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, 6 insertions, 2 deletions

diff --git a/src/Propellor/Property/Debootstrap.hs b/src/Propellor/Property/Debootstrap.hs
index 7c8e9618..c6e5c373 100644
--- a/src/Propellor/Property/Debootstrap.hs
+++ b/src/Propellor/Property/Debootstrap.hs
@@ -19,12 +19,13 @@ import Utility.FileMode
 
 import Data.List
 import Data.Char
+import qualified Data.Semigroup as Sem
 import System.Posix.Directory
 import System.Posix.Files
 
 type Url = String
 
--- | A monoid for debootstrap configuration.
+-- | A data type for debootstrap configuration.
 -- mempty is a default debootstrapped system.
 data DebootstrapConfig
 	= DefaultConfig
@@ -35,9 +36,12 @@ data DebootstrapConfig
 	| DebootstrapConfig :+ DebootstrapConfig
 	deriving (Show)
 
+instance Sem.Semigroup DebootstrapConfig where
+	(<>) = (:+)
+
 instance Monoid DebootstrapConfig where
 	mempty  = DefaultConfig
-	mappend = (:+)
+	mappend = (<>)
 
 toParams :: DebootstrapConfig -> [CommandParam]
 toParams DefaultConfig = []