remove read of the heads

and one tail

Removed head from Utility.PartialPrelude in order to avoid the build
warning with recent ghc versions as well.

Backend/VURL/Utilities.hs

@@ -31,7 +31,8 @@ migrateFromURLToVURL oldkey newbackend _af inannex
   where
 	-- Relies on the first hash being cryptographically secure, and the
 	-- default hash used by git-annex.
-	hashbackend = Prelude.head Backend.Hash.backends
+	hashbackend = fromMaybe (error "internal") $ 
+		headMaybe Backend.Hash.backends
 
 migrateFromVURLToURL :: Key -> Backend -> AssociatedFile -> Bool -> Annex (Maybe Key)
 migrateFromVURLToURL oldkey newbackend _af _

Backend/Variety.hs

@@ -29,7 +29,7 @@ regularBackendList = Backend.Hash.backends
 
 {- The default hashing backend. -}
 defaultHashBackend :: Backend
-defaultHashBackend = Prelude.head regularBackendList
+defaultHashBackend = fromMaybe (error "internal") $ headMaybe regularBackendList
 
 makeVarietyMap :: [Backend] -> M.Map KeyVariety Backend
 makeVarietyMap l = M.fromList $ zip (map backendVariety l) l

CmdLine.hs

@@ -14,6 +14,7 @@ module CmdLine (
 
 import qualified Options.Applicative as O
 import qualified Options.Applicative.Help as H
+import qualified Data.List.NonEmpty as NE
 import Control.Exception (throw)
 import Control.Monad.IO.Class (MonadIO)
 import System.Exit
@@ -91,7 +92,7 @@ dispatch' subcommandname args fuzzy cmds allargs allcmds fields getgitrepo progn
 				handleresult (parseCmd progname progdesc correctedargs allcmds getparser)
 			res -> handleresult res
 	  where
-		autocorrect = Git.AutoCorrect.prepare (fromJust subcommandname) cmdname cmds
+		autocorrect = Git.AutoCorrect.prepare (fromJust subcommandname) cmdname (NE.fromList cmds)
 		name
 			| fuzzy = case cmds of
 				(c:_) -> Just (cmdname c)

Git/AutoCorrect.hs

@@ -15,6 +15,7 @@ import qualified Git.Config
 
 import Text.EditDistance
 import Control.Concurrent
+import qualified Data.List.NonEmpty as NE
 
 {- These are the same cost values as used in git. -}
 gitEditCosts :: EditCosts
@@ -44,7 +45,7 @@ fuzzymatches input showchoice choices = fst $ unzip $
 {- Takes action based on git's autocorrect configuration, in preparation for
  - an autocorrected command being run.
  -}
-prepare :: String -> (c -> String) -> [c] -> Maybe Repo -> IO ()
+prepare :: String -> (c -> String) -> NE.NonEmpty c -> Maybe Repo -> IO ()
 prepare input showmatch matches r =
 	case readish . fromConfigValue . Git.Config.get "help.autocorrect" "0" =<< r of
 		Just n
@@ -57,7 +58,7 @@ prepare input showmatch matches r =
 		[ "Unknown command '" ++ input ++ "'"
 		, ""
 		, "Did you mean one of these?"
-		] ++ map (\m -> "\t" ++ showmatch m) matches
+		] ++ map (\m -> "\t" ++ showmatch m) (NE.toList matches)
 	warn :: Maybe Float -> IO ()
 	warn mdelaysec = hPutStr stderr $ unlines
 		[ "WARNING: You called a git-annex command named '" ++
@@ -67,7 +68,7 @@ prepare input showmatch matches r =
 			Just sec -> "Continuing in " ++ show sec ++ " seconds, assuming that you meant " ++ match
 		]
 	  where
-		match = "'" ++ showmatch (Prelude.head matches) ++ "'."
+		match = "'" ++ showmatch (NE.head matches) ++ "'."
 	sleep n = do
 		warn (Just (fromIntegral n / 10 :: Float))
 		threadDelay (n * 100000) -- deciseconds to microseconds

Git/Sha.hs

@@ -13,6 +13,7 @@ import Common
 import Git.Types
 
 import qualified Data.ByteString as S
+import qualified Data.List.NonEmpty as NE
 import Data.Char
 
 {- Runs an action that causes a git subcommand to emit a Sha, and strips
@@ -44,16 +45,15 @@ extractSha s
 		]
 
 {- Sizes of git shas. -}
-shaSizes :: [Int]
+shaSizes :: NE.NonEmpty Int
 shaSizes = 
-	[ 40 -- sha1 (must come first)
-	, 64 -- sha256
-	]
+	       40 -- sha1 (must come first)
+	NE.:| [64] -- sha256
 
 {- Git plumbing often uses a all 0 sha to represent things like a
  - deleted file. -}
-nullShas :: [Sha]
-nullShas = map (\n -> Ref (S.replicate n zero)) shaSizes
+nullShas :: NE.NonEmpty Sha
+nullShas = NE.map (\n -> Ref (S.replicate n zero)) shaSizes
   where
 	zero = fromIntegral (ord '0')
 
