throwTo: mention interruptible foreign calls

[ghc-base.git] / Data / List.hs

diff --git a/Data/List.hs b/Data/List.hs
index cc166d8..221a340 100644 (file)
--- a/Data/List.hs
+++ b/Data/List.hs
@@ -410,6 +410,8 @@ intersect               =  intersectBy (==)
 
 -- | The 'intersectBy' function is the non-overloaded version of 'intersect'.
 intersectBy             :: (a -> a -> Bool) -> [a] -> [a] -> [a]
+intersectBy _  [] _     =  []
+intersectBy _  _  []    =  []
 intersectBy eq xs ys    =  [x | x <- xs, any (eq x) ys]
 
 -- | The 'intersperse' function takes an element and a list and
@@ -420,8 +422,16 @@ intersectBy eq xs ys    =  [x | x <- xs, any (eq x) ys]
 
 intersperse             :: a -> [a] -> [a]
 intersperse _   []      = []
-intersperse _   [x]     = [x]
-intersperse sep (x:xs)  = x : sep : intersperse sep xs
+intersperse sep (x:xs)  = x : prependToAll sep xs
+
+
+-- Not exported:
+-- We want to make every element in the 'intersperse'd list available
+-- as soon as possible to avoid space leaks. Experiments suggested that
+-- a separate top-level helper is more efficient than a local worker.
+prependToAll            :: a -> [a] -> [a]
+prependToAll _   []     = []
+prependToAll sep (x:xs) = sep : x : prependToAll sep xs
 
 -- | 'intercalate' @xs xss@ is equivalent to @('concat' ('intersperse' xs xss))@.
 -- It inserts the list @xs@ in between the lists in @xss@ and concatenates the