indexed foldable-traversable instances (#134)

* indexed foldable-traversable instances

* update foldable-traversable dep

* TraversableWithIndex implementation

* Data.List: reuse mapWithIndex from Data.FunctorWithIndex

* fix List.foldrWithIndex

in turn fixes List.mapWithIndex

* simplify List right fold code

reuse left fold for reversals

* add failing mapWithIndex test for Lazy List

* fix Lazy List foldrWithIndex

* indexed instance tests

* clarify reversal in traversableWithIndexList

Author: matthewleon

bower.json

@@ -25,7 +25,7 @@
     "purescript-tailrec": "^3.3.0",
     "purescript-unfoldable": "^3.0.0",
     "purescript-partial": "^1.0.0",
-    "purescript-foldable-traversable": "^3.3.0"
+    "purescript-foldable-traversable": "^3.4.0"
   },
   "devDependencies": {
     "purescript-arrays": "^4.0.0",

src/Data/List.purs

@@ -102,6 +102,7 @@ import Control.Monad.Rec.Class (class MonadRec, Step(..), tailRecM, tailRecM2)
 
 import Data.Bifunctor (bimap)
 import Data.Foldable (class Foldable, foldr, any, foldl)
+import Data.FunctorWithIndex (mapWithIndex) as FWI
 import Data.List.Types (List(..), (:))
 import Data.List.Types (NonEmptyList(..)) as NEL
 import Data.Maybe (Maybe(..))
@@ -426,11 +427,10 @@ catMaybes = mapMaybe id
 
 
 -- | Apply a function to each element and its index in a list starting at 0.
+-- |
+-- | Deprecated. Use Data.FunctorWithIndex instead.
 mapWithIndex :: forall a b. (Int -> a -> b) -> List a -> List b
-mapWithIndex f lst = reverse $ go 0 lst Nil
-  where
-  go _ Nil acc = acc
-  go n (x : xs) acc = go (n+1) xs (f n x : acc)
+mapWithIndex = FWI.mapWithIndex
 
 --------------------------------------------------------------------------------
 -- Sorting ---------------------------------------------------------------------

src/Data/List/Lazy/Types.purs

@@ -10,9 +10,10 @@ import Control.Lazy as Z
 import Control.MonadPlus (class MonadPlus)
 import Control.MonadZero (class MonadZero)
 import Control.Plus (class Plus)
-
 import Data.Eq (class Eq1, eq1)
 import Data.Foldable (class Foldable, foldMap, foldl, foldr)
+import Data.FoldableWithIndex (class FoldableWithIndex, foldlWithIndex, foldrWithIndex)
+import Data.FunctorWithIndex (class FunctorWithIndex)
 import Data.Lazy (Lazy, defer, force)
 import Data.Maybe (Maybe(..))
 import Data.Monoid (class Monoid, mempty)
@@ -21,7 +22,8 @@ import Data.NonEmpty (NonEmpty, (:|))
 import Data.NonEmpty as NE
 import Data.Ord (class Ord1, compare1)
 import Data.Traversable (class Traversable, traverse, sequence)
-import Data.Tuple (Tuple(..))
+import Data.TraversableWithIndex (class TraversableWithIndex)
+import Data.Tuple (Tuple(..), snd)
 import Data.Unfoldable (class Unfoldable)
 
 -- | A lazy linked list.
@@ -105,6 +107,9 @@ instance functorList :: Functor List where
     go Nil = Nil
     go (Cons x xs') = Cons (f x) (f <$> xs')
 
+instance functorWithIndexList :: FunctorWithIndex Int List where
+  mapWithIndex f = foldrWithIndex (\i x acc -> f i x : acc) nil
+
 instance foldableList :: Foldable List where
   -- calls foldl on the reversed list
   foldr op z xs = foldl (flip op) z (rev xs) where
@@ -120,6 +125,22 @@ instance foldableList :: Foldable List where
 
