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.

On Mar 16, 2023, at 6:34 PM, Todd Wilson <twilson@csufresno.edu> 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

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