diff --git a/a6/a6-handout/a6.cabal b/a6/a6-handout/a6.cabal
new file mode 100644
index 0000000..7bfb1e9
--- /dev/null
+++ b/a6/a6-handout/a6.cabal
@@ -0,0 +1,30 @@
+cabal-version:      3.0
+name:               a6
+version:            1.0.0.0
+build-type:         Simple
+
+common common
+    default-language: Haskell2010
+    ghc-options: -Wall
+
+library
+    import: common
+    hs-source-dirs: src
+    build-depends:
+        base
+      , tasty
+      , tasty-hunit
+      , clock
+    exposed-modules:
+        SPC
+        SPC_Tests
+        GenServer
+
+test-suite a6-tests
+    import: common
+    type: exitcode-stdio-1.0
+    main-is: runtests.hs
+    build-depends:
+        base
+      , tasty
+      , a6
diff --git a/a6/a6-handout/runtests.hs b/a6/a6-handout/runtests.hs
new file mode 100644
index 0000000..efba441
--- /dev/null
+++ b/a6/a6-handout/runtests.hs
@@ -0,0 +1,5 @@
+import qualified SPC_Tests
+import Test.Tasty (defaultMain)
+
+main :: IO ()
+main = defaultMain SPC_Tests.tests
diff --git a/a6/a6-handout/src/GenServer.hs b/a6/a6-handout/src/GenServer.hs
new file mode 100644
index 0000000..e70e18c
--- /dev/null
+++ b/a6/a6-handout/src/GenServer.hs
@@ -0,0 +1,45 @@
+module GenServer
+  ( Chan,
+    Server,
+    receive,
+    send,
+    sendTo,
+    spawn,
+    ReplyChan,
+    requestReply,
+    reply,
+  )
+where
+
+import Control.Concurrent (Chan)
+import qualified Control.Concurrent as CC
+
+data Server msg = Server CC.ThreadId (Chan msg)
+
+data ReplyChan a = ReplyChan (Chan a)
+
+send :: Chan a -> a -> IO ()
+send chan msg =
+  CC.writeChan chan msg
+
+sendTo :: Server a -> a -> IO ()
+sendTo (Server _tid input) msg =
+  send input msg
+
+receive :: Chan a -> IO a
+receive = CC.readChan
+
+spawn :: (Chan a -> IO ()) -> IO (Server a)
+spawn server = do
+  input <- CC.newChan
+  tid <- CC.forkIO $ server input
+  pure $ Server tid input
+
+requestReply :: Server a -> (ReplyChan b -> a) -> IO b
+requestReply serv con = do
+  reply_chan <- CC.newChan
+  sendTo serv $ con $ ReplyChan reply_chan
+  receive reply_chan
+
+reply :: ReplyChan a -> a -> IO ()
+reply (ReplyChan chan) x = send chan x
diff --git a/a6/a6-handout/src/SPC.hs b/a6/a6-handout/src/SPC.hs
new file mode 100644
index 0000000..2f90853
--- /dev/null
+++ b/a6/a6-handout/src/SPC.hs
@@ -0,0 +1,255 @@
+module SPC
+  ( -- * SPC startup
+    SPC,
+    startSPC,
+
+    -- * Job functions
+    Job (..),
+    JobId,
+    JobStatus (..),
+    JobDoneReason (..),
+    jobAdd,
+    jobStatus,
+    jobWait,
+    jobCancel,
+
+    -- * Worker functions
+    WorkerName,
+    workerAdd,
+    workerStop,
+  )
+where
+
+import Control.Concurrent
+  ( forkIO,
+    killThread,
+    threadDelay,
+  )
+import Control.Monad (ap, forever, liftM, void)
+import GenServer
+import System.Clock.Seconds (Clock (Monotonic), Seconds, getTime)
+
+-- First some general utility functions.
+
+-- | Retrieve Unix time using a monotonic clock. You cannot use this
+-- to measure the actual world time, but you can use it to measure
+-- elapsed time.
+getSeconds :: IO Seconds
+getSeconds = getTime Monotonic
+
+-- | Remove mapping from association list.
