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copied!<p>Very carefully. There are two things wrong with your definition in its current state:</p> <p>The first is the one that generates the exception you're seeing. You don't actually want the assignment operator preceding <code>b(a)</code>, you want that to be the body of your anonymous function, so you'll need the <code>=></code> operator:</p> <p><code>val compose = (a: Double => Double, b: Double => Double) => b(a)</code></p> <p>However, you're still not done. In the body <code>b(a)</code>, what the compiler thinks you're doing is <code>apply</code>ing b, with <code>a</code> as the parameter, so you get:</p> <pre><code><console>:7: error: type mismatch; found : Double => Double required: Double val compose = (a: Double => Double, b: Double => Double) => b(a) </code></pre> <p>As you can see, it's expecting <code>b</code> to take a <code>Double</code>, but you're passing <code>a</code> as an argument, which is a <code>Double => Double</code> / <code>Function1[Double, Double</code>].</p> <p>So that's why you're seeing what you're seeing, and what you'd see in the next step, but I'm not entirely sure myself how to answer quite <em>how</em> to write <code>compose</code>. You can cheat somewhat by adding a parameter to <code>compose</code>:</p> <pre><code>scala> val compose = (a: Double => Double, b: Double => Double, c: Double) => b(a(c)); compose: (Double => Double, Double => Double, Double) => Double = <function3> scala> compose(x => x * 2, y => y * 2, 2) res2: Double = 8.0 </code></pre> <p>But that's cheating.</p>

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