semigroup monoid change fallout; drop ghc 7 support

Fix build with ghc 8.4, which broke due to the Semigroup Monoid change.
See https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid

Dropped support for building propellor with ghc 7 (as in debian
oldstable), to avoid needing to depend on the semigroups transitional
package, but also because it's just too old to be worth supporting.

If we indeed drop ghc 7 support entirely, some code to support "jessie"
can be removed; concurrent-output can be de-embedded, and the Singletons
code can be simplified.

This commit was sponsored by Jack Hill on Patreon.

1 files changed, 5 insertions, 1 deletions

diff --git a/src/Propellor/Property/Firewall.hs b/src/Propellor/Property/Firewall.hs
index bbc14473..ff7ffebf 100644
--- a/src/Propellor/Property/Firewall.hs
+++ b/src/Propellor/Property/Firewall.hs
@@ -18,6 +18,7 @@ module Propellor.Property.Firewall (
 ) where
 
 import Data.Monoid
+import qualified Data.Semigroup as Sem
 import Data.Char
 import Data.List
 
@@ -199,6 +200,9 @@ data Rules
 
 infixl 0 :-
 
+instance Sem.Semigroup Rules where
+	(<>) = (:-)
+
 instance Monoid Rules where
 	mempty  = Everything
-	mappend = (:-)
+	mappend = (<>)