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diff --git a/source/Checking/Cache.hs b/source/Checking/Cache.hs
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+module Checking.Cache where
+
+import Base
+import Syntax.Internal(Task)
+
+import Data.IntSet (IntSet)
+import Data.IntSet qualified as IntSet
+import Data.Binary
+
+-- The following would not work:
+-- Checking that the new task is a superset of an old task:
+-- additional assumptions may slow down the checking and
+-- lead to a failure with default timeouts. Then when you
+-- recheck the file from scratch it won't work.
+
+
+hashTasks :: [Task] -> IntSet
+hashTasks tasks = IntSet.fromList (hash <$> tasks)
+
+
+putTaskCache :: MonadIO io => FilePath -> [Task] -> io ()
+putTaskCache path tasks = liftIO $ encodeFile path $ hashTasks tasks
+
+
+notInCache :: MonadIO io => FilePath -> Task -> io Bool
+notInCache path task = do
+    eitherHashedTasks <- liftIO $ decodeFileOrFail path
+    pure case eitherHashedTasks of
+        Left _ -> True
+        Right hashedTasks -> hash task `IntSet.notMember` hashedTasks