Make Semigroup a superclass of Monoid (re #14191)

Unfortunately, this requires introducing a couple of .hs-boot files to
break up import cycles (mostly to provide class & typenames in order to
be able to write type signatures).

This does not yet re-export `(<>)` from Prelude (while the class-name
`Semigroup` is reexported); that will happen in a future commit.

Test Plan: local ./validate passed

Reviewers: ekmett, austin, bgamari, erikd, RyanGlScott

Reviewed By: ekmett, RyanGlScott

GHC Trac Issues: #14191

Differential Revision: https://phabricator.haskell.org/D3927

8 files changed, 20 insertions, 1 deletions

diff --git a/testsuite/tests/ghci/scripts/T10963.script b/testsuite/tests/ghci/scripts/T10963.script
index 357d1256ba..2bba5b36ef 100644
--- a/testsuite/tests/ghci/scripts/T10963.script
+++ b/testsuite/tests/ghci/scripts/T10963.script
@@ -3,5 +3,7 @@
 :t +d length
 let foo :: (Num a, Monoid a) => a -> a; foo = undefined
 :t +d foo
-instance Monoid Double where mempty = 0; mappend = (+)
+import Data.Semigroup
+instance Semigroup Double where (<>) = (+)
+instance Monoid Double where mempty = 0
 :t +d foo
diff --git a/testsuite/tests/ghci/scripts/T4175.stdout b/testsuite/tests/ghci/scripts/T4175.stdout
index 6f56a5f868..7b630f1ed9 100644
--- a/testsuite/tests/ghci/scripts/T4175.stdout
+++ b/testsuite/tests/ghci/scripts/T4175.stdout
@@ -21,6 +21,7 @@ instance C () -- Defined at T4175.hs:21:10
 instance Eq () -- Defined in ‘GHC.Classes’
 instance Monoid () -- Defined in ‘GHC.Base’
 instance Ord () -- Defined in ‘GHC.Classes’
+instance Semigroup () -- Defined in ‘GHC.Base’
 instance Show () -- Defined in ‘GHC.Show’
 instance Read () -- Defined in ‘GHC.Read’
 instance Enum () -- Defined in ‘GHC.Enum’
@@ -35,6 +36,8 @@ 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 Ord a => Ord (Maybe a) -- Defined in ‘GHC.Base’
+instance Semigroup a => Semigroup (Maybe a)
+  -- Defined in ‘GHC.Base’
 instance Show a => Show (Maybe a) -- Defined in ‘GHC.Show’
 instance Read a => Read (Maybe a) -- Defined in ‘GHC.Read’
 instance Foldable Maybe -- Defined in ‘Data.Foldable’
diff --git a/testsuite/tests/ghci/scripts/T7627.stdout b/testsuite/tests/ghci/scripts/T7627.stdout
index c13a3f3ba0..ff4e67005e 100644
--- a/testsuite/tests/ghci/scripts/T7627.stdout
+++ b/testsuite/tests/ghci/scripts/T7627.stdout
@@ -2,6 +2,7 @@ data () = () 	-- Defined in ‘GHC.Tuple’
 instance Eq () -- Defined in ‘GHC.Classes’
 instance Monoid () -- Defined in ‘GHC.Base’
 instance Ord () -- Defined in ‘GHC.Classes’
+instance Semigroup () -- Defined in ‘GHC.Base’
 instance Show () -- Defined in ‘GHC.Show’
 instance Read () -- Defined in ‘GHC.Read’
 instance Enum () -- Defined in ‘GHC.Enum’
@@ -19,6 +20,8 @@ instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
 instance (Monoid a, Monoid b) => Monoid (a, b)
   -- Defined in ‘GHC.Base’
 instance (Ord a, Ord b) => Ord (a, b) -- Defined in ‘GHC.Classes’
+instance (Semigroup a, Semigroup b) => Semigroup (a, b)
+  -- Defined in ‘GHC.Base’
 instance (Show a, Show b) => Show (a, b) -- Defined in ‘GHC.Show’
 instance (Read a, Read b) => Read (a, b) -- Defined in ‘GHC.Read’
 instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
diff --git a/testsuite/tests/ghci/scripts/T8535.stdout b/testsuite/tests/ghci/scripts/T8535.stdout
index 3995bc0b20..873b992a38 100644
--- a/testsuite/tests/ghci/scripts/T8535.stdout
+++ b/testsuite/tests/ghci/scripts/T8535.stdout
@@ -4,3 +4,4 @@ instance Applicative ((->) a) -- Defined in ‘GHC.Base’
 instance Functor ((->) r) -- Defined in ‘GHC.Base’
 instance Monad ((->) r) -- Defined in ‘GHC.Base’
 instance Monoid b => Monoid (a -> b) -- Defined in ‘GHC.Base’
+instance Semigroup b => Semigroup (a -> b) -- Defined in ‘GHC.Base’
