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POGeneric, type-safe way to flatten arbitrarily nested collections in Scala?
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copied!<p>On occasion I take some time to play with Scala, whose mix of features appeals to me despite an inability to use it in my own work (thus far). For kicks I decided to try the first few <a href="http://www.haskell.org/haskellwiki/99_questions/1_to_10">99 Haskell Problems</a> in the most generic way possible — operating on and returning any kind of applicable collection. The first few questions weren’t too difficult, but I find myself utterly stymied by <code>flatten</code>. I just can’t figure out how to type such a thing.</p> <p>To be specific about my question: is it possible to write a type-safe function that flattens arbitrarily-nested <code>SeqLike</code>s? So that, say,</p> <pre><code>flatten(List(Array(List(1, 2, 3), List(4, 5, 6)), Array(List(7, 8, 9), List(10, 11, 12)))) </code></pre> <p>would return</p> <pre><code>List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12): List[Int] </code></pre> <p>? Note that this isn’t quite the same question as in the Haskell and Scala problem sets; I’m trying to write a function that flattens not heterogeneous lists but, rather, homogeneous-but-nested sequences.</p> <p>Searching the web I found a <a href="http://aperiodic.net/phil/scala/s-99/p07.scala">translation into Scala</a> of that question, but it operates on and returns a List[Any]. Am I correct that this would require some kind of type recursion? Or am I making this out to be harder than it is?</p>

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