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

1 files changed, 5 insertions, 1 deletions

diff --git a/testsuite/tests/simplCore/should_run/T13429a.hs b/testsuite/tests/simplCore/should_run/T13429a.hs
index 6a838cb79c..718f26520a 100644
--- a/testsuite/tests/simplCore/should_run/T13429a.hs
+++ b/testsuite/tests/simplCore/should_run/T13429a.hs
@@ -5,6 +5,8 @@
 {-# LANGUAGE UndecidableInstances #-}
 module T13429a where -- Orignally FingerTree.hs from the ticket
 
+import Data.Semigroup (Semigroup(..))
+
 class (Monoid v) => Measured v a | a -> v where
     measure :: a -> v
 
@@ -32,9 +34,11 @@ instance Foldable (FingerTree v) where
     foldMap f (Deep _ pr m sf) =
         foldMap f pr `mappend` foldMap (foldMap f) m `mappend` foldMap f sf
 
+instance Measured v a => Semigroup (FingerTree v a) where
+    (<>) = (><)
+
 instance Measured v a => Monoid (FingerTree v a) where
     mempty = empty
-    mappend = (><)
 
 empty :: Measured v a => FingerTree v a
 empty = Empty