diff --git a/testsuite/tests/ghci/scripts/T9881.stdout b/testsuite/tests/ghci/scripts/T9881.stdout
index 18fa4d521f..68acea7c61 100644
--- a/testsuite/tests/ghci/scripts/T9881.stdout
+++ b/testsuite/tests/ghci/scripts/T9881.stdout
@@ -9,6 +9,8 @@ instance Monoid Data.ByteString.Lazy.ByteString
   -- Defined in ‘Data.ByteString.Lazy.Internal’
 instance Ord Data.ByteString.Lazy.ByteString
   -- Defined in ‘Data.ByteString.Lazy.Internal’
+instance Semigroup Data.ByteString.Lazy.ByteString
+  -- Defined in ‘Data.ByteString.Lazy.Internal’
 instance Show Data.ByteString.Lazy.ByteString
   -- Defined in ‘Data.ByteString.Lazy.Internal’
 instance Read Data.ByteString.Lazy.ByteString
@@ -26,6 +28,8 @@ instance Monoid Data.ByteString.ByteString
   -- Defined in ‘Data.ByteString.Internal’
 instance Ord Data.ByteString.ByteString
   -- Defined in ‘Data.ByteString.Internal’
+instance Semigroup Data.ByteString.ByteString
+  -- Defined in ‘Data.ByteString.Internal’
 instance Show Data.ByteString.ByteString
   -- Defined in ‘Data.ByteString.Internal’
 instance Read Data.ByteString.ByteString
diff --git a/testsuite/tests/ghci/scripts/ghci011.stdout b/testsuite/tests/ghci/scripts/ghci011.stdout
index 372930d66d..7bd58dcc24 100644
--- a/testsuite/tests/ghci/scripts/ghci011.stdout
+++ b/testsuite/tests/ghci/scripts/ghci011.stdout
@@ -5,6 +5,7 @@ instance Functor [] -- Defined in ‘GHC.Base’
 instance Monad [] -- Defined in ‘GHC.Base’
 instance Monoid [a] -- Defined in ‘GHC.Base’
 instance Ord a => Ord [a] -- Defined in ‘GHC.Classes’
+instance Semigroup [a] -- Defined in ‘GHC.Base’
 instance Show a => Show [a] -- Defined in ‘GHC.Show’
 instance Read a => Read [a] -- Defined in ‘GHC.Read’
 instance Foldable [] -- Defined in ‘Data.Foldable’
@@ -13,6 +14,7 @@ data () = () 	-- Defined in ‘GHC.Tuple’
 instance Eq () -- Defined in ‘GHC.Classes’
 instance Monoid () -- Defined in ‘GHC.Base’
 instance Ord () -- Defined in ‘GHC.Classes’
+instance Semigroup () -- Defined in ‘GHC.Base’
 instance Show () -- Defined in ‘GHC.Show’
 instance Read () -- Defined in ‘GHC.Read’
 instance Enum () -- Defined in ‘GHC.Enum’
@@ -25,6 +27,8 @@ instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’
 instance (Monoid a, Monoid b) => Monoid (a, b)
   -- Defined in ‘GHC.Base’
 instance (Ord a, Ord b) => Ord (a, b) -- Defined in ‘GHC.Classes’
+instance (Semigroup a, Semigroup b) => Semigroup (a, b)
+  -- Defined in ‘GHC.Base’
 instance (Show a, Show b) => Show (a, b) -- Defined in ‘GHC.Show’
 instance (Read a, Read b) => Read (a, b) -- Defined in ‘GHC.Read’
 instance Foldable ((,) a) -- Defined in ‘Data.Foldable’
diff --git a/testsuite/tests/ghci/scripts/ghci020.stdout b/testsuite/tests/ghci/scripts/ghci020.stdout
index 3995bc0b20..873b992a38 100644
--- a/testsuite/tests/ghci/scripts/ghci020.stdout
+++ b/testsuite/tests/ghci/scripts/ghci020.stdout
@@ -4,3 +4,4 @@ instance Applicative ((->) a) -- Defined in ‘GHC.Base’
 instance Functor ((->) r) -- Defined in ‘GHC.Base’
 instance Monad ((->) r) -- Defined in ‘GHC.Base’
 instance Monoid b => Monoid (a -> b) -- Defined in ‘GHC.Base’
+instance Semigroup b => Semigroup (a -> b) -- Defined in ‘GHC.Base’
diff --git a/testsuite/tests/ghci/should_run/T10145.stdout b/testsuite/tests/ghci/should_run/T10145.stdout
index 3995bc0b20..873b992a38 100644
--- a/testsuite/tests/ghci/should_run/T10145.stdout
+++ b/testsuite/tests/ghci/should_run/T10145.stdout
@@ -4,3 +4,4 @@ instance Applicative ((->) a) -- Defined in ‘GHC.Base’
 instance Functor ((->) r) -- Defined in ‘GHC.Base’
 instance Monad ((->) r) -- Defined in ‘GHC.Base’
 instance Monoid b => Monoid (a -> b) -- Defined in ‘GHC.Base’
+instance Semigroup b => Semigroup (a -> b) -- Defined in ‘GHC.Base’