Re: [Haskell-cafe] Basic list exercise

It seems that this solution is constantly building and breaking apart pairs.

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. 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. 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. Jeff

On Mar 16, 2023, at 6:34 PM, Todd Wilson wrote:

 Dear Cafe,

Here's a basic exercise in list processing: define a function runs :: Ord a => [a] -> [[a]] that breaks up its input list into a list of increasing "runs"; e.g., runs [3,4,5,6,2,3,4,1,2,1] ---> [[3,4,5,6],[2,3,4],[1,2],[1]] A natural solution is the following: runs [] = [] runs (x:xs) = let (ys, zs) = run x xs in (x:ys) : runs zs where run x [] = ([], []) run x (y:ys) = if x <= y then let (us, vs) = run y ys in (y:us, vs) else ([], y:ys) 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?)

Todd Wilson _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.