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
Dne 09/01/2013 09:13 PM, Harald Bögeholz napsal(a): 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 collections: ```haskell darle :: Foldable f => f a -> Maybe (Darle a) darle = getOption . foldMap (Option . Just . Darle []) ``` Best regards, Petr