From 10ca8018900364579123bf3912202176d338d3c6 Mon Sep 17 00:00:00 2001
From: Herbert Valerio Riedel <hvr@gnu.org>
Date: Tue, 19 Sep 2017 07:54:40 +0200
Subject: [PATCH] Generalise constraint on `instance Monoid (Maybe a)` to
 Semigroup

This now becomes possible due to the introduction of the
Semigroup=>Monoid superclass relation (see #14191).

Reviewers: ekmett, RyanGlScott, austin, bgamari

Reviewed By: ekmett, RyanGlScott, bgamari

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3972
---
 libraries/base/GHC/Base.hs                | 9 +++++----
 libraries/base/changelog.md               | 3 +++
 testsuite/tests/ghci/scripts/T4175.stdout | 2 +-
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/libraries/base/GHC/Base.hs b/libraries/base/GHC/Base.hs
index 82b99a88c274..052f13fbe6f6 100644
--- a/libraries/base/GHC/Base.hs
+++ b/libraries/base/GHC/Base.hs
@@ -413,12 +413,13 @@ instance Semigroup a => Semigroup (Maybe a) where
 -- | Lift a semigroup into 'Maybe' forming a 'Monoid' according to
 -- <http://en.wikipedia.org/wiki/Monoid>: \"Any semigroup @S@ may be
 -- turned into a monoid simply by adjoining an element @e@ not in @S@
--- and defining @e*e = e@ and @e*s = s = s*e@ for all @s ∈ S@.\" Since
--- there used to be no \"Semigroup\" typeclass providing just 'mappend',
--- we use 'Monoid' instead.
+-- and defining @e*e = e@ and @e*s = s = s*e@ for all @s ∈ S@.\"
+--
+-- /Since 4.11.0/: constraint on inner @a@ value generalised from
+-- 'Monoid' to 'Semigroup'.
 --
 -- @since 2.01
-instance Monoid a => Monoid (Maybe a) where
+instance Semigroup a => Monoid (Maybe a) where
     mempty = Nothing
 
 -- | For tuples, the 'Monoid' constraint on @a@ determines
diff --git a/libraries/base/changelog.md b/libraries/base/changelog.md
index eac53ba079ec..f64129965f73 100644
--- a/libraries/base/changelog.md
+++ b/libraries/base/changelog.md
@@ -14,6 +14,9 @@
     export `Semigroup` from `Prelude`; remove `Monoid` reexport
     from `Data.Semigroup` (#14191).
 
+  * Generalise `instance Monoid a => Monoid (Maybe a)` to
+    `instance Semigroup a => Monoid (Maybe a)`.
+
   * Add `infixl 9 !!` declaration for `Data.List.NonEmpty.!!`
 
   * Add `<&>` operator to `Data.Functor` (#14029)
diff --git a/testsuite/tests/ghci/scripts/T4175.stdout b/testsuite/tests/ghci/scripts/T4175.stdout
index 7b630f1ed9b9..c7421b58afc9 100644
--- a/testsuite/tests/ghci/scripts/T4175.stdout
+++ b/testsuite/tests/ghci/scripts/T4175.stdout
@@ -34,7 +34,7 @@ instance Applicative Maybe -- Defined in ‘GHC.Base’
 instance Eq a => Eq (Maybe a) -- Defined in ‘GHC.Base’
 instance Functor Maybe -- Defined in ‘GHC.Base’
 instance Monad Maybe -- Defined in ‘GHC.Base’
-instance Monoid a => Monoid (Maybe a) -- Defined in ‘GHC.Base’
+instance Semigroup a => Monoid (Maybe a) -- Defined in ‘GHC.Base’
 instance Ord a => Ord (Maybe a) -- Defined in ‘GHC.Base’
 instance Semigroup a => Semigroup (Maybe a)
   -- Defined in ‘GHC.Base’
-- 
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