   foldMap f = foldl (\b a -> b <> f a) mempty
 
+instance foldableWithIndexList :: FoldableWithIndex Int List where
+  foldrWithIndex f b xs =
+    -- as we climb the reversed list, we decrement the index
+    snd $ foldl
+            (\(Tuple i b') a -> Tuple (i - 1) (f (i - 1) a b'))
+            (Tuple len b)
+            revList
+    where
+    Tuple len revList = rev (Tuple 0 nil) xs
+      where
+      -- As we create our reversed list, we count elements.
+      rev = foldl (\(Tuple i acc) a -> Tuple (i + 1) (a : acc))
+  foldlWithIndex f acc =
+    snd <<< foldl (\(Tuple i b) a -> Tuple (i + 1) (f i b a)) (Tuple 0 acc)
+  foldMapWithIndex f = foldlWithIndex (\i acc -> append acc <<< f i) mempty
+
 instance unfoldableList :: Unfoldable List where
   unfoldr = go where
     go f b = Z.defer \_ -> case f b of
@@ -132,6 +153,10 @@ instance traversableList :: Traversable List where
 
   sequence = traverse id
 
+instance traversableWithIndexList :: TraversableWithIndex Int List where
+  traverseWithIndex f =
+    foldrWithIndex (\i a b -> cons <$> f i a <*> b) (pure nil)
+
 instance applyList :: Apply List where
   apply = ap
 

src/Data/List/Types.purs

@@ -10,9 +10,10 @@ import Control.Extend (class Extend)
 import Control.MonadPlus (class MonadPlus)
 import Control.MonadZero (class MonadZero)
 import Control.Plus (class Plus)
-
 import Data.Eq (class Eq1, eq1)
 import Data.Foldable (class Foldable, foldl, foldr, intercalate)
+import Data.FoldableWithIndex (class FoldableWithIndex, foldlWithIndex, foldrWithIndex)
+import Data.FunctorWithIndex (class FunctorWithIndex)
 import Data.Maybe (Maybe(..))
 import Data.Monoid (class Monoid, mempty)
 import Data.Newtype (class Newtype)
@@ -22,7 +23,8 @@ import Data.Ord (class Ord1, compare1)
 import Data.Semigroup.Foldable (class Foldable1)
 import Data.Semigroup.Traversable (class Traversable1, traverse1)
 import Data.Traversable (class Traversable, traverse)
-import Data.Tuple (Tuple(..))
+import Data.TraversableWithIndex (class TraversableWithIndex)
+import Data.Tuple (Tuple(..), snd)
 import Data.Unfoldable (class Unfoldable)
 
 data List a = Nil | Cons a (List a)
@@ -67,19 +69,36 @@ instance monoidList :: Monoid (List a) where
 instance functorList :: Functor List where
   map f = foldr (\x acc -> f x : acc) Nil
 
+instance functorWithIndexList :: FunctorWithIndex Int List where
+  mapWithIndex f = foldrWithIndex (\i x acc -> f i x : acc) Nil
+
 instance foldableList :: Foldable List where
-  foldr f b = foldl (flip f) b <<< rev Nil
+  foldr f b = foldl (flip f) b <<< rev
     where
-    rev acc = case _ of
-      Nil -> acc
-      a : as -> rev (a : acc) as
+    rev = foldl (flip Cons) Nil
   foldl f = go
     where
     go b = case _ of
       Nil -> b
       a : as -> go (f b a) as
   foldMap f = foldl (\acc -> append acc <<< f) mempty
 
+instance foldableWithIndexList :: FoldableWithIndex Int List where
+  foldrWithIndex f b xs =
+    -- as we climb the reversed list, we decrement the index
+    snd $ foldl
+            (\(Tuple i b') a -> Tuple (i - 1) (f (i - 1) a b'))
+            (Tuple len b)
+            revList
+    where
+    Tuple len revList = rev (Tuple 0 Nil) xs
+      where
+      -- As we create our reversed list, we count elements.
+      rev = foldl (\(Tuple i acc) a -> Tuple (i + 1) (a : acc))
+  foldlWithIndex f acc =
+    snd <<< foldl (\(Tuple i b) a -> Tuple (i + 1) (f i b a)) (Tuple 0 acc)
+  foldMapWithIndex f = foldlWithIndex (\i acc -> append acc <<< f i) mempty
+
 instance unfoldableList :: Unfoldable List where
   unfoldr f b = go b Nil
     where
@@ -91,6 +110,13 @@ instance traversableList :: Traversable List where
   traverse f = map (foldl (flip (:)) Nil) <<< foldl (\acc -> lift2 (flip (:)) acc <<< f) (pure Nil)
   sequence = traverse id
 
