diff --git a/CHANGELOG b/CHANGELOG
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,19 @@
+propellor (5.3.6) unstable; urgency=medium
+
+  * Fix build with ghc 8.4, which broke due to the Semigroup Monoid change.
+  * 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.
+  * stack.yaml: Updated to lts-9.21.
+  * Make Schroot.overlaysInTmpfs revertable
+    Thanks, Sean Whitton
+  * Update shim each time propellor is run in a container, to deal with
+    library version changes.
+  * Unbound: Added support for various DNS record types.
+    Thanks, Félix Sipma.
+
+ -- Joey Hess <id@joeyh.name>  Wed, 09 May 2018 16:24:37 -0400
+
 propellor (5.3.5) unstable; urgency=medium
 
   * Apt.stdSourcesList now adds stable-updates suite
diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -56,4 +56,4 @@
 7. Write some neat new properties and send patches!
 
 (Want to get your feet wet with propellor before plunging in?
-[try this|http://propellor.branchable.com/forum/Simple_quickstart_without_git__44___SSH__44___GPG])
+[try this](http://propellor.branchable.com/forum/Simple_quickstart_without_git__44___SSH__44___GPG])
diff --git a/debian/changelog b/debian/changelog
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,3 +1,19 @@
+propellor (5.3.6) unstable; urgency=medium
+
+  * Fix build with ghc 8.4, which broke due to the Semigroup Monoid change.
+  * 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.
+  * stack.yaml: Updated to lts-9.21.
+  * Make Schroot.overlaysInTmpfs revertable
+    Thanks, Sean Whitton
+  * Update shim each time propellor is run in a container, to deal with
+    library version changes.
+  * Unbound: Added support for various DNS record types.
+    Thanks, Félix Sipma.
+
+ -- Joey Hess <id@joeyh.name>  Wed, 09 May 2018 16:24:37 -0400
+
 propellor (5.3.5) unstable; urgency=medium
 
   * Apt.stdSourcesList now adds stable-updates suite
diff --git a/doc/README.mdwn b/doc/README.mdwn
--- a/doc/README.mdwn
+++ b/doc/README.mdwn
@@ -56,4 +56,4 @@
 7. Write some neat new properties and send patches!
 
 (Want to get your feet wet with propellor before plunging in?
-[try this|http://propellor.branchable.com/forum/Simple_quickstart_without_git__44___SSH__44___GPG])
+[try this](http://propellor.branchable.com/forum/Simple_quickstart_without_git__44___SSH__44___GPG])
diff --git a/joeyconfig.hs b/joeyconfig.hs
--- a/joeyconfig.hs
+++ b/joeyconfig.hs
@@ -99,9 +99,10 @@
 clam = host "clam.kitenet.net" $ props
 	& standardSystem Unstable X86_64
 		["Unreliable server. Anything here may be lost at any time!" ]
-	& ipv4 "45.62.211.94"
+	& ipv4 "64.137.164.186"
 
 	& CloudAtCost.decruft
+	& User.hasPassword (User "root")
 	& Ssh.hostKeys hostContext
 		[ (SshDsa, "ssh-dss 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")
 		, (SshRsa, "ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDJybAjUPUWIhvVMmer8K5ZgdfI54DM6vc8Mzw+5KmVKL0TwkvzbR1HAB4heyMGtN1F8YzkWhsI3/Txh+MQUJ+i4u8SvSYc6D1q3j3ZyCi06wZ3DJS25tZrOM/thOOA1DFA4Hhb0uI/1Kg8PguNNNSMXn8F7q3F6cFQizYgszs6z6ktiST/BTC+IXWovhcnn2vQXXU8FTcTsqBFqA5dEjZbp1WDzqp3km84ZyXGmoVlpqzXeMvlkWTIshYiQjXIwPOkALzlGYjp1lw1OaxPVI1IGFcgCbIWQQWoCReb+genX2VaR+odAYXjaOdRx0lQj7UCPTBCpqMyzBMLtT5Yiaqh")
@@ -175,6 +176,7 @@
 honeybee = host "honeybee.kitenet.net" $ props
 	& standardSystem Testing ARMHF
 		[ "Home router and arm git-annex build box." ]
+	& Apt.removed ["rsyslog"]
 	
 	& cubietech_Cubietruck
 	& hasPartition
@@ -195,7 +197,7 @@
 		[ (SshEd25519, "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIS/hDYq1MAxfOBf49htym3BOYlx4Gk9SDpiHjv7u6IC")
 		]
 
-	& JoeySites.homePowerMonitor
+	& JoeySites.homePower
 		(User "joey")
 		hosts
 		(Context "homepower.joeyh.name")
diff --git a/propellor.cabal b/propellor.cabal
--- a/propellor.cabal
+++ b/propellor.cabal
@@ -1,6 +1,6 @@
 Name: propellor
-Version: 5.3.5
-Cabal-Version: >= 1.20
+Version: 5.3.6
+Cabal-Version: 1.20
 License: BSD2
 Maintainer: Joey Hess <id@joeyh.name>
 Author: Joey Hess
@@ -45,7 +45,7 @@
   Build-Depends:
     -- propellor needs to support the ghc shipped in Debian stable,
     -- and also only depends on packages in Debian stable.
-    base >= 4.5, base < 5,
+    base >= 4.9, base < 5,
     directory, filepath, IfElse, process, bytestring, hslogger, split,
     unix, unix-compat, ansi-terminal, containers (>= 0.5), network, async,
     time, mtl, transformers, exceptions (>= 0.6), stm, text, hashable
@@ -232,9 +232,7 @@
   Default-Language: Haskell98
   Hs-Source-Dirs: executables
   Main-Is: propellor-config.hs
-  GHC-Options: -threaded -Wall -fno-warn-tabs -O0
-  if impl(ghc >= 8.0)
-    GHC-Options: -fno-warn-redundant-constraints
+  GHC-Options: -threaded -Wall -fno-warn-tabs
   Default-Extensions: TypeOperators
   Build-Depends: propellor, base
 
