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    <p>Note that <code>([0]++)</code> is the same as <code>(0:)</code>, which will make it look tidier and save us a nanosecond or two. (I'm joking with the nanosecond thing - no human can tell when something's a nanosecond faster, but it is nicer this way anyway.)</p> <p>Let's first think about making the lists you need. We want</p> <pre><code>postponeLists [[1,2,3], [7,6,8], [10,20,30,40]] = [[1,2,3], [0,7,6,8], [0,0,10,20,30,40]] = [1,2,3] : ones that should have zero in front of them </code></pre> <p>That's enough information for a definition:</p> <pre><code>postponeLists [] = [] postponeLists (l:ls) = l : map (0:) (postponeLists ls) </code></pre> <p>Now you said </p> <pre><code>foldl (zipWith +) [] [[1,2,3],[0,7,6,8],[0,0,0,3,4]] </code></pre> <p>but you mean</p> <pre><code>foldl (zipWith (+)) [] [[1,2,3],[0,7,6,8],[0,0,0,3,4]] </code></pre> <p>but unfortunately, that gives you <code>[]</code> because <code>zipWith</code> stops as soon as any of the lists run out of elements. We need some way of zipping them that doesn't stop. </p> <p>Solution 1: find the longest one, make them all that <code>maxlength</code> using <code>take maxlength.(++ repeat 0)</code><br> Solution 2: write another zipWith function that doesn't stop.</p> <p>I prefer solution 2. Let's look at the <a href="http://hackage.haskell.org/packages/archive/base/latest/doc/html/src/GHC-List.html#zipWith">definition of <code>zipWith</code></a></p> <pre><code>zipWith :: (a-&gt;b-&gt;c) -&gt; [a]-&gt;[b]-&gt;[c] zipWith f (a:as) (b:bs) = f a b : zipWith f as bs zipWith _ _ _ = [] -- here's the problem - it stops as soon as any list is empty </code></pre> <p>OK, let's not stop then:</p> <pre><code>zipWithMore :: (a -&gt; a -&gt; a) -&gt; [a] -&gt; [a] -&gt; [a] zipWithMore f (a:as) (b:bs) = f a b : zipWithMore f as bs zipWithMore f [] bs = bs -- if there's more in bs, use that zipWithMore f as [] = as -- if there's more in as, use that </code></pre> <p>Now you can replace <code>zipWith (+)</code> with <code>zipWithMore (+)</code>. I'll leave the punchline to you.</p>
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    1. COThanks, great help!
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      1. USNathan Edwards
    2. CO`(0:)` won't save you any nanoseconds over `([0]++)`, at least not with `ghc -O2`. `(:)` sometimes *is* nicer than `(++)` where either is applicable, but you shouldn't let nanosecond speculation like that shape your code.
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    3. CO@shachaf Of course not! I was pointing out by jokingly saying "save a nanosecond or two" that it's unnecessary. `(0:)` is nicer though, and I wanted to introduce that habit. Who cares about nanoseconds with a problem like this? I've made it clearer it wasn't meant seriously.
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