Remove pull array index (unsafe), add uncons and empty. (#475)

* Remove pull array index (unsafe), add uncons.

* Add pull array `empty`, add tests

Author: sjoerdvisscher

src/Data/Array/Polarized.hs

@@ -55,14 +55,12 @@
 -- vecfilter vec f = Push.alloc (transfer (loop (Pull.fromVector vec) f))
 --   where
 --     loop :: Pull.Array a -> (a -> Bool) -> Pull.Array a
---     loop arr f = case Pull.findLength arr of
---       (0,_) -> Pull.fromFunction (error "empty") 0
---       (n,_) -> case Pull.split 1 arr of
---         (head, tail) -> case Pull.index head 0 of
---           (a,_) ->
---             if f a
---             then Pull.append (Pull.singleton a) (loop tail f)
---             else loop tail f
+--     loop arr f = case Pull.uncons arr of
+--       Nothing -> Pull.empty
+--       Just (a, as) ->
+--         if f a
+--         then Pull.append (Pull.singleton a) (loop as f)
+--         else loop as f
 -- @
 --
 --

src/Data/Array/Polarized/Pull.hs

@@ -13,6 +13,7 @@ module Data.Array.Polarized.Pull
     fromVector,
     make,
     singleton,
+    empty,
 
     -- * Consumption
     toVector,
@@ -27,7 +28,7 @@ module Data.Array.Polarized.Pull
     findLength,
     split,
     reverse,
-    index,
+    uncons,
   )
 where
 
@@ -45,7 +46,7 @@ import Data.Array.Polarized.Pull.Internal
 import qualified Data.Functor.Linear as Data
 import Data.Vector (Vector)
 import qualified Data.Vector as Vector
-import Prelude.Linear hiding (foldMap, foldr, reverse, zip, zipWith)
+import Prelude.Linear hiding (foldMap, foldr, reverse, uncons, zip, zipWith)
 import qualified Unsafe.Linear as Unsafe
 
 -- | Convert a pull array into a list.

src/Data/Array/Polarized/Pull/Internal.hs

@@ -37,6 +37,10 @@ instance Data.Functor Array where
 -- is interesting in and of itself: I think this is like an n-ary With), and
 -- changing the other arrows makes no difference)
 
+-- | Create an empty pull array
+empty :: Array a
+empty = fromFunction (\_ -> error "Data.Array.Polarized.Pull.Internal.empty: this should never be called") 0
+
 -- | Produce a pull array of lenght 1 consisting of solely the given element.
 singleton :: a %1 -> Array a
 singleton = Unsafe.toLinear (\x -> fromFunction (\_ -> x) 1)
@@ -110,6 +114,7 @@ split k (Array f n) = (fromFunction f (min k n), fromFunction (\x -> f (x + k))
 reverse :: Array a %1 -> Array a
 reverse (Array f n) = Array (\x -> f (n + 1 - x)) n
 
--- | Index a pull array (without checking bounds)
-index :: Array a %1 -> Int -> (a, Array a)
-index (Array f n) ix = (f ix, Array f n)
+-- | Decompose an array into its head and tail, returns @Nothing@ if the array is empty.
+uncons :: Array a %1 -> Maybe (a, Array a)
+uncons (Array _ 0) = Nothing
+uncons (Array f n) = Just (f 0, fromFunction (\x -> f (x + 1)) (n - 1))

test/Test/Data/Polarized.hs

@@ -6,6 +6,7 @@ module Test.Data.Polarized (polarizedArrayTests) where
 import qualified Data.Array.Polarized as Polar
 import qualified Data.Array.Polarized.Pull as Pull
 import qualified Data.Array.Polarized.Push as Push
+import Data.Functor.Linear (fmap)
 import qualified Data.Vector as Vector
 import Hedgehog
 import qualified Hedgehog.Gen as Gen
@@ -34,10 +35,12 @@ polarizedArrayTests =
       testPropertyNamed "Push.make ~ Vec.replicate" "pushMake" pushMake,
       testPropertyNamed "Pull.append ~ Vec.append" "pullAppend" pullAppend,
       testPropertyNamed "Pull.asList . Pull.fromVector ~ id" "pullAsList" pullAsList,
+      testPropertyNamed "Pull.empty = []" "pullEmpty" pullEmpty,
       testPropertyNamed "Pull.singleton x = [x]" "pullSingleton" pullSingleton,
       testPropertyNamed "Pull.splitAt ~ splitAt" "pullSplitAt" pullSplitAt,
       testPropertyNamed "Pull.make ~ Vec.replicate" "pullMake" pullMake,
-      testPropertyNamed "Pull.zip ~ zip" "pullZip" pullZip
+      testPropertyNamed "Pull.zip ~ zip" "pullZip" pullZip,
+      testPropertyNamed "Pull.uncons ~ uncons" "pullUncons" pullUncons
     ]
 
 list :: Gen [Int]
@@ -88,6 +91,10 @@ pullAsList = property Prelude.$ do
   xs <- forAll list
   xs === Pull.asList (Pull.fromVector (Vector.fromList xs))
 
+pullEmpty :: Property
+pullEmpty = property Prelude.$ do
+  ([] :: [Int]) === Pull.asList Pull.empty
+
 pullSingleton :: Property
 pullSingleton = property Prelude.$ do
   x <- forAll randInt
@@ -115,3 +122,8 @@ pullZip = property Prelude.$ do
   let xs' = Pull.fromVector (Vector.fromList xs)
   let ys' = Pull.fromVector (Vector.fromList ys)
   zip xs ys === Pull.asList (Pull.zip xs' ys')
+
+pullUncons :: Property
+pullUncons = property Prelude.$ do
+  xs <- forAll list
+  uncons xs === fmap (fmap Pull.asList) (Pull.uncons (Pull.fromVector (Vector.fromList xs)))