{-# LANGUAGE BangPatterns          #-}
{-# LANGUAGE CPP                   #-}
{-# LANGUAGE DeriveGeneric         #-}
{-# LANGUAGE MagicHash             #-}
{-# LANGUAGE RankNTypes            #-}
{-# LANGUAGE RecordWildCards       #-}
{-# LANGUAGE ScopedTypeVariables   #-}
{-# LANGUAGE UnboxedTuples         #-}
{-# LANGUAGE ViewPatterns          #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Retry
-- Copyright   :  Ozgun Ataman <ozgun.ataman@soostone.com>
-- License     :  BSD3
--
-- Maintainer  :  Ozgun Ataman
-- Stability   :  provisional
--
-- This module exposes combinators that can wrap arbitrary monadic
-- actions. They run the action and potentially retry running it with
-- some configurable delay for a configurable number of times.
--
-- The express purpose of this library is to make it easier to work
-- with IO and especially network IO actions that often experience
-- temporary failure that warrant retrying of the original action. For
-- example, a database query may time out for a while, in which case
-- we should delay a bit and retry the query.
----------------------------------------------------------------------------


module Control.Retry
    (
      -- * Types and Operations
      RetryPolicyM (..)
    , RetryPolicy
    , retryPolicy
    , natTransformRetryPolicy
    , RetryStatus (..)
    , defaultRetryStatus
    , applyPolicy
    , applyAndDelay


    -- ** Lenses for 'RetryStatus'
    , rsIterNumberL
    , rsCumulativeDelayL
    , rsPreviousDelayL

    -- * Applying Retry Policies
    , retrying
    , recovering
    , stepping
    , recoverAll
    , skipAsyncExceptions
    , logRetries
    , defaultLogMsg

    -- * Retry Policies
    , constantDelay
    , exponentialBackoff
    , fullJitterBackoff
    , fibonacciBackoff
    , limitRetries

    -- * Policy Transformers
    , limitRetriesByDelay
    , limitRetriesByCumulativeDelay
    , capDelay

    -- * Development Helpers
    , simulatePolicy
    , simulatePolicyPP
    ) where

-------------------------------------------------------------------------------
import           Control.Applicative
import           Control.Concurrent
#if MIN_VERSION_base(4, 7, 0)
import           Control.Exception (AsyncException, SomeAsyncException)
#else
import           Control.Exception (AsyncException)
#endif
import           Control.Monad
import           Control.Monad.Catch
import           Control.Monad.IO.Class
import           Control.Monad.Trans.Class
import           Control.Monad.Trans.Maybe
import           Control.Monad.Trans.State
import           Data.Default.Class
import           Data.List (foldl')
import           Data.Maybe
import           GHC.Generics
import           GHC.Prim
import           GHC.Types (Int(I#))
import           System.Random
# if MIN_VERSION_base(4, 9, 0)
import           Data.Semigroup
# else
import           Data.Monoid
# endif
import           Prelude
-------------------------------------------------------------------------------


-------------------------------------------------------------------------------
-- | A 'RetryPolicyM' is a function that takes an 'RetryStatus' and
-- possibly returns a delay in microseconds.  Iteration numbers start
-- at zero and increase by one on each retry.  A *Nothing* return value from
-- the function implies we have reached the retry limit.
--
-- Please note that 'RetryPolicyM' is a 'Monoid'. You can collapse
-- multiple strategies into one using 'mappend' or '<>'. The semantics
-- of this combination are as follows:
--
-- 1. If either policy returns 'Nothing', the combined policy returns
-- 'Nothing'. This can be used to @inhibit@ after a number of retries,
-- for example.
--
-- 2. If both policies return a delay, the larger delay will be used.
-- This is quite natural when combining multiple policies to achieve a
-- certain effect.
--
-- Example:
--
-- One can easily define an exponential backoff policy with a limited
-- number of retries:
--
-- >> limitedBackoff = exponentialBackoff 50 <> limitRetries 5
--
-- Naturally, 'mempty' will retry immediately (delay 0) for an
-- unlimited number of retries, forming the identity for the 'Monoid'.
--
-- The default under 'def' implements a constant 50ms delay, up to 5 times:
--
-- >> def = constantDelay 50000 <> limitRetries 5
--
-- For anything more complex, just define your own 'RetryPolicyM':
--
-- >> myPolicy = retryPolicy $ \ rs -> if rsIterNumber n > 10 then Just 1000 else Just 10000
--
-- Since 0.7.
newtype RetryPolicyM m = RetryPolicyM { getRetryPolicyM :: RetryStatus -> m (Maybe Int) }


