home-assistant-controller/docs/superpowers/plans/2026-08-20-concurrent-runtime.md

Concurrent Runtime Implementation Plan

For agentic workers: REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (- [ ]) syntax for tracking.

Goal: Support multiple controllers over a single websocket connection: one reader thread broadcasting to per-controller channels, a writer thread sending queued service calls, supervised worker threads with backoff restarts, and reconnect-on-network-failure.

Architecture: A Bus (broadcast TChan inbound, TChan outbound, TVar (Maybe Connection), existing CallIdGen) connects a reader thread (connect/auth/subscribe/receive-broadcast; restart = reconnect) and a writer thread to N controller threads, each stepping a Mealy HASSEff machine over its own dup'd channel. Slice 1 wires them with mapConcurrently_ (any crash exits). Slice 2 wraps each worker in supervised (annotated-exception, exponential backoff, Fatal rethrow).

Tech Stack: Haskell (GHC 9.10, GHC2024), stm, async, annotated-exception, websockets, aeson, hspec, hedgehog. Nix/cabal build.

Spec: docs/superpowers/specs/2026-08-20-concurrent-runtime-design.md

Global Constraints

• Run cabal only through the devShell: nix develop -c cabal build / nix develop -c cabal test.
• After changing the .cabal file, regenerate the derivation: nix run nixpkgs#cabal2nix -- ./. > default.nix. Never edit it by hand.
• -Wall (common warnings stanza) must produce zero warnings.
• Comments are minimal, contract-style: describe what, not how.
• Never read, print, or commit secrets.yaml.
• Commit after every task. Message style: short imperative, no prefix (see git log).
• Unit tests: hspec. Property tests: hedgehog.
• Modules are vertical (split by feature/concept, not layer).

File Structure

File	Responsibility
src/HomeAssistant/Runtime/Bus.hs (create)	Shared channels + connection cell + call-id gen + channel interpreter
src/HomeAssistant/Runtime/Connection.hs (create)	Reader/writer thread actions, pure encodeService
src/HomeAssistant/Runtime/Supervisor.hs (create, slice 2)	Generic restart-with-backoff combinator, Backoff, Fatal
src/HomeAssistant/Runtime.hs (rewrite)	Glue: Controller, runController, controllers, defaultMain, step, dryRunHassEval
src/HomeAssistant/Controller.hs (1-word change)	Derive Eq on Service (tests need it)
test/Main.hs (rewrite)	hspec runner
test/BusSpec.hs, test/ConnectionSpec.hs, test/RuntimeSpec.hs, test/SupervisorSpec.hs, test/BackoffProp.hs (create)	Specs
home-assistant-controller.cabal	Deps + module lists per task

Slices: Tasks 1–3 = slice 1 (concurrency). Tasks 4–5 = slice 2 (robustness).

---

Task 1: Bus module

Files:

• Create: src/HomeAssistant/Runtime/Bus.hs
• Modify: src/HomeAssistant/Runtime.hs (move CallIdGen out, re-export from Bus)
• Modify: src/HomeAssistant/Controller.hs:43 (derive Eq)
• Modify: home-assistant-controller.cabal (exposed module, stm dep, test deps)
• Rewrite: test/Main.hs
• Create: test/BusSpec.hs

Interfaces:

• Consumes: HASSEff(..), Service from HomeAssistant.Controller.

• Produces (Bus exports): Bus(..) with fields busInbound :: TChan Value, busOutbound :: TChan Service, busConn :: TVar (Maybe WS.Connection), busGen :: CallIdGen; CallIdGen(..) (record field generateCallId :: IO Int); mkCallIdGen :: Int -> IO CallIdGen; newBus :: Int -> IO Bus; channelHassEval :: Bus -> HASSEff a -> IO a.

