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+{-# LANGUAGE TypeFamilies #-}
+module Hadrian.Oracles.ArgsHash (
+    TrackArgument, trackAllArguments, trackArgsHash, argsHashOracle
+    ) where
+
+import Control.Monad
+import Development.Shake
+import Development.Shake.Classes
+
+import Hadrian.Expression hiding (inputs, outputs)
+import Hadrian.Target
+
+-- | 'TrackArgument' is used to specify the arguments that should be tracked by
+-- the @ArgsHash@ oracle. The safest option is to track all arguments, but some
+-- arguments, such as @-jN@, do not change the build results, hence there is no
+-- need to initiate unnecessary rebuild if they are added to or removed from a
+-- command line. If all arguments should be tracked, use 'trackAllArguments'.
+type TrackArgument c b = Target c b -> String -> Bool
+
+-- | Returns 'True' for all targets and arguments, hence can be used a safe
+-- default for 'argsHashOracle'.
+trackAllArguments :: TrackArgument c b
+trackAllArguments _ _ = True
+
+-- | Given a 'Target' this 'Action' determines the corresponding argument list
+-- and computes its hash. The resulting value is tracked in a Shake oracle,
+-- hence initiating rebuilds when the hash changes (a hash change indicates
+-- changes in the build command for the given target).
+-- Note: for efficiency we replace the list of input files with its hash to
+-- avoid storing long lists of source files passed to some builders (e.g. ar)
+-- in the Shake database. This optimisation is normally harmless, because
+-- argument list constructors are assumed not to examine target sources, but
+-- only append them to argument lists where appropriate.
+trackArgsHash :: (ShakeValue c, ShakeValue b) => Target c b -> Action ()
+trackArgsHash t = do
+    let hashedInputs  = [ show $ hash (inputs t) ]
+        hashedTarget = target (context t) (builder t) hashedInputs (outputs t)
+    void (askOracle $ ArgsHash hashedTarget :: Action Int)
+
+newtype ArgsHash c b = ArgsHash (Target c b)
+    deriving (Binary, Eq, Hashable, NFData, Show, Typeable)
+type instance RuleResult (ArgsHash c b) = Int
+
+-- | This oracle stores per-target argument list hashes in the Shake database,
+-- allowing the user to track them between builds using 'trackArgsHash' queries.
+argsHashOracle :: (ShakeValue c, ShakeValue b) => TrackArgument c b -> Args c b -> Rules ()
+argsHashOracle trackArgument args = void $
+    addOracle $ \(ArgsHash target) -> do
+        argList <- interpret target args
+        let trackedArgList = filter (trackArgument target) argList
+        return $ hash trackedArgList