@@ -242,6 +240,7 @@
   Default-Language: Haskell98
   Hs-Source-Dirs: executables
   Main-Is: wrapper.hs
+  GHC-Options: -threaded -Wall -fno-warn-tabs
   Build-Depends: propellor, base
 
 source-repository head
diff --git a/src/Propellor/DotDir.hs b/src/Propellor/DotDir.hs
--- a/src/Propellor/DotDir.hs
+++ b/src/Propellor/DotDir.hs
@@ -271,7 +271,7 @@
 		, "  Main-Is: config.hs"
 		, "  GHC-Options: -threaded -Wall -fno-warn-tabs -O0"
 		, "  Extensions: TypeOperators"
-		, "  Build-Depends: propellor >= 3.0, base >= 3"
+		, "  Build-Depends: propellor >= 3.0, base >= 4.9"
 		]
 	configcontent =
 		[ "-- This is the main configuration file for Propellor, and is used to build"
@@ -316,7 +316,7 @@
 		]
 
 stackResolver :: String
-stackResolver = "lts-8.22"
+stackResolver = "lts-9.21"
 
 fullClone :: IO Result
 fullClone = do
@@ -446,9 +446,6 @@
 					)
 			Nothing -> return mref
 
--- And, if there's a remote named "upstream"
--- that does not point at the distrepo, the user must have set that up
--- and is not using the distrepo, so do nothing.
 warnoutofdate :: Bool -> IO ()
 warnoutofdate havebranch = warningMessage $ unlines
 	[ "** Your ~/.propellor/ is out of date.."
diff --git a/src/Propellor/PrivData.hs b/src/Propellor/PrivData.hs
--- a/src/Propellor/PrivData.hs
+++ b/src/Propellor/PrivData.hs
@@ -37,6 +37,7 @@
 import qualified Data.ByteString.Lazy as L
 import Control.Applicative
 import Data.Monoid
+import Data.Semigroup as Sem
 import Prelude
 
 import Propellor.Types
@@ -279,7 +280,7 @@
 
 newtype PrivInfo = PrivInfo
 	{ fromPrivInfo :: S.Set (PrivDataField, Maybe PrivDataSourceDesc, HostContext) }
-	deriving (Eq, Ord, Show, Typeable, Monoid)
+	deriving (Eq, Ord, Show, Typeable, Sem.Semigroup, Monoid)
 
 -- PrivInfo always propagates out of containers, so that propellor
 -- can see which hosts need it.
diff --git a/src/Propellor/Property/Ccache.hs b/src/Propellor/Property/Ccache.hs
--- a/src/Propellor/Property/Ccache.hs
+++ b/src/Propellor/Property/Ccache.hs
@@ -14,6 +14,7 @@
 import Utility.FileMode
 import Utility.DataUnits
 import System.Posix.Files
+import qualified Data.Semigroup as Sem
 
 -- | Limits on the size of a ccache
 data Limit
@@ -25,9 +26,12 @@
 	| NoLimit
 	| Limit :+ Limit
 
+instance Sem.Semigroup Limit where
+	(<>) = (:+)
+
 instance Monoid Limit where
 	mempty  = NoLimit
-	mappend = (:+)
+	mappend = (Sem.<>)
 
 -- | A string that will be parsed to get a data size.
 --
diff --git a/src/Propellor/Property/Chroot.hs b/src/Propellor/Property/Chroot.hs
--- a/src/Propellor/Property/Chroot.hs
+++ b/src/Propellor/Property/Chroot.hs
@@ -169,10 +169,7 @@
 propellChroot c@(Chroot loc _ _ _) mkproc systemdonly = property (chrootDesc c "provisioned") $ do
 	let d = localdir </> shimdir c
 	let me = localdir </> "propellor"
-	shim <- liftIO $ ifM (doesDirectoryExist d)
-		( pure (Shim.file me d)
-		, Shim.setup me Nothing d
-		)
+	shim <- liftIO $ Shim.setup me Nothing d
 	ifM (liftIO $ bindmount shim)
 		( chainprovision shim
 		, return FailedChange
diff --git a/src/Propellor/Property/Conductor.hs b/src/Propellor/Property/Conductor.hs
--- a/src/Propellor/Property/Conductor.hs
+++ b/src/Propellor/Property/Conductor.hs
@@ -81,6 +81,7 @@
 import qualified Propellor.Property.Ssh as Ssh
 
 import qualified Data.Set as S
+import qualified Data.Semigroup as Sem
 
 -- | Class of things that can be conducted.
 --
@@ -313,9 +314,9 @@
 -- A Host's Info indicates when it's a conductor for hosts, and when it's
 -- stopped being a conductor.
 newtype ConductorFor = ConductorFor [Host]
-	deriving (Typeable, Monoid)
+	deriving (Typeable, Sem.Semigroup, Monoid)
 newtype NotConductorFor = NotConductorFor [Host]
-	deriving (Typeable, Monoid)
+	deriving (Typeable, Sem.Semigroup, Monoid)
 
 instance Show ConductorFor where
 	show (ConductorFor l) = "ConductorFor " ++ show (map hostName l)
@@ -329,7 +330,7 @@
 
 -- Added to Info when a host has been orchestrated.
 newtype Orchestrated = Orchestrated Any
-	deriving (Typeable, Monoid, Show)
+	deriving (Typeable, Sem.Semigroup, Monoid, Show)
 instance IsInfo Orchestrated where
 	propagateInfo _ = PropagateInfo False
 
diff --git a/src/Propellor/Property/Debootstrap.hs b/src/Propellor/Property/Debootstrap.hs
--- a/src/Propellor/Property/Debootstrap.hs
+++ b/src/Propellor/Property/Debootstrap.hs
@@ -19,12 +19,13 @@
 
 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 @@
 	| DebootstrapConfig :+ DebootstrapConfig
 	deriving (Show)
 
+instance Sem.Semigroup DebootstrapConfig where
+	(<>) = (:+)
+
 instance Monoid DebootstrapConfig where
 	mempty  = DefaultConfig
-	mappend = (:+)
+	mappend = (Sem.<>)
 
 toParams :: DebootstrapConfig -> [CommandParam]
 toParams DefaultConfig = []
diff --git a/src/Propellor/Property/DiskImage/PartSpec.hs b/src/Propellor/Property/DiskImage/PartSpec.hs
--- a/src/Propellor/Property/DiskImage/PartSpec.hs
+++ b/src/Propellor/Property/DiskImage/PartSpec.hs
@@ -43,6 +43,7 @@
 
 import Data.List (sortBy)
 import Data.Ord
+import qualified Data.Semigroup as Sem
 
