Make length a good consumer

Make length into a good consumer.  Fixes Trac bug #707.

(Before length simply didn't use foldr.)

diff --git a/GHC/List.lhs b/GHC/List.lhs
index 60fc90b..689637e 100644 (file)
--- 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.,