@@ -63,7 +63,7 @@ nullShas = map (\n -> Ref (S.replicate n zero)) shaSizes
  - sha1 to the sha256, or probably just treat all null sha1 specially
  - the same as all null sha256. -}
 deleteSha :: Sha
-deleteSha = Prelude.head nullShas
+deleteSha = NE.head nullShas
 
 {- Git's magic empty tree.
  -

Git/UnionMerge.hs

@@ -107,9 +107,9 @@ mergeFile info file hashhandle h = case S8.words info of
  - generating new content.
  -}
 calcMerge :: [(Ref, [L8.ByteString])] -> Either Ref [L8.ByteString]
-calcMerge shacontents
-	| null reusable = Right new
-	| otherwise = Left $ fst $ Prelude.head reusable
+calcMerge shacontents = case reusable of
+	[] -> Right new
+	(r:_) -> Left $ fst r
   where
 	reusable = filter (\c -> sorteduniq (snd c) == new) shacontents
 	new = sorteduniq $ concat $ map snd shacontents

Remote/Helper/AWS.hs

@@ -40,7 +40,7 @@ regionMap :: Service -> M.Map Text Region
 regionMap = M.fromList . regionInfo
 
 defaultRegion :: Service -> Region
-defaultRegion = snd . Prelude.head . regionInfo
+defaultRegion = snd . fromMaybe (error "internal") . headMaybe . regionInfo
 
 data ServiceRegion = BothRegion Region | S3Region Region | GlacierRegion Region
 

Upgrade/V1.hs

@@ -167,7 +167,7 @@ readKey1' v
 		}
   where
 	bits = splitc ':' v
-	b = Prelude.head bits
+	b = fromMaybe (error "unable to parse v0 key") (headMaybe bits)
 	n = intercalate ":" $ drop (if wormy then 3 else 1) bits
 	t = if wormy
 		then readMaybe (bits !! 1) :: Maybe EpochTime

Utility/DataUnits.hs

@@ -180,16 +180,16 @@ compareSizes units abbrev old new
 
 {- Parses strings like "10 kilobytes" or "0.5tb". -}
 readSize :: [Unit] -> String -> Maybe ByteSize
-readSize units input
-	| null parsednum || null parsedunit = Nothing
-	| otherwise = Just $ round $ number * fromIntegral multiplier
+readSize units input = case parsednum of
+	[] -> Nothing
+	((number, rest):_) ->
+		let unitname = takeWhile isAlpha $ dropWhile isSpace rest
+		in case lookupUnit units unitname of
+			[] -> Nothing
+			(multiplier:_) -> 
+				Just $ round $ number * fromIntegral multiplier
   where
-	(number, rest) = head parsednum
-	multiplier = head parsedunit
-	unitname = takeWhile isAlpha $ dropWhile isSpace rest
-
 	parsednum = reads input :: [(Double, String)]
-	parsedunit = lookupUnit units unitname
 
 	lookupUnit _ [] = [1] -- no unit given, assume bytes
 	lookupUnit [] _ = []

Utility/Misc.hs

@@ -52,9 +52,8 @@ readFileStrict = readFile >=> \s -> length s `seq` return s
 separate :: (a -> Bool) -> [a] -> ([a], [a])
 separate c l = unbreak $ break c l
   where
-	unbreak r@(a, b)
-		| null b = r
-		| otherwise = (a, tail b)
+	unbreak (a, (_:b)) = (a, b)
+	unbreak r = r
 
 separate' :: (Word8 -> Bool) -> S.ByteString -> (S.ByteString, S.ByteString)
 separate' c l = unbreak $ S.break c l

Utility/PartialPrelude.hs

@@ -9,8 +9,6 @@
 
 module Utility.PartialPrelude (
 	Utility.PartialPrelude.read,
-	Utility.PartialPrelude.head,
-	Utility.PartialPrelude.tail,
 	Utility.PartialPrelude.init,
 	Utility.PartialPrelude.last,
 	Utility.PartialPrelude.readish,
@@ -27,16 +25,6 @@ import qualified Data.Maybe
 read :: Read a => String -> a
 read = Prelude.read
 
-{- head is a partial function; head [] is an error
- - Instead, use: take 1 or headMaybe -}
-head :: [a] -> a
-head = Prelude.head
-
-{- tail is also partial
- - Instead, use: drop 1 -}
-tail :: [a] -> [a]
-tail = Prelude.tail
-
 {- init too
  - Instead, use: beginning -}
 init :: [a] -> [a]

Utility/Tor.hs

@@ -80,7 +80,7 @@ addHiddenService appname uid ident = do
 		((p, _s):_) -> waithiddenservice 1 p
 		_ -> do
 			highports <- R.getStdRandom mkhighports
-			let newport = Prelude.head $
+			let newport = fromMaybe (error "internal") $ headMaybe $
 				filter (`notElem` map fst portssocks) highports
 			torrc <- findTorrc
 			writeFile torrc $ unlines $