 -- | Specifies a partition with a given filesystem.
 --
@@ -110,7 +111,7 @@
 	| AdjustPartSpecInfo MountPoint (PartSpec PartLocation -> PartSpec PartLocation)
 
 newtype PartInfo = PartInfo [PartInfoVal]
-	deriving (Monoid, Typeable)
+	deriving (Monoid, Sem.Semigroup, Typeable)
 
 instance IsInfo PartInfo where
 	propagateInfo _ = PropagateInfo False
@@ -183,9 +184,12 @@
 data PartLocation = Beginning | Middle | End
 	deriving (Eq, Ord)
 
+instance Sem.Semigroup PartLocation where
+	_ <> b = b
+
 instance Monoid PartLocation where
 	mempty = Middle
-	mappend _ b = b
+	mappend = (Sem.<>)
 
 partLocation :: PartSpec PartLocation -> PartLocation -> PartSpec PartLocation
 partLocation (mp, o, p, _) l = (mp, o, p, l)
diff --git a/src/Propellor/Property/Firewall.hs b/src/Propellor/Property/Firewall.hs
--- a/src/Propellor/Property/Firewall.hs
+++ b/src/Propellor/Property/Firewall.hs
@@ -18,6 +18,7 @@
 ) where
 
 import Data.Monoid
+import qualified Data.Semigroup as Sem
 import Data.Char
 import Data.List
 
@@ -199,6 +200,9 @@
 
 infixl 0 :-
 
+instance Sem.Semigroup Rules where
+	(<>) = (:-)
+
 instance Monoid Rules where
 	mempty  = Everything
-	mappend = (:-)
+	mappend = (Sem.<>)
diff --git a/src/Propellor/Property/FreeBSD/Pkg.hs b/src/Propellor/Property/FreeBSD/Pkg.hs
--- a/src/Propellor/Property/FreeBSD/Pkg.hs
+++ b/src/Propellor/Property/FreeBSD/Pkg.hs
@@ -9,6 +9,8 @@
 import Propellor.Base
 import Propellor.Types.Info
 
+import qualified Data.Semigroup as Sem
+
 noninteractiveEnv :: [([Char], [Char])]
 noninteractiveEnv = [("ASSUME_ALWAYS_YES", "yes")]
 
@@ -37,7 +39,7 @@
 		lines <$> readProcessEnv p a (Just noninteractiveEnv)
 
 newtype PkgUpdate = PkgUpdate String
-	deriving (Typeable, Monoid, Show)
+	deriving (Typeable, Sem.Semigroup, Monoid, Show)
 instance IsInfo PkgUpdate where
 	propagateInfo _ = PropagateInfo False
 
@@ -54,7 +56,7 @@
 			`setInfoProperty` (toInfo (PkgUpdate ""))
 
 newtype PkgUpgrade = PkgUpgrade String
-	deriving (Typeable, Monoid, Show)
+	deriving (Typeable, Sem.Semigroup, Monoid, Show)
 
 instance IsInfo PkgUpgrade where
 	propagateInfo _ = PropagateInfo False
diff --git a/src/Propellor/Property/FreeBSD/Poudriere.hs b/src/Propellor/Property/FreeBSD/Poudriere.hs
--- a/src/Propellor/Property/FreeBSD/Poudriere.hs
+++ b/src/Propellor/Property/FreeBSD/Poudriere.hs
@@ -8,17 +8,18 @@
 
 import Propellor.Base
 import Propellor.Types.Info
-import Data.List
-
 import qualified Propellor.Property.FreeBSD.Pkg as Pkg
 import qualified Propellor.Property.ZFS as ZFS
 import qualified Propellor.Property.File as File
 
+import Data.List
+import qualified Data.Semigroup as Sem
+
 poudriereConfigPath :: FilePath
 poudriereConfigPath = "/usr/local/etc/poudriere.conf"
 
 newtype PoudriereConfigured = PoudriereConfigured String
-	deriving (Typeable, Monoid, Show)
+	deriving (Typeable, Sem.Semigroup, Monoid, Show)
 
 instance IsInfo PoudriereConfigured where
 	propagateInfo _ = PropagateInfo False
diff --git a/src/Propellor/Property/Installer/Target.hs b/src/Propellor/Property/Installer/Target.hs
--- a/src/Propellor/Property/Installer/Target.hs
+++ b/src/Propellor/Property/Installer/Target.hs
@@ -112,6 +112,7 @@
 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 @@
 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 = (Sem.<>)
 
 newtype TargetFilledHandle = TargetFilledHandle Integer
 
diff --git a/src/Propellor/Property/List.hs b/src/Propellor/Property/List.hs
--- a/src/Propellor/Property/List.hs
+++ b/src/Propellor/Property/List.hs
@@ -20,6 +20,7 @@
 import Propellor.Exception
 
 import Data.Monoid
+import Prelude
 
 toProps :: [Property (MetaTypes metatypes)] -> Props (MetaTypes metatypes)
 toProps ps = Props (map toChildProperty ps)
diff --git a/src/Propellor/Property/Mount.hs b/src/Propellor/Property/Mount.hs
--- a/src/Propellor/Property/Mount.hs
+++ b/src/Propellor/Property/Mount.hs
@@ -10,6 +10,7 @@
 import Utility.Path
 
 import Data.List
+import qualified Data.Semigroup as Sem
 
 -- | type of filesystem to mount ("auto" to autodetect)
 type FsType = String
@@ -24,7 +25,7 @@
 --
 -- For default mount options, use `mempty`.
 newtype MountOpts = MountOpts [String]
-	deriving Monoid
+	deriving (Sem.Semigroup, Monoid)
 
 class ToMountOpts a where
 	toMountOpts :: a -> MountOpts
diff --git a/src/Propellor/Property/Parted.hs b/src/Propellor/Property/Parted.hs
--- a/src/Propellor/Property/Parted.hs
+++ b/src/Propellor/Property/Parted.hs
@@ -42,6 +42,7 @@
 import Utility.DataUnits
 
 import System.Posix.Files
+import qualified Data.Semigroup as Sem
 import Data.List (genericLength)
 
