[commit: ghc] master: Fixes a spaceleak in `maximumBy` and `minimumBy` (#10830). (2e43848)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/2e43848236a4b80015d8fb09a87f6f6a746c1365/ghc

>---------------------------------------------------------------

commit 2e43848236a4b80015d8fb09a87f6f6a746c1365
Author: Ryan Scott <ryan.gl.scott at gmail.com>
Date:   Fri Mar 3 17:49:35 2017 -0500

    Fixes a spaceleak in `maximumBy` and `minimumBy` (#10830).
    
    This involved changing the implementation from using
    `foldr11` to using `foldl1`.
    
    Test Plan: validate
    
    Reviewers: austin, hvr, bgamari, dalaing, dfeuer
    
    Reviewed By: bgamari
    
    Subscribers: RyanGlScott, dfeuer, thomie
    
    Differential Revision: https://phabricator.haskell.org/D3019


>---------------------------------------------------------------

2e43848236a4b80015d8fb09a87f6f6a746c1365
 libraries/base/Data/Foldable.hs | 30 ++++++++++++++++++++++++++++--
 libraries/base/changelog.md     |  4 ++++
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/libraries/base/Data/Foldable.hs b/libraries/base/Data/Foldable.hs
index 0a8b003..1d9fc92 100644
--- a/libraries/base/Data/Foldable.hs
+++ b/libraries/base/Data/Foldable.hs
@@ -551,16 +551,20 @@ all p = getAll #. foldMap (All #. p)
 
 -- | The largest element of a non-empty structure with respect to the
 -- given comparison function.
+
+-- See Note [maximumBy/minimumBy space usage]
 maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
-maximumBy cmp = foldr1 max'
+maximumBy cmp = foldl1 max'
   where max' x y = case cmp x y of
                         GT -> x
                         _  -> y
 
 -- | The least element of a non-empty structure with respect to the
 -- given comparison function.
+
+-- See Note [maximumBy/minimumBy space usage]
 minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
-minimumBy cmp = foldr1 min'
+minimumBy cmp = foldl1 min'
   where min' x y = case cmp x y of
                         GT -> y
                         _  -> x
@@ -574,3 +578,25 @@ notElem x = not . elem x
 -- 'Nothing' if there is no such element.
 find :: Foldable t => (a -> Bool) -> t a -> Maybe a
 find p = getFirst . foldMap (\ x -> First (if p x then Just x else Nothing))
+
+{-
+Note [maximumBy/minimumBy space usage]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When the type signatures of maximumBy and minimumBy were generalized to work
+over any Foldable instance (instead of just lists), they were defined using
+foldr1. This was problematic for space usage, as the semantics of maximumBy
+and minimumBy essentially require that they examine every element of the
+data structure. Using foldr1 to examine every element results in space usage
+proportional to the size of the data structure. For the common case of lists,
+this could be particularly bad (see Trac #10830).
+
+For the common case of lists, switching the implementations of maximumBy and
+minimumBy to foldl1 solves the issue, as GHC's strictness analysis can then
+make these functions only use O(1) stack space. It is perhaps not the optimal
+way to fix this problem, as there are other conceivable data structures
+(besides lists) which might benefit from specialized implementations for
+maximumBy and minimumBy (see
+https://ghc.haskell.org/trac/ghc/ticket/10830#comment:26 for a further
+discussion). But using foldl1 is at least always better than using foldr1, so
+GHC has chosen to adopt that approach for now.
+-}
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 3bb60fe..f39e149 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -73,6 +73,10 @@
     functions which solely match again `ErrorCall` do not produce
     non-exhaustive pattern-match warnings (#8779)
 
+  * Change the implementations of `maximumBy` and `minimumBy` from
+    `Data.Foldable` to use `fold11` instead of `foldr1`. This makes them run
+    in constant space when applied to lists. (#10830)
+
 ## 4.9.0.0  *May 2016*
 
   * Bundled with GHC 8.0