{-| 'Frame's extend 'Pipe's with: * The ability to fold input * Prompt and deterministic finalization 'Frame's differ from 'Pipe's because they form restricted monads rather than forming ordinary monads. This means you must rebind @do@ notation to use restricted monads from the @index-core@ package. See the \"Create Frames\" section for details. For even more details, consult the @index-core@ package. -} {-# LANGUAGE GADTs, TypeOperators #-} module Control.Frame ( -- * Types -- $types C, O, M, FrameF(..), Frame, Stack, -- * Create Frames -- $create -- ** Primitives -- $primitives yieldF, awaitF, close, -- ** Pipe-like primitives -- $pipeprims yield, await, -- * Finalize Frames -- $finalization catchD, catchF, finallyD, finallyF, -- * Compose Frames -- $compose (<-<), (>->), idF, FrameC(..), -- * Run Frames -- $run runFrame ) where import Control.Category import Control.IMonad import Control.IMonad.Trans import Control.IMonad.Trans.Free import Control.Monad.Instances () import Data.Closed (C) import Data.Maybe import Prelude hiding ((.), id) -- For documentation import Control.Pipe hiding (await, yield, Await, Yield) {- $types The first step to convert 'Pipe' code to 'Frame' code is to translate the types. All types of the form \"@Pipe a b m r@\" become \"@Frame b m (M a) C r@\". For example, given the following type signatures from the tutorial: > printer :: (Show a) => Pipe b C IO r > take' :: Int -> Pipe b b IO () > fromList :: (Monad m) => [b] -> Pipe () b m () ... you would replace them with: > printer :: (Show a) => Frame C IO (M a) C r > take' :: Int -> Frame a IO (M a) C () > fromList :: (Monad m) => [a] -> Frame a m (M ()) C () > -- To use the finalization example, change fromList's base monad to 'IO' > fromList :: [a] -> Frame a IO (M ()) C () -} -- | Index representing an open input end, receiving values of type @a@ data O a -- | Index representing an open input end, receiving values of type @Maybe a@ type M a = O (Maybe a) {-| Base functor for a pipe that can close its input end * @b@ - Output type * @x@ - Next step * @i@ - Current step's index -} data FrameF b x i where Yield :: b -> x i -> FrameF b x i Await :: (a -> x (O a)) -> FrameF b x (O a) Close :: x C -> FrameF b x (O a) instance IFunctor (FrameF b) where fmapI f p = case p of Yield b y -> Yield b (f y) Await a -> Await (f . a) Close c -> Close (f c) {-| A 'Frame' is like a 'Pipe' with an indexed input end: * @b@ - The type of the 'Frame's output * @m@ - The base monad * @i@ - The initial index of the input end ('O'pen or 'C'losed) * @j@ - The final index of the input end ('O'pen or 'C'losed) * @r@ - The return value -} type Frame b m i j r = IFreeT (FrameF (m (), b)) (U m) (r := j) i -- | A self-contained 'Frame' that is ready to be run type Stack m r = Frame C m (M ()) C r -- $create -- The second step to convert 'Pipe' code to 'Frame' code is to change your -- module header to: -- -- > {-# LANGUAGE RebindableSyntax #-} -- > -- > import Control.IMonad.Do -- > import Control.Frame -- > import Prelude hiding (Monad(..)) -- -- "Control.Frame" replaces all 'Pipe' 'await's and 'yield's with their -- corresponding 'Frame' counterparts. @Control.IMonad.Do@ rebinds @do@ -- notation to work with restricted monads, which also requires using the -- @RebindableSyntax@ extension and hiding the 'Monad' class from the @Prelude@. -- -- You also must use the restricted monad utility functions, which have the -- same name as their ordinary monad counterparts except with an \'@R@\' suffix, -- such as 'foreverR' instead of 'forever'. Finally, you must use 'liftU' -- instead of 'lift' to