+removeAssoc :: (Eq k) => k -> [(k, v)] -> [(k, v)]
+removeAssoc needle ((k, v) : kvs) =
+  if k == needle
+    then kvs
+    else (k, v) : removeAssoc needle kvs
+removeAssoc _ [] = []
+
+-- Then the definition of the glorious SPC.
+
+-- | A job that is to be enqueued in the glorious SPC.
+data Job = Job
+  { -- | The IO action that comprises the actual action of the job.
+    jobAction :: IO (),
+    -- | The maximum allowed runtime of the job, counting from when
+    -- the job begins executing (not when it is enqueued).
+    jobMaxSeconds :: Int
+  }
+
+-- | A unique identifier of a job that has been enqueued.
+newtype JobId = JobId Int
+  deriving (Eq, Ord, Show)
+
+-- | How a job finished.
+data JobDoneReason
+  = -- | Normal termination.
+    Done
+  | -- | The job was killed because it ran for too long.
+    DoneTimeout
+  | -- | The job was explicitly cancelled, or the worker
+    -- it was running on was stopped.
+    DoneCancelled
+  | -- | The job crashed due to an exception.
+    DoneCrashed
+  deriving (Eq, Ord, Show)
+
+-- | The status of a job.
+data JobStatus
+  = -- | The job is done and this is why.
+    JobDone JobDoneReason
+  | -- | The job is still running.
+    JobRunning
+  | -- | The job is enqueued, but is waiting for an idle worker.
+    JobPending
+  | -- | A job with this ID is not known to this SPC instance.
+    JobUnknown
+  deriving (Eq, Ord, Show)
+
+-- | A worker decides its own human-readable name. This is useful for
+-- debugging.
+type WorkerName = String
+
+-- | Messages sent to workers. These are sent both by SPC and by
+-- processes spawned by the workes.
+data WorkerMsg -- TODO: add messages.
+
+-- Messages sent to SPC.
+data SPCMsg
+  = -- | Add the job, and reply with the job ID.
+    MsgJobAdd Job (ReplyChan JobId)
+  | -- | Cancel the given job.
+    MsgJobCancel JobId
+  | -- | Immediately reply the status of the job.
+    MsgJobStatus JobId (ReplyChan JobStatus)
+  | -- | Reply when the job is done.
+    MsgJobWait JobId (ReplyChan JobDoneReason)
+  | -- | Some time has passed.
+    MsgTick
+
+-- | A handle to the SPC instance.
+data SPC = SPC (Server SPCMsg)
+
+-- | A handle to a worker.
+data Worker = Worker (Server WorkerMsg)
+
+-- | The central state. Must be protected from the bourgeoisie.
+data SPCState = SPCState
+  { spcJobsPending :: [(JobId, Job)],
+    spcJobsRunning :: [(JobId, Job)],
+    spcJobsDone :: [(JobId, JobDoneReason)],
+    spcJobCounter :: JobId
+    -- TODO: you will need to add more fields.
+  }
+
+-- | The monad in which the main SPC thread runs. This is a state
+-- monad with support for IO.
+newtype SPCM a = SPCM (SPCState -> IO (a, SPCState))
+
+instance Functor SPCM where
+  fmap = liftM
+
+instance Applicative SPCM where
+  pure x = SPCM $ \state -> pure (x, state)
+  (<*>) = ap
+
+instance Monad SPCM where
+  SPCM m >>= f = SPCM $ \state -> do
+    (x, state') <- m state
+    let SPCM f' = f x
+    f' state'
+
+-- | Retrieve the state.
+get :: SPCM SPCState
+get = SPCM $ \state -> pure (state, state)
+
+-- | Overwrite the state.
+put :: SPCState -> SPCM ()
+put state = SPCM $ \_ -> pure ((), state)
+
+-- | Modify the state.
+modify :: (SPCState -> SPCState) -> SPCM ()
+modify f = do
+  state <- get
+  put $ f state
+
+-- | Lift an 'IO' action into 'SPCM'.
+io :: IO a -> SPCM a
+io m = SPCM $ \state -> do
+  x <- m
+  pure (x, state)
+
+-- | Run the SPCM monad.
