Merge pull request #62 from purescript/length

length for Foldables

Author: garyb

src/Data/Foldable.purs

@@ -22,6 +22,7 @@ module Data.Foldable
   , minimum
   , minimumBy
   , null
+  , length
   ) where
 
 import Prelude
@@ -313,3 +314,9 @@ minimumBy cmp = foldl min' Nothing
 -- | is no general way to do better.
 null :: forall a f. Foldable f => f a -> Boolean
 null = foldr (\_ _ -> false) true
+
+-- | Returns the size/length of a finite structure.
+-- | Optimized for structures that are similar to cons-lists, because there
+-- | is no general way to do better.
+length :: forall a b f. Foldable f => Semiring b => f a -> b
+length = foldl (\c _ -> add one c) zero

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, null)
+import Data.Foldable (class Foldable, foldl, foldr, foldMap, foldrDefault, foldlDefault, foldMapDefaultR, foldMapDefaultL, minimumBy, minimum, maximumBy, maximum, find, findMap, length, null)
 import Data.Function (on)
 import Data.Int (toNumber)
 import Data.Maybe (Maybe(..))
@@ -110,6 +110,13 @@ main = do
   assert $ null [0] == false
   assert $ null [0,1] == false
 
+  log "Test length"
+  assert $ length Nothing == 0
+  assert $ length (Just 1) == 1
+  assert $ length [] == 0
+  assert $ length [1] == 1
+  assert $ length [1, 2] == 2
+
   log "All done!"