null for Foldables

adapted from Haskell's null in Data.Foldable

Author: matthewleon

src/Data/Foldable.purs

@@ -21,6 +21,7 @@ module Data.Foldable
   , maximumBy
   , minimum
   , minimumBy
+  , null
   ) where
 
 import Prelude
@@ -302,3 +303,9 @@ minimumBy cmp = foldl min' Nothing
   where
   min' Nothing x  = Just x
   min' (Just x) y = Just (if cmp x y == LT then x else y)
+
+-- | Test whether the structure is empty.
+-- | Optimized for structures that are similar to cons-lists, because there
+-- | is no general way to do better.
+null :: forall a f. Foldable f => f a -> Boolean
+null = foldr (\_ _ -> false) true

test/Main.purs

@@ -8,7 +8,7 @@ import Control.Monad.Eff.Console (CONSOLE, log)
 import Data.Bifoldable (class Bifoldable, bifoldl, bifoldr, bifoldMap, bifoldrDefault, bifoldlDefault, bifoldMapDefaultR, bifoldMapDefaultL)
 import Data.Bifunctor (class Bifunctor, bimap)
 import Data.Bitraversable (class Bitraversable, bisequenceDefault, bitraverse, bisequence, bitraverseDefault)
-import Data.Foldable (class Foldable, foldl, foldr, foldMap, foldrDefault, foldlDefault, foldMapDefaultR, foldMapDefaultL, minimumBy, minimum, maximumBy, maximum, find, findMap)
+import Data.Foldable (class Foldable, foldl, foldr, foldMap, foldrDefault, foldlDefault, foldMapDefaultR, foldMapDefaultL, minimumBy, minimum, maximumBy, maximum, find, findMap, null)
 import Data.Function (on)
 import Data.Int (toNumber)
 import Data.Maybe (Maybe(..))
@@ -103,6 +103,13 @@ main = do
               (map (negate <<< toNumber) (arrayFrom1UpTo 10))
       == Just (-1.0)
 
+  log "Test null"
+  assert $ null Nothing == true
+  assert $ null (Just 1) == false
+  assert $ null [] == true
+  assert $ null [0] == false
+  assert $ null [0,1] == false
+
   log "All done!"