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16786452
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2013-05-28T07:40:59.810
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2016-10-18T11:48:31.717
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2016-10-18T11:48:31.717
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485115
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16785826
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What you were told was true about "for comprehensions", but the problem with your question is that you've mixed up "for comprehensions" with "anonymous functions". "For comprehension" in Scala is a syntactic sugar for a series of <code>.flatMap</code>, <code>.map</code> and <code>.filter</code> applications. Since you are testing reduction algorithms and since it's impossible to implement a reduction algorithm using those three functions, your test cases are incorrect. Here's an example of a "for comprehension": <pre><code>val listOfLists = List(List(1,2), List(3,4), List(5)) val result = for { itemOfListOfLists <- listOfLists itemOfItemOfListOfLists <- itemOfListOfLists } yield (itemOfItemOfListOfLists + 1) assert( result == List(2,3,4,5,6) ) </code></pre> The compiler desugars the Comprehension part to the following: <pre><code>val result = listOfLists.flatMap( itemOfListOfLists => itemOfListOfLists.map( itemOfItemOfListOfLists => itemOfItemOfListOfLists + 1 ) ) </code></pre> Then it desugars the Anonymous Function syntax: <pre><code>val result = listOfLists.flatMap( new Function1[List[Int], List[Int]] { override def apply(itemOfListOfLists: List[Int]): List[Int] = itemOfListOfLists.map( new Function1[Int, Int] { override def apply(itemOfItemOfListOfLists: Int): Int = itemOfItemOfListOfLists + 1 } ) } ) </code></pre> From the desugarred code it's now evident that the <code>Function1[Int, Int]</code> class gets instantiated every time the <code>apply(itemOfListOfLists: List[Int]): List[Int]</code> method gets called. That happens for every entry of <code>listOfLists</code>. So the more complex your comprehension is the more instantiations of the <code>Function</code> objects you get.
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1. POWhy are Scala "for loop comprehensions" so very slow compared to FOR loops?
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1. PTAnswer
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1. USNikita Volkov
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1. USNikita Volkov
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