-- | Simplified 'RetryPolicyM' without any use of the monadic context in
-- determining policy. Mostly maintains backwards compatitibility with
-- type signatures pre-0.7.
type RetryPolicy = forall m . Monad m => RetryPolicyM m


instance Monad m => Default (RetryPolicyM m) where
    def = constantDelay 50000 <> limitRetries 5


-- Base 4.9.0 adds a Data.Semigroup module. This has fewer
-- dependencies than the semigroups package, so we're using base's
-- only if its available.
# if MIN_VERSION_base(4, 9, 0)
instance Monad m => Semigroup (RetryPolicyM m) where
  (RetryPolicyM a) <> (RetryPolicyM b) = RetryPolicyM $ \ n -> runMaybeT $ do
    a' <- MaybeT $ a n
    b' <- MaybeT $ b n
    return $! max a' b'


instance Monad m => Monoid (RetryPolicyM m) where
    mempty = retryPolicy $ const (Just 0)
    mappend = (<>)
# else
instance Monad m => Monoid (RetryPolicyM m) where
    mempty = retryPolicy $ const (Just 0)
    (RetryPolicyM a) `mappend` (RetryPolicyM b) = RetryPolicyM $ \ n -> runMaybeT $ do
      a' <- MaybeT $ a n
      b' <- MaybeT $ b n
      return $! max a' b'
#endif


-------------------------------------------------------------------------------
-- | Applies a natural transformation to a policy to run a RetryPolicy
-- meant for the monad @m@ in the monad @n@ provided a transformation
-- from @m@ to @n@ is available. A common case is if you have a pure
-- policy, @RetryPolicyM Identity@ and want to use it to govern an
-- @IO@ computation you could write:
--
-- @
--   purePolicyInIO :: RetryPolicyM Identity -> RetryPolicyM IO
--   purePolicyInIO = natTransformRetryPolicy (pure . runIdentity)
-- @
natTransformRetryPolicy :: (forall a. m a -> n a) -> RetryPolicyM m -> RetryPolicyM n
natTransformRetryPolicy f (RetryPolicyM p) = RetryPolicyM $ \stat -> f (p stat)


-------------------------------------------------------------------------------
-- | Datatype with stats about retries made thus far. The constructor
-- is deliberately not exported to make additional fields easier to
-- add in a backward-compatible manner. To read or modify fields in
-- RetryStatus, use the accessors or lenses below. Note that if you
-- don't want to use lenses, the exported field names can be used for
-- updates:
--
-- >> retryStatus { rsIterNumber = newIterNumber }
-- >> retryStatus & rsIterNumberL .~ newIterNumber
data RetryStatus = RetryStatus
    { rsIterNumber      :: !Int -- ^ Iteration number, where 0 is the first try
    , rsCumulativeDelay :: !Int -- ^ Delay incurred so far from retries in microseconds
    , rsPreviousDelay   :: !(Maybe Int) -- ^ Latest attempt's delay. Will always be Nothing on first run.
    } deriving (Read, Show, Eq, Generic)


-------------------------------------------------------------------------------
-- | Initial, default retry status. Exported mostly to allow user code
-- to test their handlers and retry policies. Use fields or lenses to update.
defaultRetryStatus :: RetryStatus
defaultRetryStatus = RetryStatus 0 0 Nothing

-------------------------------------------------------------------------------
rsIterNumberL :: Lens' RetryStatus Int
rsIterNumberL = lens rsIterNumber (\rs x -> rs { rsIterNumber = x })
{-# INLINE rsIterNumberL #-}


-------------------------------------------------------------------------------
rsCumulativeDelayL :: Lens' RetryStatus Int
rsCumulativeDelayL = lens rsCumulativeDelay (\rs x -> rs { rsCumulativeDelay = x })
{-# INLINE rsCumulativeDelayL #-}


-------------------------------------------------------------------------------
rsPreviousDelayL :: Lens' RetryStatus (Maybe Int)
rsPreviousDelayL = lens rsPreviousDelay (\rs x -> rs { rsPreviousDelay = x })
{-# INLINE rsPreviousDelayL #-}



