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Comment by fbonetti | Hacker News Reader

That's just a combination of pattern matching and tail call optimization. That code would look exactly this same in Haskell:

    countToTen n = countToTen (n + 1)
    countToTen 10 = done

The 10 case would have to be first, or that's an infinite loop. Cases are checked in order. That said, the compiler will give you a warning about it.