invoke operations in the base monad. -- -- Finally, every terminating 'Frame' must be 'close'd exactly once before being -- passed to composition. -- -- > printer = foreverR $ do -- > a <- await -- > liftU $ print a -- > -- > take' n = do -- > replicateMR_ n $ do -- > a <- await -- > yield a -- > close -- > liftU $ putStrLn "You shall not pass!" -- > -- > fromList xs = do -- > close -- > mapMR_ yield xs {- $primitives 'yieldF' guards against downstream termination by yielding the most up-to-date finalization alongside each value, so that downstream can call that finalizer if it terminates before requesting another value. 'awaitF' intercepts upstream termination by returning a 'Nothing' if upstream terminates before providing a value. Further attempts to request input from upstream will terminate the current 'Frame' using the return value provided from upstream. While 'awaitF' is useful for folds, 'yieldF' is less useful for end-users of this library and the higher-order 'catchF' / 'finallyF' finalization functions are much more user-friendly. Composing two 'Frame's requires that each 'Frame' invokes 'close' exactly once. Anything else will not type-check. Leave out the 'close' statement when writing library components and let the person assembling the components for composition specify where the 'close' goes. The earlier you 'close' the upstream 'Frame', the earlier it is finalized. However, once you 'close' it you may no longer 'await'. -} -- | 'Yield' the most current finalizer for this 'Frame' alongside the value yieldF :: (Monad m) => m () -> b -> Frame b m i i () yieldF m x = liftF $ Yield (m, x) (V ()) -- | 'Await' a value from upstream, returning 'Nothing' if upstream terminates awaitF :: (Monad m) => Frame b m (M a) (M a) (Maybe a) awaitF = liftF $ Await V -- | 'Close' the input end, calling the finalizers of every upstream 'Frame' close :: (Monad m) => Frame b m (M a) C () close = liftF $ Close (V ()) {- $pipeprims The following 'Pipe'-like primitives are built on top of the 'Frame' primitives. They behave identically to their 'Pipe' counterparts and can be used as drop-in replacements for them. -} -- | 'yield' a value upstream alongside an empty finalizer yield :: (Monad m) => b -> Frame b m i i () yield = yieldF (return ()) -- | 'await' a value from upstream and terminate if upstream terminates await :: (Monad m) => Frame b m (M a) (M a) a await = awaitF !>= maybe await returnR {- $finalization The third (and optional) step to convert 'Pipe' code to 'Frame' code is to register finalizers for your 'Frame'. These finalizers may be arbitrarily nested: > printer = foreverR $ catchF (putStrLn "printer interrupted") $ do > a <- await > liftU $ print a > > take' n = finallyF (putStrLn "You shall not pass!") $ do > replicateMR_ n $ do > a <- catchF (putStrLn "take' interrupted") await > yield a > close > > fromList xs = catchF (putStrLn "fromList interrupted") $ do > close > mapMR_ yield xs These convenience functions register block-level finalizers to be called if another 'Frame' terminates first. The naming conventions are: * \"catch\" functions (i.e. 'catchD' / 'catchF') call the finalizer only if another 'Frame' terminates before the block completes, but will not call the finalizer if the block terminates normally. * \"finally\" functions (i.e. 'finallyD' / 'finallyF') are like \"catch\" functions except that they also call the finalizer if the block terminates normally. * Functions that end in a \'@D@\' suffix (i.e. 'catchD' / 'finallyD') only guard against downstream termination. * Functions that end in a \'@F@\' suffix (i.e. 