 data Eep = YesReallyDeleteDiskContents
@@ -178,16 +179,21 @@
 
 data DiskSpaceUse = Percent Int | RemainingSpace
 
+instance Sem.Semigroup DiskPart where
+	FixedDiskPart <> FixedDiskPart = FixedDiskPart
+	DynamicDiskPart (Percent a) <> DynamicDiskPart (Percent b) =
+		DynamicDiskPart (Percent (a + b))
+	DynamicDiskPart RemainingSpace <> DynamicDiskPart RemainingSpace = 
+		DynamicDiskPart RemainingSpace
+	DynamicDiskPart (Percent a) <> _ = DynamicDiskPart (Percent a)
+	_ <> DynamicDiskPart (Percent b) = DynamicDiskPart (Percent b)
+	DynamicDiskPart RemainingSpace <> _ = DynamicDiskPart RemainingSpace
+	_ <> DynamicDiskPart RemainingSpace = DynamicDiskPart RemainingSpace
+
 instance Monoid DiskPart
   where
 	mempty = FixedDiskPart
-	mappend FixedDiskPart FixedDiskPart = FixedDiskPart
-	mappend (DynamicDiskPart (Percent a)) (DynamicDiskPart (Percent b)) = DynamicDiskPart (Percent (a + b))
-	mappend (DynamicDiskPart RemainingSpace) (DynamicDiskPart RemainingSpace) = DynamicDiskPart RemainingSpace
-	mappend (DynamicDiskPart (Percent a)) _ = DynamicDiskPart (Percent a)
-	mappend _ (DynamicDiskPart (Percent b)) = DynamicDiskPart (Percent b)
-	mappend (DynamicDiskPart RemainingSpace) _ = DynamicDiskPart RemainingSpace
-	mappend _ (DynamicDiskPart RemainingSpace) = DynamicDiskPart RemainingSpace
+	mappend = (Sem.<>)
 
 -- | Make a partition use some percentage of the size of the disk
 -- (less all fixed size partitions), or the remaining space in the disk.
diff --git a/src/Propellor/Property/Parted/Types.hs b/src/Propellor/Property/Parted/Types.hs
--- a/src/Propellor/Property/Parted/Types.hs
+++ b/src/Propellor/Property/Parted/Types.hs
@@ -4,6 +4,9 @@
 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 @@
 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 = (Sem.<>)
+
 -- | A partition on the disk.
 data Partition = Partition
 	{ partType :: PartType
@@ -80,11 +86,14 @@
 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 = (Sem.<>)
 
 reducePartSize :: PartSize -> PartSize -> PartSize
 reducePartSize (MegaBytes a) (MegaBytes b) = MegaBytes (a - b)
diff --git a/src/Propellor/Property/Schroot.hs b/src/Propellor/Property/Schroot.hs
--- a/src/Propellor/Property/Schroot.hs
+++ b/src/Propellor/Property/Schroot.hs
@@ -32,8 +32,8 @@
 -- Implicitly sets 'useOverlays' info property.
 --
 -- Shell script from <https://wiki.debian.org/sbuild>.
-overlaysInTmpfs :: Property (HasInfo + DebianLike)
-overlaysInTmpfs = go `requires` installed
+overlaysInTmpfs :: RevertableProperty (HasInfo + DebianLike) UnixLike
+overlaysInTmpfs = (go `requires` installed) <!> undo
   where
 	f = "/etc/schroot/setup.d/04tmpfs"
 	go :: Property (HasInfo + UnixLike)
@@ -58,6 +58,7 @@
 			, "fi"
 			]
 		`onChange` (f `File.mode` combineModes (readModes ++ executeModes))
+	undo = File.notPresent f
 
 installed :: Property DebianLike
 installed = Apt.installed ["schroot"]
diff --git a/src/Propellor/Property/SiteSpecific/JoeySites.hs b/src/Propellor/Property/SiteSpecific/JoeySites.hs
--- a/src/Propellor/Property/SiteSpecific/JoeySites.hs
+++ b/src/Propellor/Property/SiteSpecific/JoeySites.hs
@@ -909,20 +909,23 @@
 	& "/etc/systemd/logind.conf" `ConfFile.containsIniSetting`
 		("Login", "LidSwitchIgnoreInhibited", "no")
 
--- My home power monitor.
-homePowerMonitor :: IsContext c => User -> [Host] -> c -> (SshKeyType, Ssh.PubKeyText) -> Property (HasInfo + DebianLike)
-homePowerMonitor user hosts ctx sshkey = propertyList "home power monitor" $ props
+homePower :: IsContext c => User -> [Host] -> c -> (SshKeyType, Ssh.PubKeyText) -> Property (HasInfo + DebianLike)
+homePower user hosts ctx sshkey = propertyList "home power" $ props
 	& Apache.installed
 	& Apt.installed ["python", "python-pymodbus", "rrdtool", "rsync"]
 	& File.ownerGroup "/var/www/html" user (userGroup user)
-	& Git.cloned user "git://git.kitenet.net/joey/homepower" d Nothing
-	& buildpoller
+	& Git.cloned user "https://git.joeyh.name/git/joey/homepower.git" d Nothing
+	& Git.cloned user "https://git.joeyh.name/git/reactive-banana-automation.git" (d </> "reactive-banana-automation") Nothing
+	& build
 	& Systemd.enabled setupservicename
 		`requires` setupserviceinstalled
 		`onChange` Systemd.started setupservicename
-	& Systemd.enabled servicename
-		`requires` serviceinstalled
-		`onChange` Systemd.started servicename
+	& Systemd.enabled pollerservicename
+		`requires` pollerserviceinstalled
+		`onChange` Systemd.started pollerservicename
+	& Systemd.enabled controllerservicename
+		`requires` controllerserviceinstalled
+		`onChange` Systemd.started controllerservicename
 	& User.hasGroup user (Group "dialout")
 	& Group.exists (Group "gpio") Nothing
 	& User.hasGroup user (Group "gpio")
@@ -936,27 +939,57 @@
   where
 	d = "/var/www/html/homepower"
 	sshkeyfile = d </> ".ssh/key"
-	buildpoller = userScriptProperty (User "joey")
-		[ "cd " ++ d
+	build = userScriptProperty (User "joey")
+		[ "cd " ++ d </> "reactive-banana-automation"
+		, "cabal install"
+		, "cd " ++ d
 		, "make"
 		]
 		`assume` MadeChange
-		`requires` Apt.installed ["ghc", "make"]
-	servicename = "homepower"
-	servicefile = "/etc/systemd/system/" ++ servicename ++ ".service"
-	serviceinstalled = servicefile `File.hasContent`
+		`requires` Apt.installed
+			[ "ghc", "cabal-install", "make"
+			, "libghc-http-types-dev"
+			, "libghc-stm-dev"
+			, "libghc-aeson-dev"
+			, "libghc-wai-dev"
+			, "libghc-warp-dev"
+			, "libghc-http-client-dev"
+			, "libghc-reactive-banana-dev"
+			, "libghc-hinotify-dev"
+			]
+	pollerservicename = "homepower"
+	pollerservicefile = "/etc/systemd/system/" ++ pollerservicename ++ ".service"
+	pollerserviceinstalled = pollerservicefile `File.hasContent`
 		[ "[Unit]"
-		, "Description=home power monitor"
+		, "Description=home power poller"
 		, ""
 		, "[Service]"
 		, "ExecStart=" ++ d ++ "/poller"
 		, "WorkingDirectory=" ++ d
 		, "User=joey"
 		, "Group=joey"
+		, "Restart=always"
 		, ""
 		, "[Install]"
 		, "WantedBy=multi-user.target"
+		, "WantedBy=homepower-controller.target"
 		]
+	controllerservicename = "homepower-controller"
+	controllerservicefile = "/etc/systemd/system/" ++ controllerservicename ++ ".service"
+	controllerserviceinstalled = controllerservicefile `File.hasContent`
+		[ "[Unit]"
+		, "Description=home power controller"
+		, ""
+		, "[Service]"
+		, "ExecStart=" ++ d ++ "/controller"
+		, "WorkingDirectory=" ++ d
+		, "User=joey"
+		, "Group=joey"
+		, "Restart=always"
+		, ""
+		, "[Install]"
+		, "WantedBy=multi-user.target"
+		]
 	setupservicename = "homepower-setup"
 	setupservicefile = "/etc/systemd/system/" ++ setupservicename ++ ".service"
 	setupserviceinstalled = setupservicefile `File.hasContent`
@@ -976,7 +1009,7 @@
 		]
 	-- Any changes to the rsync command will need my .authorized_keys
 	-- rsync server command to be updated too.
-	rsynccommand = "rsync -e 'ssh -i" ++ sshkeyfile ++ "' -avz rrds/recent/ joey@kitenet.net:/srv/web/homepower.joeyh.name/rrds/recent/"
+	rsynccommand = "rsync -e 'ssh -i" ++ sshkeyfile ++ "' -avz rrds/ joey@kitenet.net:/srv/web/homepower.joeyh.name/rrds/"
 