+runSPCM :: SPCState -> SPCM a -> IO a
+runSPCM state (SPCM f) = fst <$> f state
+
+schedule :: SPCM ()
+schedule = undefined
+
+jobDone :: JobId -> JobDoneReason -> SPCM ()
+jobDone = undefined
+
+workerIsIdle :: WorkerName -> Worker -> SPCM ()
+workerIsIdle = undefined
+
+workerIsGone :: WorkerName -> SPCM ()
+workerIsGone = undefined
+
+checkTimeouts :: SPCM ()
+checkTimeouts = pure () -- change in Task 4
+
+workerExists :: WorkerName -> SPCM Bool
+workerExists = undefined
+
+handleMsg :: Chan SPCMsg -> SPCM ()
+handleMsg c = do
+  checkTimeouts
+  schedule
+  msg <- io $ receive c
+  case msg of
+    MsgJobAdd job rsvp -> do
+      state <- get
+      let JobId jobid = spcJobCounter state
+      put $
+        state
+          { spcJobsPending =
+              (spcJobCounter state, job) : spcJobsPending state,
+            spcJobCounter = JobId $ succ jobid
+          }
+      io $ reply rsvp $ JobId jobid
+    MsgJobStatus jobid rsvp -> do
+      state <- get
+      io $ reply rsvp $ case ( lookup jobid $ spcJobsPending state,
+                               lookup jobid $ spcJobsRunning state,
+                               lookup jobid $ spcJobsDone state
+                             ) of
+        (Just _, _, _) -> JobPending
+        (_, Just _, _) -> JobRunning
+        (_, _, Just r) -> JobDone r
+        _ -> JobUnknown
+
+startSPC :: IO SPC
+startSPC = do
+  let initial_state =
+        SPCState
+          { spcJobCounter = JobId 0,
+            spcJobsPending = [],
+            spcJobsRunning = [],
+            spcJobsDone = []
+          }
+  c <- spawn $ \c -> runSPCM initial_state $ forever $ handleMsg c
+  void $ spawn $ timer c
+  pure $ SPC c
+  where
+    timer c _ = forever $ do
+      threadDelay 1000000 -- 1 second
+      sendTo c MsgTick
+
+-- | Add a job for scheduling.
+jobAdd :: SPC -> Job -> IO JobId
+jobAdd (SPC c) job =
+  requestReply c $ MsgJobAdd job
+
+-- | Asynchronously query the job status.
+jobStatus :: SPC -> JobId -> IO JobStatus
+jobStatus (SPC c) jobid =
+  requestReply c $ MsgJobStatus jobid
+
+-- | Synchronously block until job is done and return the reason.
+jobWait :: SPC -> JobId -> IO JobDoneReason
+jobWait (SPC c) jobid =
+  requestReply c $ MsgJobWait jobid
+
+-- | Asynchronously cancel a job.
+jobCancel :: SPC -> JobId -> IO ()
+jobCancel (SPC c) jobid =
+  sendTo c $ MsgJobCancel jobid
+
+-- | Add a new worker with this name. Fails with 'Left' if a worker
+-- with that name already exists.
+workerAdd :: SPC -> WorkerName -> IO (Either String Worker)
+workerAdd = undefined
+
+-- | Shut down a running worker. No effect if the worker is already
+-- terminated.
+workerStop :: Worker -> IO ()
+workerStop = undefined
diff --git a/a6/a6-handout/src/SPC_Tests.hs b/a6/a6-handout/src/SPC_Tests.hs
new file mode 100644
index 0000000..113d738
--- /dev/null
+++ b/a6/a6-handout/src/SPC_Tests.hs
@@ -0,0 +1,15 @@
+module SPC_Tests (tests) where
+
+import Control.Concurrent (threadDelay)
+import Control.Monad (forM, forM_, replicateM)
+import Data.IORef
+import SPC
+import Test.Tasty (TestTree, localOption, mkTimeout, testGroup)
+import Test.Tasty.HUnit (assertFailure, testCase, (@?=))
+
+tests :: TestTree
+tests =
+  localOption (mkTimeout 3000000) $
+    testGroup
+      "SPC (core)"
+      []
diff --git a/a6/a6.pdf b/a6/a6.pdf
new file mode 100644
index 0000000..b85d210
Binary files /dev/null and b/a6/a6.pdf differ