Merge pull request #139 from ibara1454/issue138

Don't force more than necessary in Data.List.Lazy take

Author: garyb

src/Data/List/Lazy.purs

@@ -509,10 +509,11 @@ slice start end xs = take (end - start) (drop start xs)
 -- |
 -- | Running time: `O(n)` where `n` is the number of elements to take.
 take :: forall a. Int -> List a -> List a
-take n = List <<< map (go n) <<< unwrap
+take n = if n <= 0
+  then const nil
+  else List <<< map (go n) <<< unwrap
   where
   go :: Int -> Step a -> Step a
-  go i _ | i <= 0 = Nil
   go _ Nil = Nil
   go n' (Cons x xs) = Cons x (take (n' - 1) xs)
 

test/Test/Data/List/Lazy.purs

@@ -272,6 +272,12 @@ testListLazy = do
   assert $ (take 2 (l [1, 2, 3])) == l [1, 2]
   assert $ (take 1 nil') == nil'
 
+  log "take should evaluate exactly n items which we needed"
+  assert let oops x = 0 : oops x
+             xs = 1 : defer oops
+          in take 1 xs == l [1]
+  -- If `take` evaluate more than once, it would crash with a stack overflow
+
   log "takeWhile should keep all values that match a predicate from the front of an list"
   assert $ (takeWhile (_ /= 2) (l [1, 2, 3])) == l [1]
   assert $ (takeWhile (_ /= 3) (l [1, 2, 3])) == l [1, 2]