The logged primitive is used to save the results of the subcomputations of a transient computation (including all its threads) in a log buffer. At any point, a suspend or checkpoint can be used to save the accumulated log on a persistent storage. A restore reads the saved logs and resumes the computation from the saved checkpoint. On resumption, the saved results are used for the computations which have already been performed. The log contains purely application level state, and is therefore independent of the underlying machine architecture. The saved logs can be sent across the wire to another machine and the computation can then be resumed on that machine. We can also save the log to gather diagnostic information, especially in finish blocks.

The following example illustrates the APIs. In its first run suspend saves the state in a directory named logs and exits, in the second run it resumes from that point and then stops at the checkpoint, in the third run it resumes from the checkpoint and then finishes.

main= keep $ restore  $ do
     r <- logged $ choose [1..10 :: Int]
     logged $ liftIO $ print ("A",r)
     suspend ()
     logged $ liftIO $ print ("B",r)
     checkpoint
     liftIO $ print ("C",r)