<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"></head><body dir="auto"><div dir="ltr"></div><div dir="ltr"><blockquote type="cite"><div dir="ltr"><div dir="ltr">It seems that this solution is constantly building and breaking apart pairs.</div></div></blockquote><br></div><div dir="ltr">At first glance it seems fine. In order to pull a sublist off the front of a list you’ll need to build a new list for that part, so the disassemble/reassemble is necessary there.</div><div dir="ltr"><br></div><div dir="ltr">You can use an “as pattern” to avoid re-creating `y:ys` in your else clause, but that’s somewhat minor. I don’t see anywhere else where you are pulling something apart and then recreating the same thing.</div><div dir="ltr"><br></div><div dir="ltr">Regarding the other response, an unboxed pair is just an optimization whereby a pair of values can be returned from a function without actually allocating heap storage, but it’s just reducing memory allocation, nothing conceptually more fancy.</div><div dir="ltr"><br></div><div dir="ltr">Jeff</div><div dir="ltr"><br><blockquote type="cite">On Mar 16, 2023, at 6:34 PM, Todd Wilson <twilson@csufresno.edu> wrote:<br><br></blockquote></div><blockquote type="cite"><div dir="ltr"><div dir="ltr"><div>Dear Cafe,</div><div><br></div>Here's a basic exercise in list processing: define a function<div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="monospace">runs :: Ord a => [a] -> [[a]]</font></div></blockquote>that breaks up its input list into a list of increasing "runs"; e.g.,</div><div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="monospace">runs [3,4,5,6,2,3,4,1,2,1]</font> ---> <font face="monospace">[[3,4,5,6],[2,3,4],[1,2],[1]]</font><br></div></blockquote></div><div>A natural solution is the following:</div><div><blockquote style="margin:0 0 0 40px;border:none;padding:0px"><div><font face="monospace">runs [] = []<br>runs (x:xs) = let (ys, zs) = run x xs</font></div><div><font face="monospace"> in (x:ys) : runs zs</font></div><div><font face="monospace"> where<br> run x [] = ([], [])<br> run x (y:ys) = if x <= y</font></div><div><font face="monospace"> then let (us, vs) = run y ys</font></div><div><font face="monospace"> in (y:us, vs)</font></div><div><span style="font-family:monospace"> else ([], y:ys)</span></div></blockquote>My question: can we do better than this? It seems that this solution is constantly building and breaking apart pairs. (Or is it, when optimized?)<br></div><div><br></div><div>Todd Wilson</div></div>
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