• Step 1: Write the failing test

test/BusSpec.hs:

module BusSpec (spec) where

import Control.Concurrent.STM
  ( atomically
  , dupTChan
  , readTChan
  , writeTChan
  )
import Data.Aeson (Value (..))
import HomeAssistant.Controller (HASSEff (..), Service (..))
import HomeAssistant.Runtime.Bus
import Test.Hspec

spec :: Spec
spec = describe "Bus" $ do
  it "broadcasts inbound messages to every dup'd channel in order" $ do
    bus <- newBus 0
    p1 <- atomically (dupTChan (busInbound bus))
    p2 <- atomically (dupTChan (busInbound bus))
    atomically $ writeTChan (busInbound bus) (Number 1)
    atomically $ writeTChan (busInbound bus) (Number 2)
    r1 <- atomically $ (,) <$> readTChan p1 <*> readTChan p1
    r2 <- atomically $ (,) <$> readTChan p2 <*> readTChan p2
    r1 `shouldBe` (Number 1, Number 2)
    r2 `shouldBe` (Number 1, Number 2)

  it "channelHassEval writes CallService to the outbound channel" $ do
    bus <- newBus 0
    let svc = Service "light" "turn_on" Nothing "light.bedroom_masse"
    channelHassEval bus (CallService svc)
    atomically (readTChan (busOutbound bus)) `shouldReturn` svc

  it "channelHassEval leaves Pure untouched" $ do
    bus <- newBus 0
    channelHassEval bus (Pure 42) `shouldReturn` (42 :: Int)

  it "generates unique sequential call ids" $ do
    gen <- mkCallIdGen 0
    a <- generateCallId gen
    b <- generateCallId gen
    (a, b) `shouldBe` (1, 2)

test/Main.hs:

module Main (main) where

import Test.Hspec (hspec)
import qualified BusSpec

main :: IO ()
main = hspec BusSpec.spec

In home-assistant-controller.cabal:

Library section: add to exposed-modules: HomeAssistant.Runtime.Bus; add to build-depends: stm.

Test suite section: add other-modules: BusSpec and set:

    build-depends:
        base ^>=4.20.2.0,
        home-assistant-controller,
        hspec,
        stm,
        aeson,
        text

In src/HomeAssistant/Controller.hs change line 43 from deriving Show to deriving (Show, Eq).

• Step 2: Run test to verify it fails

Run: nix develop -c cabal test Expected: FAIL — compile error, Could not find module 'HomeAssistant.Runtime.Bus'.

• Step 3: Write the implementation

src/HomeAssistant/Runtime/Bus.hs:

{-# LANGUAGE LambdaCase #-}

module HomeAssistant.Runtime.Bus
  ( Bus(..)
  , CallIdGen(..)
  , mkCallIdGen
  , newBus
  , channelHassEval
  ) where

import Control.Concurrent.STM
  ( TChan
  , TVar
  , atomically
  , newBroadcastTChanIO
  , newTChanIO
  , newTVarIO
  , writeTChan
  )
import Data.Aeson (Value)
import Data.IORef (atomicModifyIORef', newIORef)
import HomeAssistant.Controller (HASSEff (..), Service)
import Network.WebSockets (Connection)

-- | Shared runtime state: inbound is a broadcast channel (controllers
-- read from 'dupTChan' copies), outbound queues service calls for the
-- writer, conn holds the current websocket (Nothing before first connect).
data Bus = Bus
  { busInbound  :: TChan Value
  , busOutbound :: TChan Service
  , busConn     :: TVar (Maybe Connection)
  , busGen      :: CallIdGen
  }

newBus :: Int -> IO Bus
newBus start = Bus
  <$> newBroadcastTChanIO
  <*> newTChanIO
  <*> newTVarIO Nothing
  <*> mkCallIdGen start

channelHassEval :: Bus -> HASSEff a -> IO a
channelHassEval bus = \case
  CallService svc -> atomically $ writeTChan (busOutbound bus) svc
  Pure a          -> pure a

newtype CallIdGen = CallIdGen { generateCallId :: IO Int }

mkCallIdGen :: Int -> IO CallIdGen
mkCallIdGen start = do
  gen <- newIORef start
  pure $ CallIdGen $ atomicModifyIORef' gen (\old -> let new = old + 1 in new `seq` (new, new))

Then in src/HomeAssistant/Runtime.hs:

• Delete the local newtype CallIdGen/mkCallIdGen definitions (lines 105–110).

• Add import: import HomeAssistant.Runtime.Bus (CallIdGen(..), mkCallIdGen).

• Export list stays the same (CallIdGen, mkCallIdGen now re-exported from Bus).

• Step 4: Regenerate the nix derivation

Run: nix run nixpkgs#cabal2nix -- ./. > default.nix Expected: no output; git diff default.nix shows the new stm/test dependencies.

