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?
data Darle a = Darle {getInit :: [a], getLast :: a}
deriving Show
~(Darle xs1 l1) <> ~(Darle xs2 l2) = Darle (xs1 ++ [l1] ++ xs2) l2
darle :: [a] ->Darle a
darle = foldr1 (<>) . map (Darle [])
Seems to work here. I am still puzzled, though, if this is really a good
idea performance-wise. I am afraid I don't understand it well enough.
Harald
--
Harald Bögeholz