'catchF' / 'finallyF') guard against termination in both directions. You usually want these ones. Note that finalization blocks that /begin/ after the 'close' statement may only use the \'@D@\'-suffixed version as upstream has been closed off. This is a consequence of a deficiency in Haskell's type system that will take time to work around. However an \'@F@\'-suffixed block that begins before a 'close' statement may continue through it normally. So, for code blocks after a 'close' statement, use 'catchD' \/ 'finallyD', otherwise use 'catchF' \/ 'finallyF'. In future releases, the \'@D@\'-suffixed versions will be removed and merged into the \'@F@\'-suffixed versions. -} {-| @catchD m p@ calls the finalizer @m@ if a downstream 'Frame' terminates before @p@ finishes. -} catchD :: (Monad m) => m () -> Frame b m i j r -> Frame b m i j r catchD m p = IFreeT $ U $ do x <- unU $ runIFreeT p unU $ runIFreeT $ case x of Return r -> returnI r Wrap (Close p') -> wrap $ Close (catchD m p') Wrap (Yield (m', b) p') -> wrap $ Yield (m' >> m, b) (catchD m p') Wrap (Await f ) -> wrap $ Await $ fmap (catchD m) f {-| @catchF m p@ calls the finalizer @m@ if any 'Frame' terminates before @p@ finishes. -} catchF :: (Monad m) => m () -> Frame b m (M a) j r -> Frame b m (M a) j r catchF m p = IFreeT $ U $ do x <- unU $ runIFreeT p unU $ runIFreeT $ case x of Return r -> returnI r Wrap (Close p') -> wrap $ Close $ catchD m p' Wrap (Yield (m', b) p') -> wrap $ Yield (m' >> m, b) (catchF m p') Wrap (Await f ) -> wrap $ Await $ \e -> case e of Nothing -> liftU m !> catchF m (f e) Just _ -> catchF m (f e) {-| @finallyD m p@ calls the finalizer @m@ if a downstream 'Frame' terminates before @p@ finishes or if @p@ completes normally. -} finallyD :: (Monad m) => m () -> Frame b m i j r -> Frame b m i j r finallyD m p = IFreeT $ U $ do x <- unU $ runIFreeT p unU $ runIFreeT $ case x of Return r -> liftU m !> returnI r Wrap (Close p') -> wrap $ Close (finallyD m p') Wrap (Yield (m', b) p') -> wrap $ Yield (m' >> m, b) (finallyD m p') Wrap (Await f ) -> wrap $ Await $ fmap (finallyD m) f {-| @finallyF m p@ calls the finalizer @m@ if any 'Frame' terminates before @p@ finishes or if @p@ completes normally. -} finallyF :: (Monad m) => m () -> Frame b m (M a) j r -> Frame b m (M a) j r finallyF m p = IFreeT $ U $ do x <- unU $ runIFreeT p unU $ runIFreeT $ case x of Return r -> liftU m !> returnI r Wrap (Close p') -> wrap $ Close $ finallyD m p' Wrap (Yield (m', b) p') -> wrap $ Yield (m' >> m, b) (finallyF m p') Wrap (Await f ) -> wrap $ Await $ \e -> case e of Nothing -> liftU m !> finallyF m (f e) Just _ -> finallyF m (f e) (<~<) :: (Monad m) => IFreeT (FrameF c) (U m) (r := C) (O b) -> IFreeT (FrameF b) (U m) (r := C) (O a) -> IFreeT (FrameF c) (U m) (r := C) (O a) p1 <~< p2 = IFreeT $ U $ do x1 <- unU $ runIFreeT p1 unU $ runIFreeT $ case x1 of Wrap (Close p1') -> wrap $ Close p1' Wrap (Yield c p1') -> wrap $ Yield c (p1' <~< p2) Wrap (Await f1 ) -> IFreeT $ U $ do x2 <- unU $ runIFreeT p2 let p1' = IFreeT $ returnI x1 unU $ runIFreeT $ case x2 of Wrap (Close p2') -> wrap $ Close $ p1' <~| p2' Wrap (Yield b p2') -> f1 b <~< p2' Wrap (Await f2) -> wrap $ Await $ fmap (\p2'-> p1' <~< p2') f2 (<~|) :: (Monad m) => IFreeT (FrameF c) (U m) (r := C) (O b) -> IFreeT (FrameF b) (U m) (r := C) C -> IFreeT (FrameF c) (U m) (r := C) C p1 <~| p2 = IFreeT $ U $ do x1 <- unU $ runIFreeT p1 unU $ runIFreeT $ case x1 of Wrap (Close p1') -> p1' Wrap (Yield c p1') -> wrap $ Yield c (p1' <~| p2) Wrap (Await f1 ) -> IFreeT $ U $ do x2 <- unU $ runIFreeT p2 let p1' = IFreeT $ returnI x1 unU $ runIFreeT $ case x2 of Return r -> returnI r Wrap (Yield b p2') -> f1 b <~| p2' heap :: (Monad m) => m () -> IFreeT (FrameF (m (), c)) (U m) (r := C) (M b ) -> IFreeT (FrameF (m (), c)) (U m) (r := C) (M (m (), b)) heap m p = IFreeT $ U $ do x <- unU $ runIFreeT p unU $ runIFreeT $ case x of Wrap (Close p') -> wrap $ Close $ liftU m !