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    <blockquote> <p>'yield' sucks, continuations are better</p> </blockquote> <p>Actually, Python's <code>yield</code> <em>is</em> a continuation.</p> <p>What is a continuation? A continuation is saving the present point of execution with all its state, such that one can <em>continue</em> at that point later. That's precisely what Python's <code>yield</code>, and, also, precisely how it is implemented.</p> <p>It is my understanding that Python's continuations are not <em>delimited</em>, however. I don't know much about that -- I might be wrong, in fact. Nor do I know what the implications of that may be.</p> <p>Scala's continuation do not work at run-time -- in fact, there's a continuations library for Java that work by doing stuff to bytecode at run-time, which is free of the constrains that Scala's continuation have.</p> <p>Scala's continuation are entirely done at compile time, which require quite a bit of work. It also requires that the code that will be "continued" be prepared by the compiler to do so.</p> <p>And that's why for-comprehensions do not work. A statement like this:</p> <pre><code>for { x &lt;- xs } proc(x) </code></pre> <p>If translated into</p> <pre><code>xs.foreach(x =&gt; proc(x)) </code></pre> <p>Where <code>foreach</code> is a method on <code>xs</code>'s class. Unfortunately, <code>xs</code> class has been long compiled, so it cannot be modified into supporting the continuation. As a side note, that's also why Scala doesn't have <code>continue</code>.</p> <p>Aside from that, yes, this is a duplicate question, and, yes, you should find a different way to write your code.</p>
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    1. POWhat is the preferred way to implement 'yield' in Scala?
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    1. USDaniel C. Sobral
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      1. VTUpMod
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    1. COThere are continuation based solutions that do what Urban Vagabond wants (http://stackoverflow.com/questions/2201882/implementing-yield-yield-return-using-scala-continuations/2218589#2218589 ), except they apparently don't work with `foreach` defined on existing collections. The trivial solution is to rewrite the for loops to while loops. But alternatively, could one replace existing `foreach` methods with custom methods that *are* compatible with continuations?
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      1. USKipton Barros
    2. COTiark (the Scala continuations guy) has a *really* awesome trick that makes almost every higher-order function in the collections library continuations-compatible (including `foreach`). However, a) it hasn't landed yet, and b) it's not a general solution for any higher-order function (it only works because the collections library is insanely generic).
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      1. USDaniel Spiewak
    3. CODo you have a link? Any estimated time when this will be merged into trunk?
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