL[String] =&gt; BigInt = (l: NEL[String]) =&gt; {println("g: " + l); BigInt(l.map(_.length).sum) } </code></pre> <p>Then I combine them via a <strong>cokleisli</strong> and pass in a <code>NEL[Int]</code></p> <pre><code>val k = cokleisli(f) =&gt;= cokleisli(g) println("RES: " + k( NEL(1, 2, 3) )) </code></pre> <p>What does this print?</p> <pre><code>f: NonEmptyList(1, 2, 3) f: NonEmptyList(2, 3) f: NonEmptyList(3) g: NonEmptyList(NonEmptyList(1, 2, 3)X, NonEmptyList(2, 3)X, NonEmptyList(3)X) RES: 57 </code></pre> <p>The RES value is the character count of the (String) elements in the final NEL. Two things occur to me:</p> <ol> <li>How could I have known that my NEL was going to be reduced in this manner from the method signatures involved? (I wasn't expecting the result <em>at all</em>)</li> <li>What is the point of this? Can a reasonably simple and easy-to-follow use case be distilled for me?</li> </ol> <p>This question is a thinly-veiled plea for some lovely person like <em><a href="https://stackoverflow.com/questions/2545248/function-syntax-puzzler-in-scalaz/2546400#2546400">retronym</a></em> to explain how this powerful library actually works.</p>
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