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copied!<p>I'll start by simplifying a bit. Suppose I have a function <code>f</code>. I don't know it's arity in advance, and I don't care about what it returns. I want to wrap it with some functionality and get a function with the same argument types. I also don't care what this resulting function returns, so I may as well have it return <code>Unit</code>.</p> <p>You could write a bunch (well, 22) of functions like the following:</p> <pre><code>def wrap[A](f: A => Unit): A => Unit = ??? def wrap[A, B](f: (A, B) => Unit): (A, B) => Unit = ??? def wrap[A, B, C](f: (A, B, C) => Unit): (A, B, C) => Unit = ??? </code></pre> <p>But you don't want to.</p> <p>Shapeless can definitely help you solve this problem more generically:</p> <pre><code>def wrap[F, A <: HList](f: F)( implicit h: FnHListerAux[F, A => Unit], u: FnUnHListerAux[A => Unit, F] ): F = { (args: A) => println("Before!"); f.hlisted(args); println("After!") }.unhlisted </code></pre> <p>Which gives us:</p> <pre><code>scala> def f(i: Int, s: String) { println(s * i) } f: (i: Int, s: String)Unit scala> val wf = wrap(f _) wf: (Int, String) => Unit = <function2> scala> wf(3, "ab") Before! ababab After! </code></pre> <p>Note that <code>f</code> could return something other than <code>Unit</code> and this would still work, since everything in Scala is a <code>Unit</code> and the <code>FunctionN</code> traits are covariant in their return type.</p> <p>Applying this approach to your code, we get the following:</p> <pre><code>def async[F, A <: HList]( shell: Shell, success: F, failure: FunctionTypes.Failure )( implicit h: FnHListerAux[F, A => Unit], u: FnUnHListerAux[A => Unit, F] ): F = { (args: A) => require(shell != null, "Shell cannot be null") require(shell.getDisplay() != null, "The shell must have a display") val display = shell.getDisplay() display.asyncExec(new Runnable() { def run(): Unit = { try { success.hlisted(args) } catch { case e: Throwable => failure(e) } } }) }.unhlisted </code></pre> <p>No <code>dummy</code> required.</p>

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