generalize parseDuration so it can be used in the ReadM monad

Utility/HumanTime.hs

@@ -17,7 +17,6 @@ module Utility.HumanTime (
 ) where
 
 import Utility.PartialPrelude
-import Utility.Applicative
 import Utility.QuickCheck
 
 import qualified Data.Map as M
@@ -45,8 +44,8 @@ daysToDuration :: Integer -> Duration
 daysToDuration i = Duration $ i * dsecs
 
 {- Parses a human-input time duration, of the form "5h", "1m", "5h1m", etc -}
-parseDuration :: String -> Maybe Duration
-parseDuration = Duration <$$> go 0
+parseDuration :: Monad m => String -> m Duration
+parseDuration = maybe parsefail (return . Duration) . go 0
   where
 	go n [] = return n
 	go n s = do
@@ -56,6 +55,7 @@ parseDuration = Duration <$$> 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\""
 
 fromDuration :: Duration -> String
 fromDuration Duration { durationSeconds = d }