• Step 5: Run tests to verify they pass

Run: nix develop -c cabal build && nix develop -c cabal test Expected: build with zero warnings; 4 examples, 0 failures.

• Step 6: Commit

git add src test home-assistant-controller.cabal default.nix
git commit -m "Add Runtime.Bus with channel-based effect interpreter"

---

Task 2: Connection module (reader/writer actions)

Files:

• Create: src/HomeAssistant/Runtime/Connection.hs
• Create: test/ConnectionSpec.hs
• Modify: test/Main.hs, home-assistant-controller.cabal

Interfaces:

• Consumes: Bus(..), CallIdGen(..) from Task 1.
• Produces (Connection exports): readerAction :: String -> Int -> String -> Bus -> IO Void (host, port, token; restart = reconnect); writerAction :: Bus -> IO Void; encodeService :: Int -> Service -> Data.Aeson.Value.

Background for the implementer (verified facts, do not re-verify):

• WS.runClient runs the ClientApp under bracket and closes the socket/stream when the app throws — reconnects do not leak fds.

• Exceptions from WS.receiveData (e.g. ConnectionClosed) are what make the reader restartable; connect failures throw IOException.

• HA sends a result ack for subscribe_events; we deliberately do not read it — it flows to controllers and is filtered out by their entity-id lenses (same as current behavior).

• Step 1: Write the failing test

test/ConnectionSpec.hs:

module ConnectionSpec (spec) where

import Data.Aeson (object, (.=))
import Data.Text (Text)
import HomeAssistant.Controller (Service (..))
import HomeAssistant.Runtime.Connection (encodeService)
import Test.Hspec

spec :: Spec
spec = describe "encodeService" $ do
  it "encodes a call_service message" $
    encodeService 7 (Service "light" "turn_on" Nothing "light.bedroom_masse")
      `shouldBe` object
        [ "id" .= (7 :: Int)
        , "type" .= ("call_service" :: Text)
        , "domain" .= ("light" :: Text)
        , "service" .= ("turn_on" :: Text)
        , "target" .= object ["entity_id" .= ("light.bedroom_masse" :: Text)]
        ]

  it "includes service_data when present" $
    encodeService 8 (Service "light" "turn_on" (Just (object ["brightness" .= (200 :: Int)])) "light.bedroom_masse")
      `shouldBe` object
        [ "id" .= (8 :: Int)
        , "type" .= ("call_service" :: Text)
        , "domain" .= ("light" :: Text)
        , "service" .= ("turn_on" :: Text)
        , "target" .= object ["entity_id" .= ("light.bedroom_masse" :: Text)]
        , "service_data" .= object ["brightness" .= (200 :: Int)]
        ]

test/Main.hs: add import qualified ConnectionSpec and change main to:

main :: IO ()
main = hspec $ do
  BusSpec.spec
  ConnectionSpec.spec

Cabal: library exposed-modules += HomeAssistant.Runtime.Connection; test other-modules += ConnectionSpec.

• Step 2: Run test to verify it fails

Run: nix develop -c cabal test Expected: FAIL — Could not find module 'HomeAssistant.Runtime.Connection'.

• Step 3: Write the implementation

src/HomeAssistant/Runtime/Connection.hs:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RecordWildCards #-}

module HomeAssistant.Runtime.Connection
  ( readerAction
  , writerAction
  , encodeService
  ) where

import Control.Concurrent.STM
  ( atomically
  , readTChan
  , readTVar
  , retry
  , writeTChan
  , writeTVar
  )
import Control.Lens ((^?))
import Control.Monad (forever)
import Data.Aeson (Value, eitherDecode, encode, object, (.=))
import Data.Aeson.Lens (key, _String)
import qualified Data.ByteString.Lazy as BL
import qualified Data.Text as T
import Data.Void (Void)
import HomeAssistant.Controller (Service (..))
import HomeAssistant.Runtime.Bus
import qualified Network.WebSockets as WS