 -- My home router, running hostapd and dnsmasq for wlan0,
 -- with eth0 connected to a satellite modem, and a fallback ppp connection.
@@ -1007,7 +1040,9 @@
 		, "bogus-priv"
 		, "interface=wlan0"
 		, "domain=kitenet.net"
-		, "dhcp-range=10.1.1.100,10.1.1.150,24h"
+		-- lease time is short because the homepower
+		-- controller wants to know when clients disconnect
+		, "dhcp-range=10.1.1.100,10.1.1.150,10m"
 		, "no-hosts"
 		, "address=/honeybee.kitenet.net/10.1.1.1"
 		, "address=/house.kitenet.net/10.1.1.1"
@@ -1074,7 +1109,7 @@
 	, "bsdgames", "nethack-console"
 	, "xmonad", "libghc-xmonad-dev", "libghc-xmonad-contrib-dev"
 	, "ttf-bitstream-vera"
-	, "mairix", "offlineimap", "mutt"
+	, "mairix", "offlineimap", "mutt", "slrn"
 	, "mtr", "nmap", "whois", "wireshark", "tcpdump", "iftop"
 	, "pmount", "tree", "pv"
 	, "arbtt", "hledger", "bc"
@@ -1105,7 +1140,8 @@
 	[ "build-essential", "debhelper", "devscripts"
 	, "ghc", "cabal-install", "haskell-stack"
 	, "hothasktags", "hdevtools", "hlint"
-	, "gdb", "dpkg-repack", "lintian"
+	, "gdb", "time"
+	, "dpkg-repack", "lintian"
 	, "pristine-tar", "github-backup"
 	]
 
diff --git a/src/Propellor/Property/Ssh.hs b/src/Propellor/Property/Ssh.hs
--- a/src/Propellor/Property/Ssh.hs
+++ b/src/Propellor/Property/Ssh.hs
@@ -45,6 +45,7 @@
 import System.PosixCompat
 import qualified Data.Map as M
 import qualified Data.Set as S
+import qualified Data.Semigroup as Sem
 import Data.List
 
 installed :: Property UnixLike
@@ -229,13 +230,16 @@
 instance IsInfo HostKeyInfo where
 	propagateInfo _ = PropagateInfo False
 
-instance Monoid HostKeyInfo where
-	mempty = HostKeyInfo M.empty
-	mappend (HostKeyInfo old) (HostKeyInfo new) =
+instance Sem.Semigroup HostKeyInfo where
+	HostKeyInfo old <> HostKeyInfo new =
 		-- new first because union prefers values from the first
 		-- parameter when there is a duplicate key
 		HostKeyInfo (new `M.union` old)
 
+instance Monoid HostKeyInfo where
+	mempty = HostKeyInfo M.empty
+	mappend = (Sem.<>)
+
 userPubKeys :: User -> [(SshKeyType, PubKeyText)] -> Property (HasInfo + UnixLike)
 userPubKeys u@(User n) l = pureInfoProperty ("ssh pubkey for " ++ n) $
 	UserKeyInfo (M.singleton u (S.fromList l))
@@ -250,10 +254,13 @@
 instance IsInfo UserKeyInfo where
 	propagateInfo _ = PropagateInfo False
 
+instance Sem.Semigroup UserKeyInfo where
+	UserKeyInfo old <> UserKeyInfo new =
+		UserKeyInfo (M.unionWith S.union old new)
+
 instance Monoid UserKeyInfo where
 	mempty = UserKeyInfo M.empty
-	mappend (UserKeyInfo old) (UserKeyInfo new) =
-		UserKeyInfo (M.unionWith S.union old new)
+	mappend = (Sem.<>)
 
