[commit: ghc] master: Add a strict version of foldMap to Foldable (172b00c)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/172b00c1954d8eb609ccd58a731ae89fe74a7489/ghc

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

commit 172b00c1954d8eb609ccd58a731ae89fe74a7489
Author: Simon Jakobi <simon.jakobi at gmail.com>
Date:   Mon Oct 15 13:55:37 2018 -0400

    Add a strict version of foldMap to Foldable
    
    Summary:
    Original proposal by Andrew Martin:
    https://mail.haskell.org/pipermail/libraries/2018-June/028852.html
    
    Reviewers: andrewthad, hvr, bgamari, alpmestan, tdammers
    
    Reviewed By: bgamari, alpmestan, tdammers
    
    Subscribers: alpmestan, rwbarton, thomie, carter
    
    Differential Revision: https://phabricator.haskell.org/D4924


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

172b00c1954d8eb609ccd58a731ae89fe74a7489
 libraries/base/Data/Foldable.hs | 6 ++++++
 libraries/base/changelog.md     | 2 ++
 2 files changed, 8 insertions(+)

diff --git a/libraries/base/Data/Foldable.hs b/libraries/base/Data/Foldable.hs
index f5f3112..cc0f348 100644
--- a/libraries/base/Data/Foldable.hs
+++ b/libraries/base/Data/Foldable.hs
@@ -130,6 +130,12 @@ class Foldable t where
     -- This INLINE allows more list functions to fuse. See Trac #9848.
     foldMap f = foldr (mappend . f) mempty
 
+    -- | A variant of 'foldMap' that is strict in the accumulator.
+    --
+    -- @since 4.13.0.0
+    foldMap' :: Monoid m => (a -> m) -> t a -> m
+    foldMap' f = foldl' (\ acc a -> acc <> f a) mempty
+
     -- | Right-associative fold of a structure.
     --
     -- In the case of lists, 'foldr', when applied to a binary operator, a
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index 3709a0a..bfa8cb9 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -10,6 +10,8 @@
     a representational one. There is really no reason to care about the
     type of the underlying object.
 
+  * Add `foldMap'`, a strict version of `foldMap`, to `Foldable`.
+
 ## 4.12.0.0 *TBA*
   * Bundled with GHC *TBA*