Turn a few existing folds into `Foldable`-methods (#9621)

Turn `toList`, `elem`, `sum`, `product`, `maximum`, and `minimum` into
`Foldable` methods. This helps avoiding regressions (and semantic
differences) while implementing #9586

Reviewed By: austin, dfeuer, ekmett

Differential Revision: https://phabricator.haskell.org/D231

2 files changed, 42 insertions, 37 deletions

diff --git a/libraries/base/Data/Foldable.hs b/libraries/base/Data/Foldable.hs
index cb13e5ce7e..726aa6cb24 100644
--- a/libraries/base/Data/Foldable.hs
+++ b/libraries/base/Data/Foldable.hs
@@ -39,21 +39,15 @@ module Data.Foldable (
     sequence_,
     msum,
     -- ** Specialized folds
-    toList,
     concat,
     concatMap,
     and,
     or,
     any,
     all,
-    sum,
-    product,
-    maximum,
     maximumBy,
-    minimum,
     minimumBy,
     -- ** Searches
-    elem,
     notElem,
     find
     ) where
@@ -97,6 +91,8 @@ infix  4 `elem`, `notElem`
 -- >    foldr f z (Node l k r) = foldr f (f k (foldr f z r)) l
 --
 class Foldable t where
+    {-# MINIMAL foldMap | foldr #-}
+
     -- | Combine the elements of a structure using a monoid.
     fold :: Monoid m => t m -> m
     fold = foldMap id
@@ -153,7 +149,32 @@ class Foldable t where
       where
         mf Nothing y = Just y
         mf (Just x) y = Just (f x y)
-    {-# MINIMAL foldMap | foldr #-}
+
+    -- | List of elements of a structure.
+    toList :: Foldable t => t a -> [a]
+    {-# INLINE toList #-}
+    toList t = build (\ c n -> foldr c n t)
+
+    -- | Does the element occur in the structure?
+    elem :: (Foldable t, Eq a) => a -> t a -> Bool
+    elem = any . (==)
+
+    -- | The largest element of a non-empty structure.
+    maximum :: (Foldable t, Ord a) => t a -> a
+    maximum = foldr1 max
+
+    -- | The least element of a non-empty structure.
+    minimum :: (Foldable t, Ord a) => t a -> a
+    minimum = foldr1 min
+
+    -- | The 'sum' function computes the sum of the numbers of a structure.
+    sum :: Num a => t a -> a
+    sum = getSum . foldMap Sum
+
+    -- | The 'product' function computes the product of the numbers of a
+    -- structure.
+    product :: (Foldable t, Num a) => t a -> a
+    product = getProduct . foldMap Product
 
 -- instances for Prelude types
 
@@ -165,11 +186,17 @@ instance Foldable Maybe where
     foldl f z (Just x) = f z x
 
 instance Foldable [] where
-    foldr = List.foldr
-    foldl = List.foldl
-    foldl' = List.foldl'
-    foldr1 = List.foldr1
-    foldl1 = List.foldl1
+    elem    = List.elem
+    foldl   = List.foldl
+    foldl'  = List.foldl'
+    foldl1  = List.foldl1
+    foldr   = List.foldr
+    foldr1  = List.foldr1
+    maximum = List.maximum
+    minimum = List.minimum
+    product = List.product
+    sum     = List.sum
+    toList  = id
 
 instance Foldable (Either a) where
     foldMap _ (Left _) = mempty
@@ -257,11 +284,6 @@ msum = foldr mplus mzero
 
 -- These use foldr rather than foldMap to avoid repeated concatenation.
 
--- | List of elements of a structure.
-toList :: Foldable t => t a -> [a]
-{-# INLINE toList #-}
-toList t = build (\ c n -> foldr c n t)
-
 -- | The concatenation of all the elements of a container of lists.
 concat :: Foldable t => t [a] -> [a]
 concat = fold
@@ -291,18 +313,6 @@ any p = getAny . foldMap (Any . p)
 all :: Foldable t => (a -> Bool) -> t a -> Bool
 all p = getAll . foldMap (All . p)
 
--- | The 'sum' function computes the sum of the numbers of a structure.
-sum :: (Foldable t, Num a) => t a -> a
-sum = getSum . foldMap Sum
-
--- | The 'product' function computes the product of the numbers of a structure.
-product :: (Foldable t, Num a) => t a -> a
-product = getProduct . foldMap Product
-
--- | The largest element of a non-empty structure.
-maximum :: (Foldable t, Ord a) => t a -> a
-maximum = foldr1 max
-
 -- | The largest element of a non-empty structure with respect to the
 -- given comparison function.
 maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
@@ -311,10 +321,6 @@ maximumBy cmp = foldr1 max'
                         GT -> x
                         _  -> y
 
--- | The least element of a non-empty structure.
-minimum :: (Foldable t, Ord a) => t a -> a
-minimum = foldr1 min
-
 -- | The least element of a non-empty structure with respect to the
 -- given comparison function.
 minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
@@ -323,10 +329,6 @@ minimumBy cmp = foldr1 min'
                         GT -> y
                         _  -> x
 
--- | Does the element occur in the structure?
-elem :: (Foldable t, Eq a) => a -> t a -> Bool
-elem = any . (==)
-
 -- | 'notElem' is the negation of 'elem'.
 notElem :: (Foldable t, Eq a) => a -> t a -> Bool
 notElem x = not . elem x
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 1dbada01ba..d82d354c91 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -32,6 +32,9 @@
   * Set fixity for `Data.Foldable.{elem,notElem}` to match the
     conventional one set for `Data.List.{elem,notElem}` (#9610)
 
+  * Turn `toList`, `elem`, `sum`, `product`, `maximum`, and `minimum`
+    into `Foldable` methods (#9621)
+
 ## 4.7.0.1  *Jul 2014*
 
   * Bundled with GHC 7.8.3