-- | Support for access to a write only value of a particular type.
--
-- The value is shared between multiple threads. If you want each thead to
-- manage its own version of the value, use "Effectful.Writer.Static.Local".
--
-- /Warning:/ 'Writer'\'s state will be accumulated via __left-associated__ uses
-- of '<>', which makes it unsuitable for use with types for which such pattern
-- is inefficient. __This applies, in particular, to the standard list type__,
-- which makes the 'Writer' effect pretty niche.
--
-- /Note:/ while the 'Control.Monad.Trans.Writer.Strict.Writer' from the
-- @transformers@ package includes additional operations
-- 'Control.Monad.Trans.Writer.Strict.pass' and
-- 'Control.Monad.Trans.Writer.Strict.censor', they don't cooperate with runtime
-- exceptions very well, so they're deliberately omitted here.
module Effectful.Writer.Static.Shared
  ( -- * Effect
    Writer

    -- ** Handlers
  , runWriter
  , execWriter

    -- ** Operations
  , tell
  , listen
  , listens
  ) where

import Control.Exception (onException, uninterruptibleMask)

import Effectful
import Effectful.Dispatch.Static
import Effectful.Dispatch.Static.Primitive
import Effectful.Internal.Utils

-- | Provide access to a strict (WHNF), shared, write only value of type @w@.
data Writer w :: Effect

type instance DispatchOf (Writer w) = Static NoSideEffects
newtype instance StaticRep (Writer w) = Writer (MVar' w)

-- | Run a 'Writer' effect and return the final value along with the final
-- output.
runWriter :: Monoid w => Eff (Writer w : es) a -> Eff es (a, w)
runWriter :: forall w (es :: [(Type -> Type) -> Type -> Type]) a.
Monoid w =>
Eff (Writer w : es) a -> Eff es (a, w)
runWriter Eff (Writer w : es) a
m = do
  MVar' w
v <- forall a (es :: [(Type -> Type) -> Type -> Type]). IO a -> Eff es a
unsafeEff_ forall a b. (a -> b) -> a -> b
$ forall a. a -> IO (MVar' a)
newMVar' forall a. Monoid a => a
mempty
  a
a <- forall (e :: (Type -> Type) -> Type -> Type)
       (sideEffects :: SideEffects)
       (es :: [(Type -> Type) -> Type -> Type]) a.
(DispatchOf e ~ 'Static sideEffects, MaybeIOE sideEffects es) =>
StaticRep e -> Eff (e : es) a -> Eff es a
evalStaticRep (forall w. MVar' w -> StaticRep (Writer w)
Writer MVar' w
v) Eff (Writer w : es) a
m
  (a
a, ) forall (f :: Type -> Type) a b. Functor f => (a -> b) -> f a -> f b
<$> forall a (es :: [(Type -> Type) -> Type -> Type]). IO a -> Eff es a
unsafeEff_ (forall a. MVar' a -> IO a
readMVar' MVar' w
v)

-- | Run a 'Writer' effect and return the final output, discarding the final
-- value.
execWriter :: Monoid w => Eff (Writer w : es) a -> Eff es w
execWriter :: forall w (es :: [(Type -> Type) -> Type -> Type]) a.
Monoid w =>
Eff (Writer w : es) a -> Eff es w
execWriter Eff (Writer w : es) a
m = do
  MVar' w
v <- forall a (es :: [(Type -> Type) -> Type -> Type]). IO a -> Eff es a
unsafeEff_ forall a b. (a -> b) -> a -> b
$ forall a. a -> IO (MVar' a)
newMVar' forall a. Monoid a => a
mempty
  a
_ <- forall (e :: (Type -> Type) -> Type -> Type)
       (sideEffects :: SideEffects)
       (es :: [(Type -> Type) -> Type -> Type]) a.
(DispatchOf e ~ 'Static sideEffects, MaybeIOE sideEffects es) =>
StaticRep e -> Eff (e : es) a -> Eff es a
evalStaticRep (forall w. MVar' w -> StaticRep (Writer w)
Writer MVar' w
v) Eff (Writer w : es) a
m
  forall a (es :: [(Type -> Type) -> Type -> Type]). IO a -> Eff es a
unsafeEff_ forall a b. (a -> b) -> a -> b
$ forall a. MVar' a -> IO a
readMVar' MVar' w
v

-- | Append the given output to the overall output of the 'Writer'.
tell :: (Writer w :> es, Monoid w) => w -> Eff es ()
tell :: forall w (es :: [(Type -> Type) -> Type -> Type]).
(Writer w :> es, Monoid w) =>
w -> Eff es ()
tell w
w1 = forall (es :: [(Type -> Type) -> Type -> Type]) a.
(Env es -> IO a) -> Eff es a
unsafeEff forall a b. (a -> b) -> a -> b
$ \Env es
es -> do
  Writer MVar' w
v <- forall (e :: (Type -> Type) -> Type -> Type)
       (es :: [(Type -> Type) -> Type -> Type]).
(e :> es) =>
Env es -> IO (EffectRep (DispatchOf e) e)
getEnv Env es
es
  forall a. MVar' a -> (a -> IO a) -> IO ()
modifyMVar_' MVar' w
v forall a b. (a -> b) -> a -> b
$ \w
w0 -> let w :: w
w = w
w0 forall a. Semigroup a => a -> a -> a
<> w
w1 in forall (f :: Type -> Type) a. Applicative f => a -> f a
pure w
w