 -- | Sets up a user with the specified public keys, and the corresponding
 -- private keys from the privdata.
diff --git a/src/Propellor/Property/Unbound.hs b/src/Propellor/Property/Unbound.hs
--- a/src/Propellor/Property/Unbound.hs
+++ b/src/Propellor/Property/Unbound.hs
@@ -64,6 +64,10 @@
 -- | Provided a [UnboundSection], a [UnboundZone] and a [UnboundHost],
 -- cachingDnsServer ensure unbound is configured accordingly.
 --
+-- Be carefull with CNAMEs, unbound is not a primary DNS server, so it will
+-- resolve these by itself. For a locally served zone, you probably want A/AAAA
+-- records instead.
+--
 -- Example property:
 --
 -- > cachingDnsServer
@@ -115,13 +119,47 @@
 
 genRecord :: BindDomain -> Record -> Maybe String
 genRecord dom (Address addr) = Just $ genAddressNoTtl dom addr
-genRecord dom (MX priority dest) = Just $ genMX dom priority dest
-genRecord dom (PTR revip) = Just $ genPTR dom revip
-genRecord _ (CNAME _) = Nothing
-genRecord _ (NS _) = Nothing
-genRecord _ (TXT _) = Nothing
-genRecord _ (SRV _ _ _ _) = Nothing
-genRecord _ (SSHFP _ _ _) = Nothing
+genRecord dom (MX priority dest) = Just $ unwords
+	[ dValue dom
+	, "MX"
+	, val priority
+	, dValue dest
+	]
+genRecord dom (PTR revip) = Just $ unwords
+	[ revip ++ "."
+	, "PTR"
+	, dValue dom
+	]
+genRecord dom (CNAME dest) = Just $ unwords
+	[ dValue dom
+	, "CNAME"
+	, dValue dest
+	]
+genRecord dom (NS serv) = Just $ unwords
+	[ dValue dom
+	, "NS"
+	, dValue serv
+	]
+genRecord dom (TXT txt) = Just $ unwords
+	[ dValue dom
+	, "TXT"
+	, txt
+	]
+genRecord dom (SRV priority weight port target) = Just $ unwords
+	[ dValue dom
+	, "SRV"
+	, val priority
+	, val weight
+	, val port
+	, dValue target
+	]
+genRecord dom (SSHFP algo hash fingerprint) = Just $ unwords
+	[ dValue dom
+	, "SSHFP"
+	, val algo
+	, val hash
+	, fingerprint
+	]
 genRecord _ (INCLUDE _) = Nothing
 
 genAddressNoTtl :: BindDomain -> IPAddr -> String
@@ -133,10 +171,10 @@
 	IPv6 _ -> genAddress' "AAAA" dom ttl addr
 
 genAddress' :: String -> BindDomain -> Maybe Int -> IPAddr -> String
-genAddress' recordtype dom ttl addr = dValue dom ++ " " ++ maybe "" (\ttl' -> val ttl' ++ " ") ttl ++ "IN " ++ recordtype ++ " " ++ val addr
-
-genMX :: BindDomain -> Int -> BindDomain -> String
-genMX dom priority dest = dValue dom ++ " " ++ "MX" ++ " " ++ val priority ++ " " ++ dValue dest
-
-genPTR :: BindDomain -> ReverseIP -> String
-genPTR dom revip = revip ++ ". " ++ "PTR" ++ " " ++ dValue dom
+genAddress' recordtype dom ttl addr = unwords $
+	[ dValue dom ]
+	++ maybe [] (\ttl' -> [val ttl']) ttl ++
+	[ "IN"
+	, recordtype
+	, val addr
+	]
diff --git a/src/Propellor/Shim.hs b/src/Propellor/Shim.hs
--- a/src/Propellor/Shim.hs
+++ b/src/Propellor/Shim.hs
@@ -16,11 +16,18 @@
 -- | Sets up a shimmed version of the program, in a directory, and
 -- returns its path.
 --
+-- If the shim was already set up, it's refreshed, in case newer
+-- versions of libraries are needed.
+--
 -- Propellor may be running from an existing shim, in which case it's
 -- simply reused.
 setup :: FilePath -> Maybe FilePath -> FilePath -> IO FilePath
-setup propellorbin propellorbinpath dest = checkAlreadyShimmed shim $ do
+setup propellorbin propellorbinpath dest = checkAlreadyShimmed propellorbin $ do
 	createDirectoryIfMissing True dest
+
+	-- Remove all old libraries inside dest, but do not delete the
+	-- directory itself, since it may be bind-mounted inside a chroot.
+	mapM_ nukeFile =<< dirContentsRecursive dest
 
 	libs <- parseLdd <$> readProcess "ldd" [propellorbin]
 	glibclibs <- glibcLibs
diff --git a/src/Propellor/Types.hs b/src/Propellor/Types.hs
--- a/src/Propellor/Types.hs
+++ b/src/Propellor/Types.hs
@@ -43,6 +43,7 @@
 	, module Propellor.Types.ZFS
 	) where
 
+import qualified Data.Semigroup as Sem
 import Data.Monoid
 import Control.Applicative
 import Prelude
@@ -212,14 +213,13 @@
 instance TightenTargets Property where
 	tightenTargets (Property _ d a i c) = Property sing d a i c
 
--- | Any type of Property is a monoid. When properties x and y are
+-- | Any type of Property is a Semigroup. When properties x and y are
 -- appended together, the resulting property has a description like
 -- "x and y". Note that when x fails to be ensured, it will not
 -- try to ensure y.
-instance SingI metatypes => Monoid (Property (MetaTypes metatypes))
+instance SingI metatypes => Sem.Semigroup (Property (MetaTypes metatypes))
   where
-	mempty = Property sing "noop property" Nothing mempty mempty
-	mappend (Property _ d1 a1 i1 c1) (Property _ d2 a2 i2 c2) =
+	Property _ d1 a1 i1 c1 <> Property _ d2 a2 i2 c2 =
 	  	Property sing d (a1 <> a2) (i1 <> i2) (c1 <> c2)
 	  where
 		-- Avoid including "noop property" in description
@@ -230,16 +230,35 @@
 			(Nothing, Just _) -> d2
 			(Nothing, Nothing) -> d1
 
