[Haskell-cafe] Performance of delete-and-return-last-element

Petr Pudlák petr.mvd at gmail.com
Thu Sep 5 20:29:53 CEST 2013

Dne 09/01/2013 09:13 PM, Harald Bögeholz napsal(a):
> Am 31.08.13 14:35, schrieb Petr Pudlák:
>> One solution would be to fold over a specific semigroup instead of a
>> recursive function:
>> |import  Data.Semigroup
>> import  Data.Foldable(foldMap)
>> import  Data.Maybe(maybeToList)
>> data  Darle  a =Darle  {getInit  :: [a],getLast  ::a  }
>>    deriving  Show
>> instance  Semigroup  (Darle  a)where
>>      ~(Darle  xs1 l1) <> ~(Darle  xs2 l2) =Darle  (xs1 ++ [l1] ++ xs2) l2
>> darle  :: [a] ->Darle  a
>> darle  = foldr1 (<>) . map (Darle  [])|
>> It's somewhat more verbose, but the core idea is clearly expressed in
>> the one line that defines |<>|, and IMHO it better shows /what/ are we
>> doing rather than /how/. It's sufficiently lazy so that you can do
>> something like |head . getInit $ darle [1..]|.
> I am wondering why you put the Semigroup instance there and what the
> other imports are for. Doesn't this work just as well?
Sorry, the two other imports are redundant, I forgot to erase them when 
playing with various ideas.

The Semigroup instance of course isn't necessary for this particular 
purpose. But having it (1) signals that the operation satisfies some 
laws (associativity) and (2) allows the structure to be reused anywhere 
where a Semigroup is required.

For example, we can wrap it into `Option` to get a monoid, and perhaps 
use it in `foldMap`. This way we extend the functionality to empty 
darle :: Foldable f => f a -> Maybe (Darle a)
darle = getOption . foldMap (Option . Just . Darle [])

   Best regards,

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