-- | Connect, authenticate, subscribe, then receive and broadcast forever.
-- Restarting this action reconnects. All setup sends happen before the
-- connection is published in the bus, so only the writer sends afterwards.
readerAction :: String -> Int -> String -> Bus -> IO Void
readerAction host port token bus =
  WS.runClient host port "/api/websocket" $ \conn -> do
    handshake conn token
    subscribe bus conn
    atomically $ writeTVar (busConn bus) (Just conn)
    putStrLn "[reader] connected"
    receiveLoop bus conn

handshake :: WS.Connection -> String -> IO ()
handshake conn token = do
  required <- receiveJSON conn
  expectType "auth_required" required
  WS.sendTextData conn $ encode $ object
    [ "type"         .= ("auth" :: T.Text)
    , "access_token" .= token
    ]
  ok <- receiveJSON conn
  expectType "auth_ok" ok

expectType :: T.Text -> Value -> IO ()
expectType expected msg =
  case msg ^? key "type" . _String of
    Just t | t == expected -> pure ()
    _ -> fail $ "expected " <> T.unpack expected <> ", got: " <> show msg

subscribe :: Bus -> WS.Connection -> IO ()
subscribe bus conn = do
  sid <- generateCallId (busGen bus)
  WS.sendTextData conn $ encode $ object
    [ "id"         .= sid
    , "type"       .= ("subscribe_events" :: T.Text)
    , "event_type" .= ("state_changed" :: T.Text)
    ]

-- | Undecodable messages are skipped: reconnecting cannot fix a decode
-- problem, so crashing here would only produce a hot restart loop.
receiveLoop :: Bus -> WS.Connection -> IO Void
receiveLoop bus conn = forever $ do
  msg <- WS.receiveData conn :: IO BL.ByteString
  case eitherDecode msg of
    Left err -> putStrLn $ "[reader] skipping undecodable message: " <> err
    Right v  -> atomically $ writeTChan (busInbound bus) v

receiveJSON :: WS.Connection -> IO Value
receiveJSON conn = do
  msg <- WS.receiveData conn
  case eitherDecode msg of
    Left err -> fail $ "Invalid JSON from Home Assistant: " ++ err
    Right x  -> pure x

writerAction :: Bus -> IO Void
writerAction bus = forever $ do
  svc <- atomically $ readTChan (busOutbound bus)
  conn <- atomically $ readTVar (busConn bus) >>= maybe retry pure
  callId <- generateCallId (busGen bus)
  WS.sendTextData conn $ encode $ encodeService callId svc

encodeService :: Int -> Service -> Value
encodeService callId Service{..} = object $
  [ "id"      .= callId
  , "type"    .= ("call_service" :: T.Text)
  , "domain"  .= serviceDomain
  , "service" .= serviceName
  , "target"  .= object ["entity_id" .= serviceTarget]
  ] <> maybe [] (\d -> ["service_data" .= d]) serviceData

• Step 4: Run tests to verify they pass

Run: nix develop -c cabal build && nix develop -c cabal test Expected: zero warnings; 6 examples, 0 failures.

• Step 5: Commit

git add src test home-assistant-controller.cabal
git commit -m "Add Runtime.Connection reader and writer actions"

---

Task 3: Runtime rewrite — controller threads and wiring (completes slice 1)

Files:

• Rewrite: src/HomeAssistant/Runtime.hs
• Create: test/RuntimeSpec.hs
• Modify: test/Main.hs, home-assistant-controller.cabal

Interfaces:

• Consumes: Bus(..), newBus, channelHassEval, CallIdGen(..), mkCallIdGen (Task 1); readerAction, writerAction (Task 2).
• Produces (Runtime exports): defaultMain :: IO (), step, dryRunHassEval, CallIdGen, mkCallIdGen, Controller(..) with data Controller = forall b. Controller T.Text (HASS (Event Value) b), runController :: Bus -> Controller -> IO Void.
• Removes exports: app, hassEval, wsCallService, receiveJSON (deleted or internal to Connection). The get_states debug dump goes away.

Background: mapConcurrently_ rethrows the first worker exception and cancels the rest — a crash exits the process, same as today. That is intended for slice 1.

