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copied!<p>You want to transform each element in the input list into a sequence of as many equal <code>Bool</code>s as the element says, and you want the <code>Bool</code> be <code>True</code> if the index of the number in the input list is even, and <code>False</code> if the index is odd.</p> <p>For that, you don't need the index, and it is better avoided - that gives simpler and usually more efficient code. The point is that the value alternates, it has a periodic pattern. To construct such periodic patterns, the Prelude offers the useful</p> <pre><code>cycle :: [a] -> [a] Prelude> take 10 $ cycle [1,2,3] [1,2,3,1,2,3,1,2,3,1] Prelude> take 10 $ cycle [True,False] [True,False,True,False,True,False,True,False,True,False] </code></pre> <p>Neat, that's exactly what we need.</p> <p>Now, we can pair up each element of the input list with the corresponding <code>Bool</code>:</p> <pre><code>[ 2, 2, 4, 3] [True,False,True,False,... </code></pre> <p>We could use <code>zip</code> to produce pairs, <code>[(2,True), (2,False), ...]</code> and then use a function that transforms the pair into the appropriate sequence of <code>Bool</code>s.</p> <p>But that pattern is so common that we have a special higher-order function for that, <code>zipWith</code>.</p> <p>So if the type of list elements is <code>Int</code>, we can write</p> <pre><code>row :: [Int] -> [Bool] row xs = concat $ zipWith replicate xs (cycle [True,False]) </code></pre> <p>For the type <code>Integer</code>, we can't use <code>replicate</code>, but we can use <code>genericReplicate</code> from <code>Data.List</code>.</p>

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