-------------------------------------------------------------------------------
-- | Apply policy on status to see what the decision would be.
-- 'Nothing' implies no retry, 'Just' returns updated status.
applyPolicy
    :: Monad m
    => RetryPolicyM m
    -> RetryStatus
    -> m (Maybe RetryStatus)
applyPolicy (RetryPolicyM policy) s = do
    res <- policy s
    case res of
      Just delay -> return $! Just $! RetryStatus
          { rsIterNumber = rsIterNumber s + 1
          , rsCumulativeDelay = rsCumulativeDelay s `boundedPlus` delay
          , rsPreviousDelay = Just delay }
      Nothing -> return Nothing


-------------------------------------------------------------------------------
-- | Apply policy and delay by its amount if it results in a retry.
-- Return updated status.
applyAndDelay
    :: MonadIO m
    => RetryPolicyM m
    -> RetryStatus
    -> m (Maybe RetryStatus)
applyAndDelay policy s = do
    chk <- applyPolicy policy s
    case chk of
      Just rs -> do
        case (rsPreviousDelay rs) of
          Nothing -> return ()
          Just delay -> liftIO $ threadDelay delay
        return (Just rs)
      Nothing -> return Nothing



-------------------------------------------------------------------------------
-- | Helper for making simplified policies that don't use the monadic
-- context.
retryPolicy :: (RetryStatus -> Maybe Int) -> RetryPolicy
retryPolicy f = RetryPolicyM $ \ s -> return (f s)


-------------------------------------------------------------------------------
-- | Retry immediately, but only up to @n@ times.
limitRetries
    :: Int
    -- ^ Maximum number of retries.
    -> RetryPolicy
limitRetries i = retryPolicy $ \ RetryStatus { rsIterNumber = n} -> if n >= i then Nothing else (Just 0)


-------------------------------------------------------------------------------
-- | Add an upperbound to a policy such that once the given time-delay
-- amount *per try* has been reached or exceeded, the policy will stop
-- retrying and fail. If you need to stop retrying once *cumulative*
-- delay reaches a time-delay amount, use
-- 'limitRetriesByCumulativeDelay'
limitRetriesByDelay
    :: Monad m
    => Int
    -- ^ Time-delay limit in microseconds.
    -> RetryPolicyM m
    -> RetryPolicyM m
limitRetriesByDelay i p = RetryPolicyM $ \ n ->
    (>>= limit) `liftM` getRetryPolicyM p n
  where
    limit delay = if delay >= i then Nothing else Just delay


-------------------------------------------------------------------------------
-- | Add an upperbound to a policy such that once the cumulative delay
-- over all retries has reached or exceeded the given limit, the
-- policy will stop retrying and fail.
limitRetriesByCumulativeDelay
    :: Monad m
    => Int
    -- ^ Time-delay limit in microseconds.
    -> RetryPolicyM m
    -> RetryPolicyM m
limitRetriesByCumulativeDelay cumulativeLimit p = RetryPolicyM $ \ stat ->
  (>>= limit stat) `liftM` getRetryPolicyM p stat
  where
    limit status curDelay
      | rsCumulativeDelay status `boundedPlus` curDelay > cumulativeLimit = Nothing
      | otherwise = Just curDelay


-------------------------------------------------------------------------------
-- | Implement a constant delay with unlimited retries.
constantDelay
    :: Int
    -- ^ Base delay in microseconds
    -> RetryPolicy
constantDelay delay = retryPolicy (const (Just delay))


-------------------------------------------------------------------------------
-- | Grow delay exponentially each iteration.  Each delay will
-- increase by a factor of two.
exponentialBackoff
    :: Int
    -- ^ Base delay in microseconds
    -> RetryPolicy
exponentialBackoff base = retryPolicy $ \ RetryStatus { rsIterNumber = n } ->
  Just $! base `boundedMult` boundedPow 2 n

-------------------------------------------------------------------------------
-- | FullJitter exponential backoff as explained in AWS Architecture
-- Blog article.
--
-- @http:\/\/www.awsarchitectureblog.com\/2015\/03\/backoff.html@
--
-- temp = min(cap, base * 2 ** attempt)
--
-- sleep = temp \/ 2 + random_between(0, temp \/ 2)
fullJitterBackoff
    :: MonadIO m
    => Int
    -- ^ Base delay in microseconds
    -> RetryPolicyM m
fullJitterBackoff base = RetryPolicyM $ \ RetryStatus { rsIterNumber = n } -> do
  let d = (base `boundedMult` boundedPow 2 n) `div` 2
  rand <- liftIO $ randomRIO (0, d)
  return $! Just $! d `boundedPlus` rand


