foldl1 and foldr1 #110
Description
Should foldl1 and foldr1 be added either to the Foldable1 class or as helpers in the Data.Semigroup.Foldable module? They can be implemented, if awkwardly, in terms of foldMap1:
import Prelude
import Data.Monoid.Dual (Dual(..))
import Data.Newtype (alaF)
import Data.Semigroup.Foldable (class Foldable1, foldMap1)
data FoldRight1 a = FoldRight1 (a -> (a -> a -> a) -> a) a
instance foldRight1Semigroup :: Semigroup (FoldRight1 a) where
append (FoldRight1 lf lr) (FoldRight1 rf rr) = FoldRight1 (\a f -> lf (f lr (rf a f)) f) rr
mkFoldRight1 :: forall a. a -> FoldRight1 a
mkFoldRight1 = FoldRight1 const
runFoldRight1 :: forall a. FoldRight1 a -> (a -> a -> a) -> a
runFoldRight1 (FoldRight1 f a) = f a
foldr1 :: forall a f. Foldable1 f => (a -> a -> a) -> f a -> a
foldr1 = flip (runFoldRight1 <<< foldMap1 mkFoldRight1)
foldl1 :: forall a f. Foldable1 f => (a -> a -> a) -> f a -> a
foldl1 = flip (runFoldRight1 <<< alaF Dual foldMap1 mkFoldRight1) <<< flipbut allowing instances to define their own for efficiency, as Foldable does, would be nice (in which case these could also be added as default implementations).