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
collections:
```haskell
darle :: Foldable f => f a -> Maybe (Darle a)
darle = getOption . foldMap (Option . Just . Darle [])
```
Best regards,
Petr
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