-- | Execute an action and append its output to the overall output of the
-- 'Writer'.
--
-- /Note:/ if an exception is received while the action is executed, the partial
-- output of the action will still be appended to the overall output of the
-- 'Writer':
--
-- >>> :{
--   runEff . execWriter @String $ do
--     tell "Hi"
--     handle (\(_::ErrorCall) -> pure ((), "")) $ do
--       tell " there"
--       listen $ do
--         tell "!"
--         error "oops"
-- :}
-- "Hi there!"
listen :: (Writer w :> es, Monoid w) => Eff es a -> Eff es (a, w)
listen :: forall w (es :: [(Type -> Type) -> Type -> Type]) a.
(Writer w :> es, Monoid w) =>
Eff es a -> Eff es (a, w)
listen Eff es a
m = forall (es :: [(Type -> Type) -> Type -> Type]) a.
(Env es -> IO a) -> Eff es a
unsafeEff forall a b. (a -> b) -> a -> b
$ \Env es
es -> do
  -- The mask is uninterruptible because modifyMVar_ v0 in the merge function
  -- might block and if an async exception is received while waiting, w1 will be
  -- lost.
  forall b. ((forall a. IO a -> IO a) -> IO b) -> IO b
uninterruptibleMask forall a b. (a -> b) -> a -> b
$ \forall a. IO a -> IO a
unmask -> do
    MVar' w
v1 <- forall a. a -> IO (MVar' a)
newMVar' forall a. Monoid a => a
mempty
    -- Replace thread local MVar with a fresh one for isolated listening.
    MVar' w
v0 <- forall (e :: (Type -> Type) -> Type -> Type)
       (es :: [(Type -> Type) -> Type -> Type]) a.
(e :> es) =>
Env es
-> (EffectRep (DispatchOf e) e
    -> IO (a, EffectRep (DispatchOf e) e))
-> IO a
stateEnv Env es
es forall a b. (a -> b) -> a -> b
$ \(Writer MVar' w
v) -> forall (f :: Type -> Type) a. Applicative f => a -> f a
pure (MVar' w
v, forall w. MVar' w -> StaticRep (Writer w)
Writer MVar' w
v1)
    a
a <- forall a. IO a -> IO a
unmask (forall (es :: [(Type -> Type) -> Type -> Type]) a.
Eff es a -> Env es -> IO a
unEff Eff es a
m Env es
es) forall a b. IO a -> IO b -> IO a
`onException` forall {b} {es :: [(Type -> Type) -> Type -> Type]}.
(Writer b :> es, Semigroup b) =>
Env es -> MVar' b -> MVar' b -> IO b
merge Env es
es MVar' w
v0 MVar' w
v1
    (a
a, ) forall (f :: Type -> Type) a b. Functor f => (a -> b) -> f a -> f b
<$> forall {b} {es :: [(Type -> Type) -> Type -> Type]}.
(Writer b :> es, Semigroup b) =>
Env es -> MVar' b -> MVar' b -> IO b
merge Env es
es MVar' w
v0 MVar' w
v1
  where
    -- Merge results accumulated in the local MVar with the mainline. If an
    -- exception was received while listening, merge results recorded so far.
    merge :: Env es -> MVar' b -> MVar' b -> IO b
merge Env es
es MVar' b
v0 MVar' b
v1 = do
      forall (e :: (Type -> Type) -> Type -> Type)
       (es :: [(Type -> Type) -> Type -> Type]).
(e :> es) =>
Env es -> EffectRep (DispatchOf e) e -> IO ()
putEnv Env es
es forall a b. (a -> b) -> a -> b
$ forall w. MVar' w -> StaticRep (Writer w)
Writer MVar' b
v0
      b
w1 <- forall a. MVar' a -> IO a
readMVar' MVar' b
v1
      forall a. MVar' a -> (a -> IO a) -> IO ()
modifyMVar_' MVar' b
v0 forall a b. (a -> b) -> a -> b
$ \b
w0 -> let w :: b
w = b
w0 forall a. Semigroup a => a -> a -> a
<> b
w1 in forall (f :: Type -> Type) a. Applicative f => a -> f a
pure b
w
      forall (f :: Type -> Type) a. Applicative f => a -> f a
pure b
w1

-- | Execute an action and append its output to the overall output of the
-- 'Writer', then return the final value along with a function of the recorded
-- output.
--
-- @'listens' f m ≡ 'Data.Bifunctor.second' f '<$>' 'listen' m@
listens :: (Writer w :> es, Monoid w) => (w -> b) -> Eff es a -> Eff es (a, b)
listens :: forall w (es :: [(Type -> Type) -> Type -> Type]) b a.
(Writer w :> es, Monoid w) =>
(w -> b) -> Eff es a -> Eff es (a, b)
listens w -> b
f Eff es a
m = do
  (a
a, w
w) <- forall w (es :: [(Type -> Type) -> Type -> Type]) a.
(Writer w :> es, Monoid w) =>
Eff es a -> Eff es (a, w)
listen Eff es a
m
  forall (f :: Type -> Type) a. Applicative f => a -> f a
pure (a
a, w -> b
f w
w)

-- $setup
-- >>> import Control.Exception (ErrorCall)
-- >>> import Control.Monad.Catch