--- | Any type of RevertableProperty is a monoid. When revertable 
+-- | Any type of Property is a Monoid.
+instance SingI metatypes => Monoid (Property (MetaTypes metatypes))
+  where
+	-- | A property that does nothing.
+	mempty = Property sing "noop property" Nothing mempty mempty
+	mappend = (Sem.<>)
+
+-- | Any type of RevertableProperty is a Semigroup. When revertable 
 -- properties x and y are appended together, the resulting revertable
 -- property has a description like "x and y".
 -- Note that when x fails to be ensured, it will not try to ensure y.
 instance
-	( Monoid (Property setupmetatypes)
-	, Monoid (Property undometatypes)
+	( Sem.Semigroup (Property (MetaTypes setupmetatypes))
+	, Sem.Semigroup (Property (MetaTypes undometatypes))
+	, SingI setupmetatypes
+	, SingI undometatypes
 	)
-	=> Monoid (RevertableProperty setupmetatypes undometatypes)
+	=> Sem.Semigroup (RevertableProperty (MetaTypes setupmetatypes) (MetaTypes undometatypes))
   where
-	mempty = RevertableProperty mempty mempty
-	mappend (RevertableProperty s1 u1) (RevertableProperty s2 u2) =
+	RevertableProperty s1 u1 <> RevertableProperty s2 u2 =
 		RevertableProperty (s1 <> s2) (u2 <> u1)
+
+instance
+	( Monoid (Property (MetaTypes setupmetatypes))
+	, Monoid (Property (MetaTypes undometatypes))
+	, SingI setupmetatypes
+	, SingI undometatypes
+	)
+	=> Monoid (RevertableProperty (MetaTypes setupmetatypes) (MetaTypes undometatypes))
+  where
+	mempty = RevertableProperty mempty mempty
+	mappend = (Sem.<>)
diff --git a/src/Propellor/Types/Chroot.hs b/src/Propellor/Types/Chroot.hs
--- a/src/Propellor/Types/Chroot.hs
+++ b/src/Propellor/Types/Chroot.hs
@@ -6,8 +6,10 @@
 import Propellor.Types.Empty
 import Propellor.Types.Info
 
-import Data.Monoid
+import qualified Data.Semigroup as Sem
 import qualified Data.Map as M
+import Data.Monoid
+import Prelude
 
 data ChrootInfo = ChrootInfo
 	{ _chroots :: M.Map FilePath Host
@@ -18,13 +20,16 @@
 instance IsInfo ChrootInfo where
 	propagateInfo _ = PropagateInfo False
 
-instance Monoid ChrootInfo where
-	mempty = ChrootInfo mempty mempty
-	mappend old new = ChrootInfo
+instance Sem.Semigroup ChrootInfo where
+	old <> new = ChrootInfo
 		{ _chroots = M.union (_chroots old) (_chroots new)
 		, _chrootCfg = _chrootCfg old <> _chrootCfg new
 		}
 
+instance Monoid ChrootInfo where
+	mempty = ChrootInfo mempty mempty
+	mappend = (Sem.<>)
+
 instance Empty ChrootInfo where
 	isEmpty i = and
 		[ isEmpty (_chroots i)
@@ -36,12 +41,15 @@
 	| SystemdNspawnCfg [(String, Bool)]
 	deriving (Show, Eq)
 
+instance Sem.Semigroup ChrootCfg where
+	v <> NoChrootCfg = v
+	NoChrootCfg <> v = v
+	SystemdNspawnCfg l1 <> SystemdNspawnCfg l2 =
+		SystemdNspawnCfg (l1 <> l2)
+
 instance Monoid ChrootCfg where
 	mempty = NoChrootCfg
-	mappend v NoChrootCfg = v
-	mappend NoChrootCfg v = v
-	mappend (SystemdNspawnCfg l1) (SystemdNspawnCfg l2) =
-		SystemdNspawnCfg (l1 <> l2)
+	mappend = (Sem.<>)
 
 instance Empty ChrootCfg where
 	isEmpty c= c == NoChrootCfg
diff --git a/src/Propellor/Types/Core.hs b/src/Propellor/Types/Core.hs
--- a/src/Propellor/Types/Core.hs
+++ b/src/Propellor/Types/Core.hs
@@ -10,6 +10,7 @@
 import Propellor.Types.Result
 
 import Data.Monoid
+import qualified Data.Semigroup as Sem
 import "mtl" Control.Monad.RWS.Strict
 import Control.Monad.Catch
 import Control.Applicative
@@ -50,15 +51,18 @@
 
 -- | When two actions are appended together, the second action
 -- is only run if the first action does not fail.
-instance Monoid (Propellor Result) where
-	mempty = return NoChange
-	mappend x y = do
+instance Sem.Semigroup (Propellor Result) where
+	x <> y = do
 		rx <- x
 		case rx of
 			FailedChange -> return FailedChange
 			_ -> do
 				ry <- y
 				return (rx <> ry)
+	
+instance Monoid (Propellor Result) where
+	mempty = return NoChange
+	mappend = (Sem.<>)
 
 -- | An action that Propellor runs at the end, after trying to satisfy all
 -- properties. It's passed the combined Result of the entire Propellor run.
diff --git a/src/Propellor/Types/Dns.hs b/src/Propellor/Types/Dns.hs
--- a/src/Propellor/Types/Dns.hs
+++ b/src/Propellor/Types/Dns.hs
@@ -12,6 +12,7 @@
 import Data.Word
 import qualified Data.Map as M
 import qualified Data.Set as S
+import qualified Data.Semigroup as Sem
 import Data.List
 import Data.Monoid
 import Prelude
@@ -26,7 +27,7 @@
 	val (IPv6 addr) = addr
 
 newtype AliasesInfo = AliasesInfo (S.Set HostName)
-	deriving (Show, Eq, Ord, Monoid, Typeable)
+	deriving (Show, Eq, Ord, Sem.Semigroup, Monoid, Typeable)
 
 instance IsInfo AliasesInfo where
 	propagateInfo _ = PropagateInfo False
@@ -42,7 +43,7 @@
 -- of the containers in the host be reflected in the DNS.
 newtype DnsInfoPropagated = DnsInfoPropagated
 	{ fromDnsInfoPropagated :: S.Set Record }
-	deriving (Show, Eq, Ord, Monoid, Typeable)
+	deriving (Show, Eq, Ord, Sem.Semigroup, Monoid, Typeable)
 
 toDnsInfoPropagated :: S.Set Record -> DnsInfoPropagated
 toDnsInfoPropagated = DnsInfoPropagated
@@ -55,7 +56,7 @@
 -- the host.
 newtype DnsInfoUnpropagated = DnsInfoUnpropagated
 	{ fromDnsInfoUnpropagated :: S.Set Record }
-	deriving (Show, Eq, Ord, Monoid, Typeable)
+	deriving (Show, Eq, Ord, Sem.Semigroup, Monoid, Typeable)
 