-------------------------------------------------------------------------------
-- | Implement Fibonacci backoff.
fibonacciBackoff
    :: Int
    -- ^ Base delay in microseconds
    -> RetryPolicy
fibonacciBackoff base = retryPolicy $ \RetryStatus { rsIterNumber = n } ->
  Just $ fib (n + 1) (0, base)
    where
      fib 0 (a, _) = a
      fib !m (!a, !b) = fib (m-1) (b, a `boundedPlus` b)


-------------------------------------------------------------------------------
-- | Set a time-upperbound for any delays that may be directed by the
-- given policy.  This function does not terminate the retrying.  The policy
-- `capDelay maxDelay (exponentialBackoff n)` will never stop retrying.  It
-- will reach a state where it retries forever with a delay of `maxDelay`
-- between each one.  To get termination you need to use one of the
-- 'limitRetries' function variants.
capDelay
    :: Monad m
    => Int
    -- ^ A maximum delay in microseconds
    -> RetryPolicyM m
    -> RetryPolicyM m
capDelay limit p = RetryPolicyM $ \ n ->
  (fmap (min limit)) `liftM` (getRetryPolicyM p) n


-------------------------------------------------------------------------------
-- | Retry combinator for actions that don't raise exceptions, but
-- signal in their type the outcome has failed. Examples are the
-- 'Maybe', 'Either' and 'EitherT' monads.
--
-- Let's write a function that always fails and watch this combinator
-- retry it 5 additional times following the initial run:
--
-- >>> import Data.Maybe
-- >>> let f _ = putStrLn "Running action" >> return Nothing
-- >>> retrying def (const $ return . isNothing) f
-- Running action
-- Running action
-- Running action
-- Running action
-- Running action
-- Running action
-- Nothing
--
-- Note how the latest failing result is returned after all retries
-- have been exhausted.
retrying  :: MonadIO m
          => RetryPolicyM m
          -> (RetryStatus -> b -> m Bool)
          -- ^ An action to check whether the result should be retried.
          -- If True, we delay and retry the operation.
          -> (RetryStatus -> m b)
          -- ^ Action to run
          -> m b
retrying policy chk f = go defaultRetryStatus
  where
    go s = do
        res <- f s
        chk' <- chk s res
        case chk' of
          True -> do
            rs <- applyAndDelay policy s
            case rs of
              Nothing -> return res
              Just rs' -> go $! rs'
          False -> return res


-------------------------------------------------------------------------------
-- | Retry ALL exceptions that may be raised. To be used with caution;
-- this matches the exception on 'SomeException'. Note that this
-- handler explicitly does not handle 'AsyncException' nor
-- 'SomeAsyncException' (for versions of base >= 4.7). It is not a
-- good idea to catch async exceptions as it can result in hanging
-- threads and programs. Note that if you just throw an exception to
-- this thread that does not descend from SomeException, recoverAll
-- will not catch it.
--
-- See how the action below is run once and retried 5 more times
-- before finally failing for good:
--
-- >>> let f _ = putStrLn "Running action" >> error "this is an error"
-- >>> recoverAll def f
-- Running action
-- Running action
-- Running action
-- Running action
-- Running action
-- Running action
-- *** Exception: this is an error
recoverAll
#if MIN_VERSION_exceptions(0, 6, 0)
         :: (MonadIO m, MonadMask m)
#else
         :: (MonadIO m, MonadCatch m)
#endif
         => RetryPolicyM m
         -> (RetryStatus -> m a)
         -> m a
recoverAll set f = recovering set handlers f
    where
      handlers = skipAsyncExceptions ++ [h]
      h _ = Handler $ \ (_ :: SomeException) -> return True


-------------------------------------------------------------------------------
-- | List of pre-made handlers that will skip retries on
-- 'AsyncException' and 'SomeAsyncException'. Append your handlers to
-- this list as a convenient way to make sure you're not catching
-- async exceptions like user interrupt.
skipAsyncExceptions
    :: ( MonadIO m
       )
    => [RetryStatus -> Handler m Bool]
skipAsyncExceptions = handlers
  where
    asyncH _ = Handler $ \ (_ :: AsyncException) -> return False
#if MIN_VERSION_base(4, 7, 0)
    someAsyncH _ = Handler $ \(_ :: SomeAsyncException) -> return False
    handlers = [asyncH, someAsyncH]
#else
    handlers = [asyncH]
#endif


