[Haskell-cafe] Basic list exercise

[Anthony Clayden]

On Thu, Mar 16, 2023, 20:33 Todd Wilson <twilson at csufresno.edu
<http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe>>
wrote:

> ...

> 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?)


I don't think we can do better (as others have commented). Laziness is a
benefit for peeling off only the beginning(s) of possibly-infinite
(sub-)lists.

Possibly splitting off the head to call sub-function `run x xs` is also
"breaking apart". If you don't like pairs, you can split the returned list
itself to give the pairing.

Using `l@(y:zs)` as did Viktor might help a little if the compiler is
particularly dumb. (GHC isn't.)

>    runs :: Ord a => [a] -> [[a]]
>
>    runs   []  = []
>    runs l@[x] = [l]
>    runs (x: l@(y:zs))
>        | x <= y    = let (ys:zs) = runs l in ((x:ys) : zs)
>        | otherwise = ([x]: runs l)

This solution appears to  achieve the same level of laziness as Viktor's.

AntC
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