Fix build with ghc-8.8.x.

The 'fail' method has been moved to the 'MonadFail' class. I made the changes
so that the code still compiles with previous versions of 'base' that don't
have the new MonadFail class exported by Prelude yet.

CmdLine/GitAnnex/Options.hs

@@ -9,6 +9,7 @@
 
 module CmdLine.GitAnnex.Options where
 
+import Control.Monad.Fail as Fail ( MonadFail(..) )
 import Options.Applicative
 import qualified Data.Map as M
 
@@ -215,8 +216,8 @@ parseAllOption = flag' WantAllKeys
 	<> help "operate on all versions of all files"
 	)
 
-parseKey :: Monad m => String -> m Key
-parseKey = maybe (fail "invalid key") return . deserializeKey
+parseKey :: MonadFail m => String -> m Key
+parseKey = maybe (Fail.fail "invalid key") return . deserializeKey
 
 -- Options to match properties of annexed files.
 annexedMatchingOptions :: [GlobalOption]

Command/Expire.hs

@@ -17,6 +17,7 @@ import Annex.VectorClock
 import qualified Remote
 import Utility.HumanTime
 
+import Control.Monad.Fail as Fail ( MonadFail(..) )
 import Data.Time.Clock.POSIX
 import qualified Data.Map as M
 
@@ -105,9 +106,9 @@ parseExpire ps = do
 		Nothing -> giveup $ "bad expire time: " ++ s
 		Just d -> Just (now - durationToPOSIXTime d)
 
-parseActivity :: Monad m => String -> m Activity
+parseActivity :: MonadFail m => String -> m Activity
 parseActivity s = case readish s of
-	Nothing -> fail $ "Unknown activity. Choose from: " ++ 
+	Nothing -> Fail.fail $ "Unknown activity. Choose from: " ++ 
 		unwords (map show [minBound..maxBound :: Activity])
 	Just v -> return v
 

Command/Init.hs

@@ -13,6 +13,7 @@ import Annex.Version
 import Types.RepoVersion
 import qualified Annex.SpecialRemote
 
+import Control.Monad.Fail as Fail ( MonadFail(..) )
 import qualified Data.Map as M
 	
 cmd :: Command
@@ -33,14 +34,14 @@ optParser desc = InitOptions
 		<> help "Override default annex.version"
 		))
 
-parseRepoVersion :: Monad m => String -> m RepoVersion
+parseRepoVersion :: MonadFail m => String -> m RepoVersion
 parseRepoVersion s = case RepoVersion <$> readish s of
-	Nothing -> fail $ "version parse error"
+	Nothing -> Fail.fail $ "version parse error"
 	Just v
 		| v `elem` supportedVersions -> return v
 		| otherwise -> case M.lookup v autoUpgradeableVersions of
 			Just v' -> return v'
-			Nothing -> fail $ s ++ " is not a currently supported repository version"
+			Nothing -> Fail.fail $ s ++ " is not a currently supported repository version"
 
 seek :: InitOptions -> CommandSeek
 seek = commandAction . start

Utility/HumanTime.hs

@@ -19,6 +19,7 @@ module Utility.HumanTime (
 import Utility.PartialPrelude
 import Utility.QuickCheck
 
+import Control.Monad.Fail as Fail ( MonadFail(..) )
 import qualified Data.Map as M
 import Data.Time.Clock
 import Data.Time.Clock.POSIX (POSIXTime)
@@ -44,7 +45,7 @@ daysToDuration :: Integer -> Duration
 daysToDuration i = Duration $ i * dsecs
 
 {- Parses a human-input time duration, of the form "5h", "1m", "5h1m", etc -}
-parseDuration :: Monad m => String -> m Duration
+parseDuration :: MonadFail m => String -> m Duration
 parseDuration = maybe parsefail (return . Duration) . go 0
   where
 	go n [] = return n
@@ -55,7 +56,7 @@ parseDuration = maybe parsefail (return . Duration) . go 0
 				u <- M.lookup c unitmap
 				go (n + num * u) rest
 			_ -> return $ n + num
-	parsefail = fail "duration parse error; expected eg \"5m\" or \"1h5m\""
+	parsefail = Fail.fail "duration parse error; expected eg \"5m\" or \"1h5m\""
 
 fromDuration :: Duration -> String
 fromDuration Duration { durationSeconds = d }