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, 16 insertions, 7 deletions

diff --git a/src/Propellor/Property/Parted/Types.hs b/src/Propellor/Property/Parted/Types.hs
index cfd8760d..5891cc16 100644
--- a/src/Propellor/Property/Parted/Types.hs
+++ b/src/Propellor/Property/Parted/Types.hs
@@ -4,6 +4,9 @@ import qualified Propellor.Property.Partition as Partition
 import Utility.DataUnits
 
 import Data.Char
+import qualified Data.Semigroup as Sem
+import Data.Monoid
+import Prelude
 
 class PartedVal a where
 	pval :: a -> String
@@ -19,14 +22,17 @@ instance PartedVal TableType where
 data PartTable = PartTable TableType Alignment [Partition]
 	deriving (Show)
 
-instance Monoid PartTable where
-	-- | default TableType is MSDOS, with a `safeAlignment`.
-	mempty = PartTable MSDOS safeAlignment []
+instance Sem.Semigroup PartTable where
 	-- | uses the TableType of the second parameter
 	-- and the larger alignment,
-	mappend (PartTable _l1 a1 ps1) (PartTable l2 a2 ps2) =
+	PartTable _l1 a1 ps1 <> PartTable l2 a2 ps2 =
 		PartTable l2 (max a1 a2) (ps1 ++ ps2)
 
+instance Monoid PartTable where
+	-- | default TableType is MSDOS, with a `safeAlignment`.
+	mempty = PartTable MSDOS safeAlignment []
+	mappend = (<>)
+
 -- | A partition on the disk.
 data Partition = Partition
 	{ partType :: PartType
@@ -80,11 +86,14 @@ fromPartSize :: PartSize -> ByteSize
 fromPartSize (MegaBytes b) = b * 1000000
 fromPartSize (Bytes n) = n
 
+instance Sem.Semigroup PartSize where
+	MegaBytes a <> MegaBytes b = MegaBytes (a + b)
+	Bytes a <> b = Bytes (a + fromPartSize b)
+	a <> Bytes b = Bytes (b + fromPartSize a)
+
 instance Monoid PartSize where
 	mempty = MegaBytes 0
-	mappend (MegaBytes a) (MegaBytes b) = MegaBytes (a + b)
-	mappend (Bytes a) b = Bytes (a + fromPartSize b)
-	mappend a (Bytes b) = Bytes (b + fromPartSize a)
+	mappend = (<>)
 
 reducePartSize :: PartSize -> PartSize -> PartSize
 reducePartSize (MegaBytes a) (MegaBytes b) = MegaBytes (a - b)