[Haskell-beginners] list splitting - nice implementation?

[KC]

Lists are good if they are short; otherwise, lists are good if you are
only traversing them from head to tail or decapitating them.

You want a more complex data structure.


On Sat, Nov 17, 2012 at 11:51 PM, Emmanuel Touzery <etouzery at gmail.com> wrote:
> well for isSorted, better use the implementation from Data.List.Ordered.
> That part was poor in performance for sure, but it wasn't my main focus, I
> was more interested in the rest.
>
>
> On Sun, Nov 18, 2012 at 8:45 AM, Emmanuel Touzery <etouzery at gmail.com>
> wrote:
>>
>> Hello,
>>
>>  i wonder what would be the idiomatic way to achieve that algorithm in
>> haskell:
>>
>> [1,4,56,450,23,46,52] => [1,4,56,450]
>> [1,4,56,450,23,46,52] => [23,46,52]
>>
>>  in other words split the list when one element gets smaller than the
>> previous one. Tge rest of the time the list is sorted. There would be only
>> two lists, not N. I always need the first or second sublist, I don't need
>> both at once. But of course a more complete algorithm handling the N case
>> and/or returning both sublists would be good.
>>
>>  i could code this by hand, but i'm trying to use as much as possible
>> builtin higher-order functions. However in this case so far I've only come
>> up with this:
>>
>> import Data.List
>>
>> isSorted :: Ord a => [a] -> Bool
>> isSorted l = (sort l) == l
>>
>> secondPart :: Ord a => [a] -> [a]
>> secondPart l = head $ filter isSorted (tails l)
>>
>> firstPart :: Ord a => [a] -> [a]
>> firstPart l = last $ filter isSorted (inits l)
>>
>>  It is concise alright, but it seems contrived and also in terms of
>> performance I don't think it's OK (for small lists sure but for big lists?).
>>
>>  Anyway, somehow I think something as simple as this must be doable very
>> concisely and with optimal performance using only builtin higher-order
>> functions. Any idea?
>>
>>  Thanks!
>>
>> Emmanuel
>
>
>
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-- 
--
Regards,
KC