Improve performance of 'toUnfoldable'

Just use an integer index for the unfolding, as opposed to the uncons'
implementation, which was constructing all of the tails of the input
array.

Author: hdgarrood

src/Data/Array.purs

@@ -128,7 +128,12 @@ import Partial.Unsafe (unsafePartial)
 
 -- | Convert an `Array` into an `Unfoldable` structure.
 toUnfoldable :: forall f a. Unfoldable f => Array a -> f a
-toUnfoldable = unfoldr $ uncons' (const Nothing) (\h t -> Just (Tuple h t))
+toUnfoldable xs = unfoldr f 0
+  where
+  len = length xs
+  f i
+    | i < len   = Just (Tuple (unsafePartial (unsafeIndex xs i)) (i+1))
+    | otherwise = Nothing
 
 -- | Convert a `Foldable` structure into an `Array`.
 fromFoldable :: forall f a. Foldable f => f a -> Array a

test/Test/Data/Array.purs

@@ -5,8 +5,8 @@ import Prelude
 import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Console (log, CONSOLE)
 
-import Data.Array (range, replicate, foldM, unzip, zip, zipWithA, zipWith, intersectBy, intersect, (\\), deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group', group, span, dropWhile, drop, takeWhile, take, sortBy, sort, catMaybes, mapMaybe, mapWithIndex, filterM, filter, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, (!!), uncons, init, tail, last, head, insertBy, insert, snoc, (:), length, null, singleton, fromFoldable)
-import Data.Foldable (for_, foldMapDefaultR, class Foldable, all)
+import Data.Array (range, replicate, foldM, unzip, zip, zipWithA, zipWith, intersectBy, intersect, (\\), deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group', group, span, dropWhile, drop, takeWhile, take, sortBy, sort, catMaybes, mapMaybe, mapWithIndex, filterM, filter, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, (!!), uncons, init, tail, last, head, insertBy, insert, snoc, (:), length, null, singleton, fromFoldable, toUnfoldable)
+import Data.Foldable (for_, foldMapDefaultR, class Foldable, all, traverse_)
 import Data.Maybe (Maybe(..), isNothing, fromJust)
 import Data.NonEmpty ((:|))
 import Data.NonEmpty as NE
@@ -308,6 +308,17 @@ testArray = do
     assert $ length arr == n
     assert $ all (_ == elem) arr
 
+  log "toUnfoldable"
+  let toUnfoldableId xs = toUnfoldable xs == xs
+  traverse_ (assert <<< toUnfoldableId)
+    [ []
+    , [1]
+    , [1,2,3]
+    , [2,3,1]
+    , [4,0,0,1,25,36,458,5842,23757]
+    ]
+
+
 nil :: Array Int
 nil = []