• Step 1: Write the failing test

test/RuntimeSpec.hs:

module RuntimeSpec (spec) where

import Control.Concurrent (threadDelay)
import Control.Concurrent.Async (async)
import Control.Concurrent.STM (atomically, readTChan, writeTChan)
import Data.Aeson (Value, object, (.=))
import Data.Text (Text)
import HomeAssistant.Controller (light, lightController)
import HomeAssistant.Runtime (Controller (..), runController)
import HomeAssistant.Runtime.Bus
import Test.Hspec

spec :: Spec
spec = describe "runController" $ do
  it "feeds inbound events through the machine and forwards service calls" $ do
    bus <- newBus 0
    _ <- async (runController bus (Controller "test" lightController))
    threadDelay 100000 -- let the controller dup its inbound channel
    atomically $ writeTChan (busInbound bus) (doorEvent "on")  -- initial value: no change event
    atomically $ writeTChan (busInbound bus) (doorEvent "off") -- door closes: lights on
    atomically $ writeTChan (busInbound bus) (doorEvent "on")  -- door opens: lights off
    svc1 <- atomically (readTChan (busOutbound bus))
    svc2 <- atomically (readTChan (busOutbound bus))
    svc1 `shouldBe` light True
    svc2 `shouldBe` light False

doorEvent :: Text -> Value
doorEvent state = object
  [ "event" .= object
    [ "data" .= object
      [ "entity_id" .= ("binary_sensor.makuuhuone_ovi_contact" :: Text)
      , "new_state" .= object ["state" .= state]
      ]
    ]
  ]

Note: the first event only seeds the machine (changes does not fire on the initial value), hence two expected service calls, not three.

test/Main.hs: add import qualified RuntimeSpec; run all three specs. Cabal: test other-modules += RuntimeSpec, test build-depends += async.

• Step 2: Run test to verify it fails

Run: nix develop -c cabal test Expected: FAIL — Controller / runController not in scope.

• Step 3: Rewrite the Runtime module

Replace src/HomeAssistant/Runtime.hs entirely with:

{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE OverloadedStrings #-}

module HomeAssistant.Runtime
  ( defaultMain
  , step
  , CallIdGen
  , mkCallIdGen
  , dryRunHassEval
  , Controller(..)
  , runController
  ) where

import AFRP (Event (..), Mealy (..))
import Control.Concurrent.Async (mapConcurrently_)
import Control.Concurrent.STM (atomically, dupTChan, readTChan)
import Data.Aeson (Value)
import qualified Data.Text as T
import Data.Time (getCurrentTime)
import Data.Void (Void)
import HomeAssistant.Controller (HASS, HASSEff (..), lightController)
import HomeAssistant.Runtime.Bus
import HomeAssistant.Runtime.Connection (readerAction, writerAction)
import Network.Socket (withSocketsDo)
import System.Environment (getEnv)

step :: (forall x. eff x -> IO x) -> Mealy eff a b -> a -> IO (b, Mealy eff a b)
step nt (Mealy f) a = do
  now <- getCurrentTime
  f nt now a

data Controller = forall b. Controller T.Text (HASS (Event Value) b)

controllers :: [Controller]
controllers = [Controller "light" lightController]

-- | Steps the machine for every inbound message; service calls go to the
-- bus. A restart re-dups the inbound channel and starts from the machine's
-- initial state; messages broadcast during the restart window are lost.
runController :: Bus -> Controller -> IO Void
runController bus (Controller _name machine) = do
  inbound <- atomically (dupTChan (busInbound bus))
  go inbound machine
  where
    go inbound f = do
      msg <- atomically (readTChan inbound)
      (_, f') <- step (channelHassEval bus) f (Event msg)
      go inbound f'

defaultMain :: IO ()
defaultMain = withSocketsDo $ do
  token <- getEnv "HA_TOKEN"
  bus <- newBus 0
  mapConcurrently_ id $
    [ readerAction "last-resort-redux" 8123 token bus
    , writerAction bus
    ] ++ map (runController bus) controllers

dryRunHassEval :: CallIdGen -> HASSEff a -> IO a
dryRunHassEval gen = \case
  CallService x -> do
    callId <- generateCallId gen
    print (callId, x)
  Pure a -> pure a

• Step 4: Regenerate derivation and run tests

Run: nix run nixpkgs#cabal2nix -- ./. > default.nix && nix develop -c cabal build && nix develop -c cabal test Expected: zero warnings; 7 examples, 0 failures.