> p' Wrap (Yield (m', c) p') -> wrap $ Yield (m >> m', c) (heap m p') Wrap (Await f ) -> wrap $ Await $ \e -> case e of Nothing -> heap (return ()) (f Nothing) Just (m', b) -> heap m' (f $ Just b) stack :: (Monad m) => Bool -> IFreeT (FrameF b ) (U m) (r := C) (M a) -> IFreeT (FrameF (Maybe b)) (U m) (r := C) (M a) stack t p = IFreeT $ U $ do x <- unU $ runIFreeT p unU $ runIFreeT $ case x of Wrap (Close p') -> wrap $ Close $ warn p' Wrap (Yield b p') -> wrap $ Yield (Just b) (stack t p') Wrap (Await f ) -> let p' = wrap $ Await $ \e -> stack (isNothing e) (f e) in case t of False -> p' True -> wrap $ Yield Nothing p' warn :: (Monad m) => IFreeT (FrameF b ) (U m) (r := C) C -> IFreeT (FrameF (Maybe b)) (U m) (r := C) C warn p = IFreeT $ U $ do x <- unU $ runIFreeT p unU $ runIFreeT $ case x of Return r -> wrap $ Yield Nothing (returnI r) Wrap (Yield b p') -> wrap $ Yield (Just b) (warn p') {- $compose The fourth step to convert 'Pipe' code to 'Frame' code is to replace ('<+<') with ('<-<'): > printer <-< take' 3 <-< fromList [1..] Like 'Pipe's, Frames form a 'Category' where composition pipes the output from the upstream 'Frame' to the input of the downstream 'Frame'. Additionally, composition guarantees the following behaviors: * 'Frame's receive exactly one 'Nothing' if an upstream 'Frame' terminates. * Registered finalizers get called exactly once if a downstream 'Frame' terminates. * Finalizers are always ordered from upstream to downstream. The 'Category' laws cannot be broken, so you don't have to be careful when using 'Frame's. Note that you may only compose 'Frame's that begin open and end closed. -} -- | Corresponds to ('<<<')/('.') from @Control.Category@ (<-<) :: Monad m => Frame c m (M b) C r -> Frame b m (M a) C r -> Frame c m (M a) C r p1 <-< p2 = heap (return ()) p1 <~< stack False p2 -- | Corresponds to ('>>>') from @Control.Category@ (>->) :: Monad m => Frame b m (M a) C r -> Frame c m (M b) C r -> Frame c m (M a) C r (>->) = flip (<-<) infixr 9 <-< infixr 9 >-> -- | Corresponds to 'id' from @Control.Category@ idF :: (Monad m) => Frame a m (M a) C r idF = foreverR $ await !>= yield -- | 'Frame's form a 'Category' instance when you rearrange the type variables newtype FrameC m r a b = FrameC { unFrameC :: Frame b m (M a) C r } instance (Monad m) => Category (FrameC m r) where id = FrameC idF (FrameC p1) . (FrameC p2) = FrameC (p1 <-< p2) {- $run The fifth step to convert 'Pipe' code to 'Frame' code is to use 'runFrame' instead of 'runPipe': >>> runFrame $ printer <-< take' 3 <-< fromList [1..] 1 2 3 fromList interrupted You shall not pass! printer interrupted >>> runFrame $ printer <-< take' 3 <-< fromList [1] 1 You shall not pass! take' interrupted printer interrupted -} {-| Run the 'Frame' monad transformer, converting it back to the base monad. 'runFrame' is the 'Frame' equivalent to 'runPipe' and requires a self-contained 'Stack'. -} runFrame :: (Monad m) => Stack m r -> m r runFrame p = do x <- unU $ runIFreeT p case x of Wrap (Close p') -> runFrame' p' Wrap (Yield _ p') -> runFrame p' Wrap (Await f ) -> runFrame (f $ Just ()) runFrame' :: (Monad m) => Frame C m C C r -> m r runFrame' p = do x <- unU $ runIFreeT p case x of Return (V r) -> return r Wrap (Yield _ p') -> runFrame' p'