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POFunction syntax puzzler in scalaz
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  1. POFunction syntax puzzler in scalaz
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    copied!<p>Following watching <a href="http://vimeo.com/10482466" rel="noreferrer">Nick Partidge's presentation</a> on deriving <a href="http://github.com/scalaz/scalaz" rel="noreferrer">scalaz</a>, I got to looking at this example, which is just awesome:</p> <pre><code>import scalaz._ import Scalaz._ def even(x: Int) : Validation[NonEmptyList[String], Int] = if (x % 2 ==0) x.success else "not even: %d".format(x).wrapNel.fail println( even(3) &lt;|*|&gt; even(5) ) //prints: Failure(NonEmptyList(not even: 3, not even: 5)) </code></pre> <p>I was trying to understand what the <code>&lt;|*|&gt;</code> method was doing, here is the source code:</p> <pre><code>def &lt;|*|&gt;[B](b: M[B])(implicit t: Functor[M], a: Apply[M]): M[(A, B)] = &lt;**&gt;(b, (_: A, _: B)) </code></pre> <p>OK, that is fairly confusing (!) - but it references the <code>&lt;**&gt;</code> method, which is declared thus:</p> <pre><code>def &lt;**&gt;[B, C](b: M[B], z: (A, B) =&gt; C)(implicit t: Functor[M], a: Apply[M]): M[C] = a(t.fmap(value, z.curried), b) </code></pre> <p>So I have a few questions:</p> <ol> <li>How come the method appears to take a <em>higher-kinded type</em> of one type parameter (<code>M[B]</code>) but can get passed a <code>Validation</code> (which has two type paremeters)?</li> <li>The syntax <code>(_: A, _: B)</code> defines the function <code>(A, B) =&gt; Pair[A,B]</code> which the 2nd method expects: <em>what is happening to the Tuple2/Pair in the failure case? There's no tuple in sight!</em></li> </ol>
 

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