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/Installer/Target.hs b/src/Propellor/Property/Installer/Target.hs
index 80e660ad..54e9075e 100644
--- a/src/Propellor/Property/Installer/Target.hs
+++ b/src/Propellor/Property/Installer/Target.hs
@@ -112,6 +112,7 @@ import Data.List
 import Data.Char
 import Data.Ord
 import Data.Ratio
+import qualified Data.Semigroup as Sem
 import System.Process (readProcess)
 
 -- | Partition table for the target disk.
@@ -438,9 +439,12 @@ getMountsSizes = mapMaybe (parse . words) . lines <$> readProcess "findmnt" ps "
 data TargetFilled = TargetFilled (Ratio Integer)
 	deriving (Show, Eq)
 
+instance Sem.Semigroup TargetFilled where
+	TargetFilled n <> TargetFilled m = TargetFilled (n+m) 
+
 instance Monoid TargetFilled where
 	mempty = TargetFilled (0 % 1)
-	mappend (TargetFilled n) (TargetFilled m) = TargetFilled (n+m)
+	mappend = (<>)
 
 newtype TargetFilledHandle = TargetFilledHandle Integer