+instance traversableWithIndexList :: TraversableWithIndex Int List where
+  traverseWithIndex f =
+    map rev
+    <<< foldlWithIndex (\i acc -> lift2 (flip (:)) acc <<< f i) (pure Nil)
+    where
+    rev = foldl (flip Cons) Nil
+
 instance applyList :: Apply List where
   apply Nil _ = Nil
   apply (f : fs) xs = (f <$> xs) <> (fs <*> xs)

test/Test/Data/List.purs

@@ -1,15 +1,18 @@
 module Test.Data.List (testList) where
 
 import Prelude
-import Data.List.NonEmpty as NEL
+
 import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Console (CONSOLE, log)
 import Data.Foldable (foldMap, foldl)
+import Data.FoldableWithIndex (foldMapWithIndex, foldlWithIndex, foldrWithIndex)
 import Data.List (List(..), (..), stripPrefix, Pattern(..), length, range, foldM, unzip, zip, zipWithA, zipWith, intersectBy, intersect, (\\), deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group', group, partition, span, dropWhile, drop, dropEnd, takeWhile, take, takeEnd, sortBy, sort, catMaybes, mapMaybe, filterM, filter, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, (!!), uncons, unsnoc, init, tail, last, head, insertBy, insert, snoc, null, singleton, fromFoldable, transpose, mapWithIndex, (:))
+import Data.List.NonEmpty as NEL
 import Data.Maybe (Maybe(..), isNothing, fromJust)
 import Data.Monoid.Additive (Additive(..))
 import Data.NonEmpty ((:|))
 import Data.Traversable (traverse)
+import Data.TraversableWithIndex (traverseWithIndex)
 import Data.Tuple (Tuple(..))
 import Data.Unfoldable (replicate, replicateA, unfoldr)
 import Partial.Unsafe (unsafePartial)
@@ -324,12 +327,30 @@ testList = do
   log "foldl should be stack-safe"
   void $ pure $ foldl (+) 0 $ range 1 100000
 
+  log "foldlWithIndex should be correct"
+  assert $ foldlWithIndex (\i b _ -> i + b) 0 (range 0 10000) == 50005000
+
+  log "foldlWithIndex should be stack-safe"
+  void $ pure $ foldlWithIndex (\i b _ -> i + b) 0 $ range 0 100000
+
+  log "foldrWithIndex should be correct"
+  assert $ foldrWithIndex (\i _ b -> i + b) 0 (range 0 10000) == 50005000
+
+  log "foldrWithIndex should be stack-safe"
+  void $ pure $ foldrWithIndex (\i _ b -> i + b) 0 $ range 0 100000
+
   log "foldMap should be stack-safe"
   void $ pure $ foldMap Additive $ range 1 100000
 
   log "foldMap should be left-to-right"
   assert $ foldMap show (range 1 5) == "12345"
 
+  log "foldMapWithIndex should be stack-safe"
+  void $ pure $ foldMapWithIndex (\i _ -> Additive i) $ range 1 100000
+
+  log "foldMapWithIndex should be left-to-right"
+  assert $ foldMapWithIndex (\i _ -> show i) (fromFoldable [0, 0, 0]) == "012"
+
   log "unfoldable replicate should be stack-safe"
   void $ pure $ length $ replicate 100000 1
 
@@ -354,6 +375,13 @@ testList = do
   let xs = fromFoldable (range 1 100000)
   assert $ traverse Just xs == Just xs
 
