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POUsing scalaz state in a more complicated computation
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2012-02-08T13:02:50
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2012-02-09T09:30:50.450
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2012-02-08T13:18:00.637
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I'm trying to understand how to use scalaz <code>State</code> to perform a complicated stateful computation. Here is the problem: <blockquote> Given a <code>List[Int]</code> of potential divisors and a <code>List[Int]</code> of numbers, find a <code>List[(Int, Int)</code>] of matching pairs (divisor, number) where a divisor is allowed to match at most one number. </blockquote> As a test: <pre><code>def findMatches(divs: List[Int], nums: List[Int]): List[(Int, Int)] </code></pre> And with the following input: <pre><code>findMatches( List(2, 3, 4), List(1, 6, 7, 8, 9) ) </code></pre> We can get at most 3 matches. If we stipulate that the matches must be made in the order in which they occur traversing the lists l-r, then the matches must be: <pre><code>List( (2, 6) , (3, 9) , (4, 8) ) </code></pre> So the following two tests need to pass: <pre><code>assert(findMatches(List(2, 3, 4), List(1, 6, 7, 8, 9)) == List((2, 6), (3, 9), (4, 8))) assert(findMatches(List(2, 3, 4), List(1, 6, 7, 8, 11)) == List((2, 6), (4, 8))) </code></pre> Here's an imperative solution: <pre><code>scala> def findMatches(divs: List[Int], nums: List[Int]): List[(Int, Int)] = { | var matches = List.empty[(Int, Int)] | var remaining = nums | divs foreach { div => | remaining find (_ % div == 0) foreach { n => | remaining = remaining filterNot (_ == n) | matches = matches ::: List(div -> n) | } | } | matches | } findMatches: (divs: List[Int], nums: List[Int])List[(Int, Int)] </code></pre> Notice that I have to update the state of <code>remaining</code> as well as accumulating <code>matches</code>. It sounds like a job for scalaz traverse! My useless working has got me this far: <pre><code>scala> def findMatches(divs: List[Int], nums: List[Int]): List[(Int, Int)] = { | divs.traverse[({type l[a] = State[List[Int], a]})#l, Int]( div => | state { (rem: List[Int]) => rem.find(_ % div == 0).map(n => rem.filterNot(_ == n) -> List(div -> n)).getOrElse(rem -> List.empty[(Int, Int)]) } | ) ~> nums | } <console>:15: error: type mismatch; found : List[(Int, Int)] required: Int state { (rem: List[Int]) => rem.find(_ % div == 0).map(n => rem.filterNot(_ == n) -> List(div -> n)).getOrElse(rem -> List.empty[(Int, Int)]) } ^ </code></pre>
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<scala><functional-programming><scalaz>
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Using scalaz state in a more complicated computation
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