Make scanr a good producer and consumer

This fixes #9355.

libraries/base/GHC/List.lhs

@@ -229,11 +229,29 @@ foldr1 _ []             =  errorEmptyList "foldr1"
 --
 -- > head (scanr f z xs) == foldr f z xs.
 
+{-# NOINLINE [1] scanr #-}
 scanr                   :: (a -> b -> b) -> b -> [a] -> [b]
 scanr _ q0 []           =  [q0]
 scanr f q0 (x:xs)       =  f x q : qs
                            where qs@(q:_) = scanr f q0 xs
 
+{-# INLINE [0] strictUncurryScanr #-}
+strictUncurryScanr :: (a -> b -> c) -> (a, b) -> c
+strictUncurryScanr f pair = case pair of
+                              (x, y) -> f x y
+
+{-# INLINE [0] scanrFB #-}
+scanrFB :: (a -> b -> b) -> (b -> c -> c) -> a -> (b, c) -> (b, c)
+scanrFB f c = \x (r, est) -> (f x r, r `c` est)
+
+{-# RULES
+"scanr" [~1] forall f q0 ls . scanr f q0 ls =
+  build (\c n -> strictUncurryScanr c (foldr (scanrFB f c) (q0,n) ls))
+"scanrList" [1] forall f q0 ls .
+               strictUncurryScanr (:) (foldr (scanrFB f (:)) (q0,[]) ls) =
+                 scanr f q0 ls
+ #-}
+
 -- | 'scanr1' is a variant of 'scanr' that has no starting value argument.
 
 scanr1                  :: (a -> a -> a) -> [a] -> [a]

libraries/base/changelog.md

@@ -77,6 +77,8 @@
     second argument, so that the fusion RULES for it do not change the
     semantics. (#9596)
 
+  * `scanr` now takes part in list fusion (#9355)
+
 ## 4.7.0.1  *Jul 2014*
 
   * Bundled with GHC 7.8.3