• Step 5: Commit

git add src test home-assistant-controller.cabal default.nix
git commit -m "Run controllers on their own threads over the bus"

---

Task 4: Supervisor module (starts slice 2)

Files:

• Create: src/HomeAssistant/Runtime/Supervisor.hs
• Create: test/SupervisorSpec.hs, test/BackoffProp.hs
• Modify: test/Main.hs, home-assistant-controller.cabal

Interfaces:

• Consumes: nothing from other runtime modules (generic).
• Produces: supervised :: Text -> Backoff -> IO Void -> IO Void; Backoff(..) (backoffBase, backoffCap, backoffQuiet, all NominalDiffTime); defaultBackoff (= Backoff 0.1 30 30); backoffDelay :: Backoff -> Int -> NominalDiffTime; nextAttempt :: Backoff -> NominalDiffTime -> Int -> Int; Fatal (..) (newtype Fatal = Fatal Text).

Background (verified, do not re-verify): Control.Exception.Annotated is built on safe-exceptions: its catch/catches/try only catch synchronous exceptions, so async exceptions propagate without any manual SomeAsyncException filtering. catch @(AnnotatedException e) sees through a single AnnotatedException wrapper (bare e gets an empty annotation set), which is how the Fatal handler below works.

Invariant (document in code): actions passed to supervised must not be wrapped in checkpoints around Fatal-throwing code — a double-wrapped Fatal is invisible to the handler and would restart instead of crashing. The reader honors this (Task 5).

• Step 1: Write the failing tests

test/SupervisorSpec.hs:

module SupervisorSpec (spec) where

import Control.Concurrent (newEmptyMVar, putMVar, readMVar, threadDelay)
import Control.Concurrent.Async (async, cancel, poll, waitCatch)
import Control.Exception (SomeException, fromException)
import Control.Exception.Annotated (AnnotatedException (..), throw)
import Control.Monad (forever)
import Data.IORef (atomicModifyIORef', newIORef, readIORef)
import Data.Maybe (isJust, isNothing)
import HomeAssistant.Runtime.Supervisor
import System.IO.Error (ioError, userError)
import Test.Hspec

tinyBackoff :: Backoff
tinyBackoff = Backoff 0.001 0.002 0.001

spec :: Spec
spec = describe "supervised" $ do
  it "restarts a crashing action until it stays up" $ do
    counter <- newIORef (0 :: Int)
    up <- newEmptyMVar
    let action = do
          n <- atomicModifyIORef' counter (\c -> (c + 1, c + 1))
          if n < 3
            then ioError (userError "boom")
            else do putMVar up (); forever (threadDelay 1000000)
    sup <- async (supervised "test" tinyBackoff action)
    readMVar up
    threadDelay 50000
    status <- poll sup
    isNothing status `shouldBe` True
    readIORef counter `shouldReturn` 3
    cancel sup

  it "rethrows Fatal instead of restarting" $ do
    counter <- newIORef (0 :: Int)
    let action = do
          atomicModifyIORef' counter (\c -> (c + 1, c + 1))
          throw (Fatal "auth_invalid")
    sup <- async (supervised "test" tinyBackoff action)
    res <- waitCatch sup
    case res of
      Left se -> case fromException se :: Maybe (AnnotatedException Fatal) of
        Just _  -> pure ()
        Nothing -> expectationFailure "expected Fatal to propagate"
      Right _ -> expectationFailure "supervised returned"
    threadDelay 50000
    readIORef counter `shouldReturn` 1

  it "does not restart on async exceptions" $ do
    counter <- newIORef (0 :: Int)
    let action = do
          atomicModifyIORef' counter (\c -> (c + 1, c + 1))
          forever (threadDelay 1000000)
    sup <- async (supervised "test" tinyBackoff action)
    threadDelay 100000
    cancel sup
    threadDelay 100000
    readIORef counter `shouldReturn` 1
    status <- poll sup
    isJust status `shouldBe` True

  describe "nextAttempt" $ do
    it "resets after a quiet period" $
      nextAttempt tinyBackoff 0.001 5 `shouldBe` 1
    it "increments otherwise" $
      nextAttempt tinyBackoff 0.0005 5 `shouldBe` 6

  describe "backoffDelay" $ do
    it "starts at base" $ backoffDelay tinyBackoff 1 `shouldBe` 0.001
    it "doubles" $ backoffDelay tinyBackoff 2 `shouldBe` 0.002
    it "clamps at cap" $ backoffDelay tinyBackoff 3 `shouldBe` 0.002

test/BackoffProp.hs:

