Merge pull request #42 from zudov/stack-safe

Make functions in Data.List stack-safe

Author: paf31

src/Data/List.purs

@@ -185,8 +185,7 @@ null _ = false
 -- |
 -- | Running time: `O(n)`
 length :: forall a. List a -> Int
-length Nil = 0
-length (Cons _ xs) = 1 + length xs
+length = foldl (\acc _ -> acc + 1) 0
 
 --------------------------------------------------------------------------------
 -- Extending arrays ------------------------------------------------------------
@@ -712,10 +711,11 @@ instance functorList :: Functor List where
 instance foldableList :: Foldable List where
   foldr _ b Nil = b
   foldr o b (Cons a as) = a `o` foldr o b as
-  foldl _ b Nil = b
-  foldl o b (Cons a as) = foldl o (b `o` a) as
-  foldMap _ Nil = mempty
-  foldMap f (Cons x xs) = f x <> foldMap f xs
+  foldl = go
+    -- FIXME: Helper function is needed until purescript/purescript#1413 is fixed
+    where go _ b Nil = b
+          go o b (Cons a as) = go o (b `o` a) as
+  foldMap f = foldl (\acc -> append acc <<< f) mempty
 
 instance unfoldableList :: Unfoldable List where
   unfoldr f b = go (f b)

test/Test/Data/List.purs

@@ -1,7 +1,10 @@
 module Test.Data.List (testList) where
 
 import Prelude
+import Control.Monad
 import Control.Monad.Eff.Console (log)
+import Data.Foldable
+import Data.Monoid.Additive
 import Data.List
 import Data.Maybe (Maybe(..), isNothing)
 import Data.Maybe.Unsafe (fromJust)
@@ -46,6 +49,9 @@ testList = do
   assert $ length (toList [1]) == 1
   assert $ length (toList [1, 2, 3, 4, 5]) == 5
 
+  log "length should be stack-safe"
+  void $ pure $ length (range 1 100000)
+
   log "snoc should add an item to the end of an list"
   assert $ toList [1, 2, 3] `snoc` 4 == toList [1, 2, 3, 4]
   assert $ nil `snoc` 1 == toList [1]
@@ -271,6 +277,15 @@ testList = do
   assert $ foldM (\x y -> Just (x + y)) 0 (range 1 10) == Just 55
   assert $ foldM (\_ _ -> Nothing) 0 (range 1 10) == Nothing
 
+  log "foldl should be stack-safe"
+  void $ pure $ foldl (+) 0 $ range 1 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 "can find the first 10 primes using lazy lists"
   -- let eratos :: L.List Number -> L.List Number
   --     eratos xs = Control.Lazy.defer \_ ->