add unsupportedOS

3 files changed, 13 insertions, 6 deletions

diff --git a/src/Propellor/Property.hs b/src/Propellor/Property.hs
index fe99a3fd..b6b8dc0d 100644
--- a/src/Propellor/Property.hs
+++ b/src/Propellor/Property.hs
@@ -20,6 +20,7 @@ module Propellor.Property (
 	, property
 	, ensureProperty
 	, withOS
+	, unsupportedOS
 	, makeChange
 	, noChange
 	, doNothing
@@ -256,10 +257,18 @@ isNewerThan x y = do
 -- > myproperty = withOS "foo installed" $ \o -> case o of
 -- > 	(Just (System (Debian suite) arch)) -> ...
 -- > 	(Just (System (Buntish release) arch)) -> ...
--- >	Nothing -> ...
+-- >	Nothing -> unsupportedOS
 withOS :: Desc -> (Maybe System -> Propellor Result) -> Property NoInfo
 withOS desc a = property desc $ a =<< getOS
 
+-- | Throws an error, for use in `withOS` when a property is lacking
+-- support for an OS.
+unsupportedOS :: Propellor a
+unsupportedOS = go =<< getOS
+  where
+	go Nothing = error "Unknown host OS is not supported by this property."
+	go (Just o) = error $ "This property is not implemented for " ++ show o
+
 -- | Undoes the effect of a RevertableProperty.
 revert :: RevertableProperty i -> RevertableProperty i
 revert (RevertableProperty p1 p2) = RevertableProperty p2 p1
diff --git a/src/Propellor/Property/Apt.hs b/src/Propellor/Property/Apt.hs
index eae56ab5..7301a6ae 100644
--- a/src/Propellor/Property/Apt.hs
+++ b/src/Propellor/Property/Apt.hs
@@ -160,16 +160,14 @@ installed' params ps = robustly $ check (isInstallable ps) go
 
 installedBackport :: [Package] -> Property NoInfo
 installedBackport ps = withOS desc $ \o -> case o of
-	Nothing -> error "cannot install backports; os not declared"
 	(Just (System (Debian suite) _)) -> case backportSuite suite of
-		Nothing -> notsupported o
+		Nothing -> unsupportedOS
 		Just bs -> ensureProperty $
 			runApt (["install", "-t", bs, "-y"] ++ ps)
 				`changesFile` dpkgStatus
-	_ -> notsupported o
+	_ -> unsupportedOS
   where
 	desc = unwords ("apt installed backport":ps)
-	notsupported o = error $ "backports not supported on " ++ show o
 
 -- | Minimal install of package, without recommends.
 installedMin :: [Package] -> Property NoInfo
diff --git a/src/Propellor/Property/OS.hs b/src/Propellor/Property/OS.hs
index 5f1adddb..e5da0921 100644
--- a/src/Propellor/Property/OS.hs
+++ b/src/Propellor/Property/OS.hs
@@ -86,7 +86,7 @@ cleanInstallOnce confirmation = check (not <$> doesFileExist flagfile) $
 	osbootstrapped = withOS (newOSDir ++ " bootstrapped") $ \o -> case o of
 		(Just d@(System (Debian _) _)) -> debootstrap d
 		(Just u@(System (Buntish _) _)) -> debootstrap u
-		_ -> error "os is not declared to be Debian or *buntu"
+		_ -> unsupportedOS
 	
 	debootstrap targetos = ensureProperty $
 		-- Ignore the os setting, and install debootstrap from