module BackoffProp (spec) where

import Data.Time (NominalDiffTime)
import qualified Hedgehog.Gen as Gen
import qualified Hedgehog.Range as Range
import HomeAssistant.Runtime.Supervisor (Backoff (..), backoffDelay)
import Test.Hspec (Spec, describe, it)
import Test.Hspec.Hedgehog (hedgehog, forAll, (===))

spec :: Spec
spec = describe "backoffDelay" $
  it "doubles from base, clamped at cap" $ hedgehog $ do
    baseD <- forAll $ Gen.double (Range.constant 0.0001 10)
    ratio <- forAll $ Gen.double (Range.constant 1 100)
    let base = realToFrac baseD :: NominalDiffTime
        cap = realToFrac (baseD * ratio) :: NominalDiffTime
        backoff = Backoff base cap 1
        delays = map (backoffDelay backoff) [1 .. 100 :: Int]
    head delays === min cap base
    mapM_ (\(a, b) -> b === min cap (a * 2)) (zip delays (drop 1 delays))

(hspec-hedgehog provides the hedgehog bridge — plain hedgehog 1.5 has no hspec integration — and re-exports forAll and (===).)

test/Main.hs: add imports and run all five specs. Cabal: library exposed-modules += HomeAssistant.Runtime.Supervisor, library build-depends += annotated-exception; test other-modules += SupervisorSpec, BackoffProp, test build-depends += hedgehog, hspec-hedgehog, annotated-exception, time.

• Step 2: Run tests to verify they fail

Run: nix develop -c cabal test Expected: FAIL — Could not find module 'HomeAssistant.Runtime.Supervisor'.

• Step 3: Write the implementation

src/HomeAssistant/Runtime/Supervisor.hs:

{-# LANGUAGE ScopedTypeVariables #-}

module HomeAssistant.Runtime.Supervisor
  ( supervised
  , Backoff(..)
  , defaultBackoff
  , backoffDelay
  , nextAttempt
  , Fatal(..)
  ) where

import Control.Concurrent (threadDelay)
import Control.Exception.Annotated
  ( Exception
  , Handler (..)
  , SomeException
  , catches
  , displayException
  , throw
  )
import Data.Text (Text)
import qualified Data.Text as T
import Data.Time (NominalDiffTime, diffUTCTime, getCurrentTime)
import Data.Void (Void)

-- | A failure that cannot be fixed by restarting; propagates out of
-- 'supervised' and terminates the process.
newtype Fatal = Fatal Text
  deriving (Show, Exception)

data Backoff = Backoff
  { backoffBase  :: NominalDiffTime -- ^ Delay before the first restart
  , backoffCap   :: NominalDiffTime -- ^ Maximum delay between restarts
  , backoffQuiet :: NominalDiffTime -- ^ Uptime after which the delay resets
  }
  deriving (Eq, Show)

defaultBackoff :: Backoff
defaultBackoff = Backoff 0.1 30 30

-- | Delay before the @attempt@-th restart: doubles from base, clamped at cap.
backoffDelay :: Backoff -> Int -> NominalDiffTime
backoffDelay (Backoff base cap _) attempt = go (attempt - 1) base
  where
    go 0 d = d
    go n d = go (n - 1) (min cap (d * 2))

-- | Attempt number to use after a crash that ran for the given uptime.
nextAttempt :: Backoff -> NominalDiffTime -> Int -> Int
nextAttempt (Backoff _ _ quiet) uptime attempt
  | uptime >= quiet = 1
  | otherwise = attempt + 1

-- | Runs the action forever, restarting it with backoff after synchronous
-- exceptions; async exceptions propagate. 'Fatal' is rethrown (crashing the
-- caller) rather than restarted. The action must never return normally and
-- must not be wrapped in checkpoints around 'Fatal'-throwing code: a
-- doubly-wrapped 'Fatal' is indistinguishable from a crash and would be
-- restarted instead of escalated.
supervised :: Text -> Backoff -> IO Void -> IO Void
supervised name backoff action = go 1
  where
    go attempt = do
      start <- getCurrentTime
      outcome <- (action >> pure (Nothing :: Maybe (Either Fatal SomeException))) `catches`
        [ Handler $ \(f :: Fatal) -> pure (Just (Left f))
        , Handler $ \(e :: SomeException) -> pure (Just (Right e))
        ]
      case outcome of
        Nothing -> error "unreachable: supervised action returned"
        Just (Left f) -> throw f
        Just (Right e) -> do
          putStrLn $ "[" <> T.unpack name <> "] attempt " <> show attempt <> " crashed: " <> displayException e
          let delay = backoffDelay backoff attempt
          putStrLn $ "[" <> T.unpack name <> "] restarting in " <> show delay <> "s"
          threadDelay (round (realToFrac delay * 1000000))
          end <- getCurrentTime
          go (nextAttempt backoff (diffUTCTime end start) attempt)

