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 689637e..60fc90b 100644 (file)
--- a/GHC/List.lhs
+++ b/GHC/List.lhs
@@ -111,21 +111,12 @@ 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 = 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
+length                  :: [a] -> Int
+length l                =  len l 0#
+  where
+    len :: [a] -> Int# -> Int
+    len []     a# = I# a#
+    len (_:xs) a# = len xs (a# +# 1#)
 
 -- | 'filter', applied to a predicate and a list, returns the list of
 -- those elements that satisfy the predicate; i.e.,