 toDnsInfoUnpropagated :: S.Set Record -> DnsInfoUnpropagated
 toDnsInfoUnpropagated = DnsInfoUnpropagated
@@ -183,14 +184,17 @@
 -- | Adding a Master NamedConf stanza for a particulr domain always
 -- overrides an existing Secondary stanza for that domain, while a
 -- Secondary stanza is only added when there is no existing Master stanza.
-instance Monoid NamedConfMap where
-	mempty = NamedConfMap M.empty
-	mappend (NamedConfMap old) (NamedConfMap new) = NamedConfMap $
+instance Sem.Semigroup NamedConfMap where
+	NamedConfMap old <> NamedConfMap new = NamedConfMap $
 		M.unionWith combiner new old
 	  where
 		combiner n o = case (confDnsServerType n, confDnsServerType o) of
 			(Secondary, Master) -> o
 			_  -> n
+
+instance Monoid NamedConfMap where
+	mempty = NamedConfMap M.empty
+	mappend = (Sem.<>)
 
 instance Empty NamedConfMap where
 	isEmpty (NamedConfMap m) = isEmpty m
diff --git a/src/Propellor/Types/Docker.hs b/src/Propellor/Types/Docker.hs
--- a/src/Propellor/Types/Docker.hs
+++ b/src/Propellor/Types/Docker.hs
@@ -6,8 +6,10 @@
 import Propellor.Types.Empty
 import Propellor.Types.Info
 
-import Data.Monoid
+import qualified Data.Semigroup as Sem
 import qualified Data.Map as M
+import Data.Monoid
+import Prelude
 
 data DockerInfo = DockerInfo
 	{ _dockerRunParams :: [DockerRunParam]
@@ -18,12 +20,15 @@
 instance IsInfo DockerInfo where
 	propagateInfo _ = PropagateInfo False
 
-instance Monoid DockerInfo where
-	mempty = DockerInfo mempty mempty
-	mappend old new = DockerInfo
+instance Sem.Semigroup DockerInfo where
+	old <> new = DockerInfo
 		{ _dockerRunParams = _dockerRunParams old <> _dockerRunParams new
 		, _dockerContainers = M.union (_dockerContainers old) (_dockerContainers new)
 		}
+
+instance Monoid DockerInfo where
+	mempty = DockerInfo mempty mempty
+	mappend = (Sem.<>)
 
 instance Empty DockerInfo where
 	isEmpty i = and
diff --git a/src/Propellor/Types/Info.hs b/src/Propellor/Types/Info.hs
--- a/src/Propellor/Types/Info.hs
+++ b/src/Propellor/Types/Info.hs
@@ -17,6 +17,7 @@
 import Data.Dynamic
 import Data.Maybe
 import Data.Monoid
+import qualified Data.Semigroup as Sem
 import qualified Data.Typeable as T
 import Prelude
 
@@ -25,7 +26,7 @@
 -- Many different types of data can be contained in the same Info value
 -- at the same time. See `toInfo` and `fromInfo`.
 newtype Info = Info [InfoEntry]
-	deriving (Monoid, Show)
+	deriving (Sem.Semigroup, Monoid, Show)
 
 data InfoEntry where
 	InfoEntry :: (IsInfo v, Typeable v) => v -> InfoEntry
@@ -80,10 +81,13 @@
 data InfoVal v = NoInfoVal | InfoVal v
 	deriving (Typeable, Show)
 
+instance Sem.Semigroup (InfoVal v) where
+	_ <> v@(InfoVal _) = v
+	v <> NoInfoVal = v
+
 instance Monoid (InfoVal v) where
 	mempty = NoInfoVal
-	mappend _ v@(InfoVal _) = v
-	mappend v NoInfoVal = v
+	mappend = (Sem.<>)
 
 instance (Typeable v, Show v) => IsInfo (InfoVal v) where
 	propagateInfo _ = PropagateInfo False
diff --git a/src/Propellor/Types/MetaTypes.hs b/src/Propellor/Types/MetaTypes.hs
--- a/src/Propellor/Types/MetaTypes.hs
+++ b/src/Propellor/Types/MetaTypes.hs
@@ -66,7 +66,7 @@
 type MetaTypes = Sing
 
 -- This boilerplate would not be needed if the singletons library were
--- used. However, we're targeting too old a version of ghc to use it yet.
+-- used.
 data instance Sing (x :: MetaType) where
 	OSDebianS :: Sing ('Targeting 'OSDebian)
 	OSBuntishS :: Sing ('Targeting 'OSBuntish)
diff --git a/src/Propellor/Types/Result.hs b/src/Propellor/Types/Result.hs
--- a/src/Propellor/Types/Result.hs
+++ b/src/Propellor/Types/Result.hs
@@ -1,6 +1,7 @@
 module Propellor.Types.Result where
 
 import System.Console.ANSI
+import qualified Data.Semigroup as Sem
 import Data.Monoid
 import Prelude
 
@@ -8,14 +9,16 @@
 data Result = NoChange | MadeChange | FailedChange
 	deriving (Read, Show, Eq)
 
+instance Sem.Semigroup Result where
+	FailedChange <> _ = FailedChange
+	_ <> FailedChange = FailedChange
+	MadeChange <> _ = MadeChange
+	_ <> MadeChange = MadeChange
+	NoChange <> NoChange = NoChange
+
 instance Monoid Result where
 	mempty = NoChange
-
-	mappend FailedChange _ = FailedChange
-	mappend _ FailedChange = FailedChange
-	mappend MadeChange _ = MadeChange
-	mappend _ MadeChange = MadeChange
-	mappend NoChange NoChange = NoChange
+	mappend = (Sem.<>)
 
 class ToResult t where
 	toResult :: t -> Result
diff --git a/src/Propellor/Types/ResultCheck.hs b/src/Propellor/Types/ResultCheck.hs
--- a/src/Propellor/Types/ResultCheck.hs
+++ b/src/Propellor/Types/ResultCheck.hs
@@ -14,6 +14,7 @@
 import Utility.Monad
 
 import Data.Monoid
+import Prelude
 
 -- | This is a `Property` but its `Result` is not accurate; in particular
 -- it may return `NoChange` despite having made a change. 
diff --git a/stack.yaml b/stack.yaml
--- a/stack.yaml
+++ b/stack.yaml
@@ -1,4 +1,4 @@
 # When updating the resolver here, also update stackResolver in Propellor.DotDir
-resolver: lts-8.22
+resolver: lts-9.21
 packages:
 - '.'