-------------------------------------------------------------------------------
-- | Run an action and recover from a raised exception by potentially
-- retrying the action a number of times. Note that if you're going to
-- use a handler for 'SomeException', you should add explicit cases
-- *earlier* in the list of handlers to reject 'AsyncException' and
-- 'SomeAsyncException', as catching these can cause thread and
-- program hangs. 'recoverAll' already does this for you so if you
-- just plan on catching 'SomeException', you may as well ues
-- 'recoverAll'
recovering
#if MIN_VERSION_exceptions(0, 6, 0)
    :: (MonadIO m, MonadMask m)
#else
    :: (MonadIO m, MonadCatch m)
#endif
    => RetryPolicyM m
    -- ^ Just use 'def' for default settings
    -> [(RetryStatus -> Handler m Bool)]
    -- ^ Should a given exception be retried? Action will be
    -- retried if this returns True *and* the policy allows it.
    -- This action will be consulted first even if the policy
    -- later blocks it.
    -> (RetryStatus -> m a)
    -- ^ Action to perform
    -> m a
recovering policy hs f = mask $ \restore -> go restore defaultRetryStatus
    where
      go restore = loop
        where
          loop s = do
            r <- try $ restore (f s)
            case r of
              Right x -> return x
              Left e -> recover (e :: SomeException) hs
            where
              recover e [] = throwM e
              recover e ((($ s) -> Handler h) : hs')
                | Just e' <- fromException e = do
                    chk <- h e'
                    case chk of
                      True -> do
                        rs <- applyAndDelay policy s
                        case rs of
                          Just rs' -> loop $! rs'
                          Nothing -> throwM e'
                      False -> throwM e'
                | otherwise = recover e hs'



-------------------------------------------------------------------------------
-- | A version of 'recovering' that tries to run the action only a
-- single time. The control will return immediately upon both success
-- and failure. Useful for implementing retry logic in distributed
-- queues and similar external-interfacing systems.
stepping
#if MIN_VERSION_exceptions(0, 6, 0)
    :: (MonadIO m, MonadMask m)
#else
    :: (MonadIO m, MonadCatch m)
#endif
    => RetryPolicyM m
    -- ^ Just use 'def' for default settings
    -> [(RetryStatus -> Handler m Bool)]
    -- ^ Should a given exception be retried? Action will be
    -- retried if this returns True *and* the policy allows it.
    -- This action will be consulted first even if the policy
    -- later blocks it.
    -> (RetryStatus -> m ())
    -- ^ Action to run with updated status upon failure.
    -> (RetryStatus -> m a)
    -- ^ Main action to perform with current status.
    -> RetryStatus
    -- ^ Current status of this step
    -> m (Maybe a)
stepping policy hs schedule f s = do
    r <- try $ f s
    case r of
      Right x -> return $ Just x
      Left e -> recover (e :: SomeException) hs
    where
      recover e [] = throwM e
      recover e ((($ s) -> Handler h) : hs')
        | Just e' <- fromException e = do
            chk <- h e'
            case chk of
              True -> do
                res <- applyPolicy policy s
                case res of
                  Just rs -> do
                    schedule $! rs
                    return Nothing
                  Nothing -> throwM e'
              False -> throwM e'
        | otherwise = recover e hs'


-------------------------------------------------------------------------------
-- | Helper function for constructing handler functions of the form required
-- by 'recovering'.
logRetries
    :: ( Monad m
       , Exception e)
    => (e -> m Bool)
    -- ^ Test for whether action is to be retried
    -> (Bool -> e -> RetryStatus -> m ())
    -- ^ How to report the generated warning message. Boolean is
    -- whether it's being retried or crashed.
    -> RetryStatus
    -- ^ Retry number
    -> Handler m Bool
logRetries test reporter status = Handler $ \ err -> do
    result <- test err
    reporter result err status
    return result

-- | For use with 'logRetries'.
defaultLogMsg :: (Exception e) => Bool -> e -> RetryStatus -> String
defaultLogMsg shouldRetry err status =
    "[retry:" <> iter <> "] Encountered " <> show err <> ". " <> nextMsg
  where
    iter = show $ rsIterNumber status
    nextMsg = if shouldRetry then "Retrying." else "Crashing."


