better git version checking

Git/Version.hs

@@ -1,6 +1,6 @@
-{- git version checking
+{- git versions
  -
- - Copyright 2011 Joey Hess <joey@kitenet.net>
+ - Copyright 2011, 2013 Joey Hess <joey@kitenet.net>
  -
  - Licensed under the GNU GPL version 3 or higher.
  -}
@@ -8,24 +8,29 @@
 module Git.Version where
 
 import Common
-import qualified Build.SysConfig
 
-{- Using the version it was configured for avoids running git to check its
- - version, at the cost that upgrading git won't be noticed.
- - This is only acceptable because it's rare that git's version influences
- - code's behavior. -}
-version :: String
-version = Build.SysConfig.gitversion
+data GitVersion = GitVersion String Integer
+	deriving (Eq)
 
-older :: String -> Bool
-older v = normalize version < normalize v
+instance Ord GitVersion where
+	compare (GitVersion _ x) (GitVersion _ y) = compare x y
+
+instance Show GitVersion where
+	show (GitVersion s _) = s
+
+installed :: IO GitVersion
+installed = normalize . extract <$> readProcess "git" ["--version"]
+  where
+  	extract s = case lines s of
+		[] -> ""
+		(l:_) -> unwords $ drop 2 $ words l
 
 {- To compare dotted versions like 1.7.7 and 1.8, they are normalized to
  - a somewhat arbitrary integer representation. -}
-normalize :: String -> Integer
-normalize = sum . mult 1 . reverse .
-		extend precision . take precision .
-		map readi . split "."
+normalize :: String -> GitVersion
+normalize v = GitVersion v $ 
+	sum $ mult 1 $ reverse $ extend precision $ take precision $
+		map readi $ split "." v
   where
 	extend n l = l ++ replicate (n - length l) 0
 	mult _ [] = []