• Step 4: Regenerate derivation and run tests

Run: nix run nixpkgs#cabal2nix -- ./. > default.nix && nix develop -c cabal build && nix develop -c cabal test Expected: zero warnings; 15 examples and 1 property, 0 failures.

• Step 5: Commit

git add src test home-assistant-controller.cabal default.nix
git commit -m "Add supervisor with backoff restarts"

---

Task 5: Supervised wiring and fatal auth (completes slice 2)

Files:

• Modify: src/HomeAssistant/Runtime.hs (defaultMain only)
• Modify: src/HomeAssistant/Runtime/Connection.hs (Fatal in handshake)

Interfaces:

• Consumes: supervised, defaultBackoff, Fatal (Task 4).
• Produces: no new exports; defaultMain behavior changes: workers are supervised, auth failure exits the process with Fatal.

Background: waitAny rethrows the exception of the first completed async. Supervised workers only complete by rethrowing Fatal, so waitAny blocks forever in normal operation and propagates Fatal otherwise.

• Step 1: Change defaultMain in src/HomeAssistant/Runtime.hs

Update imports: replace mapConcurrently_ with async, waitAny from Control.Concurrent.Async; add absurd to the Data.Void import; add import HomeAssistant.Runtime.Supervisor (defaultBackoff, supervised). Replace defaultMain with:

defaultMain :: IO ()
defaultMain = withSocketsDo $ do
  token <- getEnv "HA_TOKEN"
  bus <- newBus 0
  let workers =
        [ ("reader", readerAction "last-resort-redux" 8123 token bus)
        , ("writer", writerAction bus)
        ] ++ [ (name, runController bus c) | c@(Controller name _) <- controllers ]
  as <- mapM (\(name, act) -> async (supervised name defaultBackoff act)) workers
  (_, v) <- waitAny as
  absurd v

• Step 2: Make handshake failures fatal in src/HomeAssistant/Runtime/Connection.hs

Add imports: import HomeAssistant.Runtime.Supervisor (Fatal) and throw from Control.Exception.Annotated. Change expectType and receiveJSON:

expectType :: T.Text -> Value -> IO ()
expectType expected msg =
  case msg ^? key "type" . _String of
    Just t | t == expected -> pure ()
    _ -> throw (Fatal $ "expected " <> expected <> ", got: " <> T.pack (show msg))

receiveJSON :: WS.Connection -> IO Value
receiveJSON conn = do
  msg <- WS.receiveData conn
  case eitherDecode msg of
    Left err -> throw (Fatal $ "Invalid JSON from Home Assistant: " <> T.pack err)
    Right x  -> pure x

The fail-based behavior (crash the process) is preserved for slice 1's semantics but now carries a Fatal marker the supervisor escalates. Do not add checkpoints around the handshake — see the supervised contract.

• Step 3: Build and test

Run: nix develop -c cabal build && nix develop -c cabal test Expected: zero warnings, zero test failures.

• Step 4: Manual smoke check (optional, needs real HA)

Run: HA_TOKEN=... nix develop -c cabal run home-assistant-controller (only if a Home Assistant instance is reachable; otherwise skip — unit tests cover the wiring logic).

• Step 5: Commit

git add src
git commit -m "Supervise workers and make auth failures fatal"

---

Verification (all tasks)

• nix develop -c cabal build — zero warnings under -Wall.
• nix develop -c cabal test — all specs green.
• git status clean after each commit.

Non-goals (from spec)

• Request/response correlation for service calls; dynamic controller registration; structured logging; env-based host/port config; state pre-seeding via get_states; multiple websocket connections.

Gaps accepted by spec

• Reader's connect/auth/subscribe loop has no integration test (thin IO glue over WS.runClient; localhost fake-server scaffolding judged not worth the complexity).
• Fatal detection relies on the no-checkpoints-around-handshake invariant (documented in supervised's contract and Task 5).