-------------------------------------------------------------------------------
-- | Run given policy up to N iterations and gather results. In the
-- pair, the @Int@ is the iteration number and the @Maybe Int@ is the
-- delay in microseconds.
simulatePolicy :: Monad m => Int -> RetryPolicyM m -> m [(Int, Maybe Int)]
simulatePolicy n (RetryPolicyM f) = flip evalStateT defaultRetryStatus $ forM [0..n] $ \i -> do
  stat <- get
  delay <- lift (f stat)
  put $! stat
    { rsIterNumber = i + 1
    , rsCumulativeDelay = rsCumulativeDelay stat `boundedPlus` fromMaybe 0 delay
    , rsPreviousDelay = delay
    }
  return (i, delay)


-------------------------------------------------------------------------------
-- | Run given policy up to N iterations and pretty print results on
-- the console.
simulatePolicyPP :: Int -> RetryPolicyM IO -> IO ()
simulatePolicyPP n p = do
    ps <- simulatePolicy n p
    forM_ ps $ \ (iterNo, res) -> putStrLn $
      show iterNo <> ": " <> maybe "Inhibit" ppTime res
    putStrLn $ "Total cumulative delay would be: " <>
      (ppTime $ boundedSum $ (mapMaybe snd) ps)


-------------------------------------------------------------------------------
ppTime :: (Integral a, Show a) => a -> String
ppTime n | n < 1000 = show n <> "us"
         | n < 1000000 = show ((fromIntegral n / 1000) :: Double) <> "ms"
         | otherwise = show ((fromIntegral n / 1000) :: Double) <> "ms"

-------------------------------------------------------------------------------
-- Bounded arithmetic
-------------------------------------------------------------------------------

-- | Same as '+' on 'Int' but it maxes out at @'maxBound' :: 'Int'@ or
-- @'minBound' :: 'Int'@ rather than rolling over
boundedPlus :: Int -> Int -> Int
boundedPlus i@(I# i#) j@(I# j#) = case addIntC# i# j# of
  (# k#, 0# #) -> I# k#
  (# _, _ #)
    | maxBy abs i j < 0 -> minBound
    | otherwise -> maxBound
  where
    maxBy f a b = if f a >= f b then a else b

-- | Same as '*' on 'Int' but it maxes out at @'maxBound' :: 'Int'@ or
-- @'minBound' :: 'Int'@ rather than rolling over
boundedMult :: Int -> Int -> Int
boundedMult i@(I# i#) j@(I# j#) = case mulIntMayOflo# i# j# of
  0# -> I# (i# *# j#)
  _ | signum i * signum j < 0 -> minBound
    | otherwise -> maxBound

-- | Same as 'sum' on 'Int' but it maxes out at @'maxBound' :: 'Int'@ or
-- @'minBound' :: 'Int'@ rather than rolling over
boundedSum :: [Int] -> Int
boundedSum = foldl' boundedPlus 0

-- | Same as '^' on 'Int' but it maxes out at @'maxBound' :: 'Int'@ or
-- @'MinBound' :: 'Int'@ rather than rolling over
boundedPow :: Int -> Int -> Int
boundedPow x0 y0
  | y0 < 0 = error "Negative exponent"
  | y0 == 0 = 1
  | otherwise = f x0 y0
  where
    f x y
      | even y = f (x `boundedMult` x) (y `quot` 2)
      | y == 1 = x
      | otherwise = g (x `boundedMult` x) ((y - 1) `quot` 2) x
    g x y z
      | even y = g (x `boundedMult` x) (y `quot` 2) z
      | y == 1 = x `boundedMult` z
      | otherwise = g (x `boundedMult` x) ((y - 1) `quot` 2) (x `boundedMult` z)

-------------------------------------------------------------------------------
-- Lens machinery
-------------------------------------------------------------------------------
-- Unexported type aliases to clean up the documentation
type Lens s t a b = forall f. Functor f => (a -> f b) -> s -> f t

type Lens' s a = Lens s s a a


-------------------------------------------------------------------------------
lens :: (s -> a) -> (s -> b -> t) -> Lens s t a b
lens sa sbt afb s = sbt s <$> afb (sa s)
{-# INLINE lens #-}


                              ------------------
                              -- Simple Tests --
                              ------------------



-- data TestException = TestException deriving (Show, Typeable)
-- data AnotherException = AnotherException deriving (Show, Typeable)

-- instance Exception TestException
-- instance Exception AnotherException


-- test = retrying def [h1,h2] f
--     where
--       f = putStrLn "Running action" >> throwM AnotherException
--       h1 = Handler $ \ (e :: TestException) -> return False
--       h2 = Handler $ \ (e :: AnotherException) -> return True
