X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=GHC%2FList.lhs;h=689637e2ea42a52e077cc2ae8862e55b917efc0f;hb=30464c0cb915c2ae900909568fa8677bba341e45;hp=60fc90b2fb9ede137e9632e1b44f6e1ce1a005ac;hpb=f59ef9948524e4f7e3a947b89306ab377fd56849;p=haskell-directory.git

diff --git a/GHC/List.lhs b/GHC/List.lhs
index 60fc90b..689637e 100644
--- a/GHC/List.lhs
+++ b/GHC/List.lhs
@@ -111,12 +111,21 @@ null (_:_)              =  False
 -- | 'length' returns the length of a finite list as an 'Int'.
 -- It is an instance of the more general 'Data.List.genericLength',
 -- the result type of which may be any kind of number.
-length                  :: [a] -> Int
-length l                =  len l 0#
-  where
-    len :: [a] -> Int# -> Int
-    len []     a# = I# a#
-    len (_:xs) a# = len xs (a# +# 1#)
+length :: [a] -> Int
+length l = lenAcc 0# l
+
+{-# RULES
+"length" [~1] forall xs. length xs = foldr incL (I# 0#) xs
+"lenAcc" [1]  forall n#. foldr incL (I# n#) = lenAcc n#
+ #-}
+
+incL :: a -> Int -> Int		-- Internal
+{-# NOINLINE [0] incL #-}
+incL x n = n `plusInt` oneInt
+
+lenAcc :: Int# -> [a] -> Int	-- Internal
+lenAcc a# []     = I# a#
+lenAcc a# (_:xs) = lenAcc (a# +# 1#) xs
 
 -- | 'filter', applied to a predicate and a list, returns the list of
 -- those elements that satisfy the predicate; i.e.,