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POHow do I create a partial function with generics in scala?
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  1. POHow do I create a partial function with generics in scala?
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    copied!<p>I'm trying to write a performance measurements library for Scala. My idea is to transparently 'mark' sections so that the execution time can be collected. Unfortunately I wasn't able to bend the compiler to my will.</p> <p>An admittedly contrived example of what I have in mind:</p> <pre><code>// generate a timing function val myTimer = mkTimer('myTimer) // see how the timing function returns the right type depending on the // type of the function it is passed to it val act = actor { loop { receive { case 'Int =&gt; val calc = myTimer { (1 to 100000).sum } val result = calc + 10 // calc must be Int self reply (result) case 'String =&gt; val calc = myTimer { (1 to 100000).mkString } val result = calc + " String" // calc must be String self reply (result) } </code></pre> <p>Now, this is the farthest I got: </p> <pre><code>trait Timing { def time[T &lt;: Any](name: Symbol)(op: =&gt; T) :T = { val start = System.nanoTime val result = op val elapsed = System.nanoTime - start println(name + ": " + elapsed) result } def mkTimer[T &lt;: Any](name: Symbol) : (() =&gt; T) =&gt; () =&gt; T = { type c = () =&gt; T time(name)(_ : c) } } </code></pre> <p>Using the <code>time</code> function directly works and the compiler correctly uses the return type of the anonymous function to type the 'time' function:</p> <pre><code>val bigString = time('timerBigString) { (1 to 100000).mkString("-") } println (bigString) </code></pre> <p>Great as it seems, this pattern has a number of shortcomings:</p> <ul> <li>forces the user to reuse the same symbol at each invocation</li> <li>makes it more difficult to do more advanced stuff like predefined project-level timers</li> <li>does not allow the library to initialize once a data structure for 'timerBigString</li> </ul> <p>So here it comes mkTimer, that would allow me to partially apply the time function and reuse it. I use mkTimer like this:</p> <pre><code>val myTimer = mkTimer('aTimer) val myString= myTimer { (1 to 100000).mkString("-") } println (myString) </code></pre> <p>But I get a compiler error:</p> <pre><code>error: type mismatch; found : String required: () =&gt; Nothing (1 to 100000).mkString("-") </code></pre> <p>I get the same error if I inline the currying:</p> <pre><code>val timerBigString = time('timerBigString) _ val bigString = timerBigString { (1 to 100000).mkString("-") } println (bigString) </code></pre> <p>This works if I do <code>val timerBigString = time('timerBigString) (_: String)</code>, but this is not what I want. I'd like to defer typing of the partially applied function until application.</p> <p>I conclude that the compiler is deciding the return type of the partial function when I first create it, chosing "Nothing" because it can't make a better informed choice.</p> <p>So I guess what I'm looking for is a sort of late-binding of the partially applied function. Is there any way to do this? Or maybe is there a completely different path I could follow?</p> <p>Well, thanks for reading this far</p> <p>-teo</p>
 

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