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POWhat is the FunctionN trait that represents a function taking a by-name parameter?
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  1. POWhat is the FunctionN trait that represents a function taking a by-name parameter?
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    copied!<p>A function in Scala is an object that implements one of the <code>FunctionN</code> traits. For example:</p> <pre><code>scala&gt; def f(x: Int) = x * x f: (x: Int)Int scala&gt; val ff = f _ ff: Int =&gt; Int = &lt;function1&gt; scala&gt; val fff: Function1[Int, Int] = f _ fff: Int =&gt; Int = &lt;function1&gt; </code></pre> <p>So far, so good. But what if we have a function that takes a by-name parameter? It certainly does still implement one of the <code>FunctionN</code> traits:</p> <pre><code>scala&gt; def g(x: =&gt; Int) = x * x g: (x: =&gt; Int)Int scala&gt; val gg = g _ gg: =&gt; Int =&gt; Int = &lt;function1&gt; scala&gt; gg.isInstanceOf[Function1[_, _]] res0: Boolean = true </code></pre> <p>But what type is it, exactly? It's not <code>Function1[Int, Int]</code>:</p> <pre><code>scala&gt; val ggg: Function1[Int, Int] = g _ &lt;console&gt;:8: error: type mismatch; found : =&gt; Int =&gt; Int required: Int =&gt; Int val ggg: Function1[Int, Int] = g _ ^ </code></pre> <p>Nor is it <code>Function1[Function0[Int], Int]</code>:</p> <pre><code>scala&gt; val ggg: Function1[Function0[Int], Int] = g _ &lt;console&gt;:8: error: type mismatch; found : =&gt; Int =&gt; Int required: () =&gt; Int =&gt; Int val ggg: Function1[Function0[Int], Int] = g _ ^ </code></pre> <p>And <code>Function1[=&gt; Int, Int]</code> fails to compile:</p> <pre><code>scala&gt; val ggg: Function1[=&gt; Int, Int] = g _ &lt;console&gt;:1: error: identifier expected but '=&gt;' found. val ggg: Function1[=&gt; Int, Int] = g _ ^ </code></pre> <p>So what is it?</p>
 

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