+  log "traverseWithIndex should be stack-safe"
+  assert $ traverseWithIndex (const Just) xs == Just xs
+
+  log "traverseWithIndex should be correct"
+  assert $ traverseWithIndex (\i a -> Just $ i + a) (fromFoldable [2, 2, 2])
+           == Just (fromFoldable [2, 3, 4])
+
   log "append should concatenate two lists"
   assert $ (l [1, 2]) <> (l [3, 4]) == (l [1, 2, 3, 4])
 

test/Test/Data/List/Lazy.purs

@@ -5,18 +5,18 @@ import Prelude
 import Control.Lazy (defer)
 import Control.Monad.Eff (Eff)
 import Control.Monad.Eff.Console (CONSOLE, log)
-
+import Data.FoldableWithIndex (foldMapWithIndex, foldlWithIndex, foldrWithIndex)
+import Data.FunctorWithIndex (mapWithIndex)
 import Data.Lazy as Z
 import Data.List.Lazy (List, nil, stripPrefix, Pattern(..), cons, foldl, foldr, foldMap, singleton, transpose, take, iterate, filter, uncons, foldM, foldrLazy, range, unzip, zip, length, zipWithA, replicate, repeat, zipWith, intersectBy, intersect, deleteBy, delete, unionBy, union, nubBy, nub, groupBy, group, partition, span, dropWhile, drop, takeWhile, slice, catMaybes, mapMaybe, filterM, concat, concatMap, reverse, alterAt, modifyAt, updateAt, deleteAt, insertAt, findLastIndex, findIndex, elemLastIndex, elemIndex, init, tail, last, head, insertBy, insert, snoc, null, replicateM, fromFoldable, (:), (\\), (!!))
 import Data.List.Lazy.NonEmpty as NEL
 import Data.Maybe (Maybe(..), isNothing, fromJust)
 import Data.Monoid.Additive (Additive(..))
 import Data.NonEmpty ((:|))
 import Data.Traversable (traverse)
+import Data.TraversableWithIndex (traverseWithIndex)
 import Data.Tuple (Tuple(..))
-
 import Partial.Unsafe (unsafePartial)
-
 import Test.Assert (ASSERT, assert)
 
 testListLazy :: forall eff. Eff (assert :: ASSERT, console :: CONSOLE | eff) Unit
@@ -48,9 +48,34 @@ testListLazy = do
   log "foldMap should be left-to-right"
   assert $ foldMap show (range 1 5) == "12345"
 
+  log "foldlWithIndex should be correct"
+  assert $ foldlWithIndex (\i b _ -> i + b) 0 (range 0 10000) == 50005000
+
+  log "foldlWithIndex should be stack-safe"
+  void $ pure $ foldlWithIndex (\i b _ -> i + b) 0 $ range 0 100000
+
+  log "foldrWithIndex should be correct"
+  assert $ foldrWithIndex (\i _ b -> i + b) 0 (range 0 10000) == 50005000
+
+  log "foldrWithIndex should be stack-safe"
+  void $ pure $ foldrWithIndex (\i _ b -> i + b) 0 $ range 0 100000
+
+  log "foldMapWithIndex should be stack-safe"
+  void $ pure $ foldMapWithIndex (\i _ -> Additive i) $ range 1 100000
+
+  log "foldMapWithIndex should be left-to-right"
+  assert $ foldMapWithIndex (\i _ -> show i) (fromFoldable [0, 0, 0]) == "012"
+
   log "traverse should be stack-safe"
   assert $ ((traverse Just longList) >>= last) == last longList
 
+  log "traverseWithIndex should be stack-safe"
+  assert $ traverseWithIndex (const Just) longList == Just longList
+
+  log "traverseWithIndex should be correct"
+  assert $ traverseWithIndex (\i a -> Just $ i + a) (fromFoldable [2, 2, 2])
+           == Just (fromFoldable [2, 3, 4])
+
   log "bind should be stack-safe"
   void $ pure $ last $ longList >>= pure
 
@@ -228,6 +253,9 @@ testListLazy = do
   log "catMaybe should take an list of Maybe values and throw out Nothings"
   assert $ catMaybes (l [Nothing, Just 2, Nothing, Just 4]) == l [2, 4]
 
+  log "mapWithIndex should take a list of values and apply a function which also takes the index into account"
+  assert $ mapWithIndex (\x ix -> x + ix) (fromFoldable [0, 1, 2, 3]) == fromFoldable [0, 2, 4, 6]
+
   -- log "sort should reorder a list into ascending order based on the result of compare"
   -- assert $ sort (l [1, 3, 2, 5, 6, 4]) == l [1, 2, 3, 4, 5, 6]