Lazier intersperse

A lazier implementation of intersperse, and consequentially intercalate, to
avoid space leaks.

1 files changed, 10 insertions, 2 deletions

diff --git a/libraries/base/Data/List.hs b/libraries/base/Data/List.hs
index c954757ddd..221a340a24 100644
--- a/libraries/base/Data/List.hs
+++ b/libraries/base/Data/List.hs
@@ -422,8 +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