) where
import {-# SOURCE #-} GHC.Err ( error )
-import Data.Tuple
+import Data.Tuple() -- Instances
import Data.Maybe
import GHC.Base
-- | '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.,