{-| 'Pipe' is a monad transformer that enriches the base monad with the ability to 'await' or 'yield' data to and from other 'Pipe's. -} module Control.Pipe ( -- * Introduction -- $summary -- * Types -- $types PipeF(..), Pipe, Producer, Consumer, Pipeline, -- * Create Pipes -- $create await, yield, pipe, -- * Compose Pipes -- $category (<+<), (>+>), idP, PipeC(..), -- * Run Pipes -- $runpipe runPipe ) where import Control.Applicative import Control.Category import Control.Monad (forever) import Control.Monad.Trans.Class (lift) import Control.Monad.Trans.Free import Data.Void (Void) import Prelude hiding ((.), id) {- $summary I completely expose the 'Pipe' data type and internals in order to encourage people to write their own 'Pipe' functions. This does not compromise the correctness or safety of the library at all and you can feel free to use the constructors directly without violating any laws or invariants. I promote using the 'Monad' and 'Category' instances to build and compose pipes, but this does not mean that they are the only option. In fact, any combinator provided by other iteratee libraries can be recreated for pipes, too. However, this core library does not provide many of the functions found in other libraries in order to encourage people to find principled and theoretically grounded solutions rather than devise ad-hoc solutions characteristic of other iteratee implementations. -} {- $types The 'Pipe' type is strongly inspired by Mario Blazevic's @Coroutine@ type in his concurrency article from Issue 19 of The Monad Reader and is formulated in the exact same way. His @Coroutine@ type is actually a free monad transformer (i.e. 'FreeT') and his @InOrOut@ functor corresponds to 'PipeF'. -} -- | The base functor for the 'Pipe' type data PipeF a b x = Await (a -> x) | Yield (b, x) -- I could use the "DerivingFunctor" extension, but I want to remain portable instance Functor (PipeF a b) where fmap f (Await a) = Await $ fmap f a fmap f (Yield y) = Yield $ fmap f y {-| The base type for pipes * @a@ - The type of input received from upstream pipes * @b@ - The type of output delivered to downstream pipes * @m@ - The base monad * @r@ - The type of the return value -} type Pipe a b = FreeT (PipeF a b) -- | A pipe that produces values type Producer b = Pipe () b -- | A pipe that consumes values type Consumer b = Pipe b Void -- | A self-contained pipeline that is ready to be run type Pipeline = Pipe () Void {- $create 'yield' and 'await' are the only two primitives you need to create pipes. Since @Pipe a b m@ is a monad, you can assemble 'yield' and 'await' statements using ordinary @do@ notation. Since @Pipe a b@ is also a monad transformer, you can use 'lift' to invoke the base monad. For example, you could write a pipe stage that requests permission before forwarding any output: > check :: (Show a) => Pipe a a IO r > check = forever $ do > x <- await > lift $ putStrLn $ "Can '" ++ (show x) ++ "' pass?" > ok <- read <$> lift getLine > when ok (yield x) -} {-| Wait for input from upstream. 'await' blocks until input is available from upstream. -} await :: (Monad m) => Pipe a b m a await = wrap $ Await return {-| Deliver output downstream. 'yield' restores control back upstream and binds the result to 'await'. -} yield :: (Monad m) => b -> Pipe a b m () yield b = wrap $ Yield (b, return ()) {-| Convert a pure function into a pipe > pipe = forever $ do > x <- await > yield (f x) -} pipe :: (Monad m) => (a -> b) -> Pipe a b m r pipe f = forever $ await >>= yield . f {- $category 'Pipe's form a 'Category', meaning that you can compose 'Pipe's and also define an identity 'Pipe'. 'Pipe' composition binds the output of the upstream 'Pipe' to the input of the downstream 'Pipe'. Like Haskell functions, 'Pipe's are lazy, meaning that upstream 'Pipe's are only evaluated as far as necessary to generate enough input for downstream 'Pipe's. If any 'Pipe' terminates, it also terminates every 'Pipe' composed with it. If you want to define a proper 'Category' instance you have to wrap the 'Pipe' type using the newtype 'PipeC' in order to rearrange the type variables. This means that if you want to compose pipes using ('.') from the 'Category' type class, you end up with a newtype mess: > unPipeC (PipeC p1 . PipeC p2) You can avoid this by using convenient operators that do this newtype wrapping and unwrapping for you: > p1 <+< p2 = unPipeC $ PipeC p1 . PipeC p2 > > idP = unPipeC id The 'Category' instance obeys the 'Category' laws. In other words: * Composition is truly associative. The result of composition produces the exact same composite 'Pipe' regardless of how you group composition, so it is perfectly safe to omit the parentheses altogether: > (p1 <+< p2) <+< p3 = p1 <+< (p2 <+< p3) = p1 <+< p2 <+< p3 * 'idP' is a true identity pipe. Composing a pipe with 'idP' returns the exact same original pipe: > p <+< idP = p > idP <+< p = p The 'Category' laws are \"correct by construction\", meaning that you cannot break them despite the library's internals being fully exposed. The above equalities are true using the strongest denotational semantics possible in Haskell, namely that both sides of the equals sign correspond to the exact same value in Haskell, constructor-for-constructor, value-for-value. You cannot create a function that can distinguish the results. Actually, all other class instances in this library provide the same strong guarantees for their corresponding laws. I only emphasize the guarantee for the 'Category' instance because it is one of the most distinguishing features of this library, making it far more extensible than other implementations. -} -- | 'Pipe's form a 'Category' instance when you rearrange the type variables newtype PipeC m r a b = PipeC { unPipeC :: Pipe a b m r} instance (Monad m) => Category (PipeC m r) where id = PipeC idP PipeC p1 . PipeC p2 = PipeC $ p1 <+< p2 -- | Corresponds to ('<<<')/('.') from @Control.Category@ (<+<) :: (Monad m) => Pipe b c m r -> Pipe a b m r -> Pipe a c m r p1 <+< p2 = FreeT $ do x1 <- runFreeT p1 let p1' = FreeT $ return x1 runFreeT $ case x1 of Pure r -> return r Free (Yield y ) -> wrap $ Yield $ fmap (<+< p2) y Free (Await f1) -> FreeT $ do x2 <- runFreeT p2 runFreeT $ case x2 of Pure r -> return r Free (Yield (x, p)) -> f1 x <+< p Free (Await f2 ) -> wrap $ Await $ fmap (p1' <+<) f2 -- | Corresponds to ('>>>') from @Control.Category@ (>+>) :: (Monad m) => Pipe a b m r -> Pipe b c m r -> Pipe a c m r (>+>) = flip (<+<) {- These associativities might help performance since pipe evaluation is downstream-biased. I set them to the same priority as (.). -} infixr 9 <+< infixl 9 >+> -- | Corresponds to 'id' from @Control.Category@ idP :: (Monad m) => Pipe a a m r idP = pipe id {- $runpipe Note that you can also unwrap a 'Pipe' a single step at a time using 'runFreeT' (since 'Pipe' is just a type synonym for a free monad transformer). This will take you to the next /external/ 'await' or 'yield' statement. This means that a closed 'Pipeline' will unwrap to a single step, in which case you would have been better served by 'runPipe'. This directly follows from the 'Category' laws, which guarantee that you cannot resolve a composite pipe into its component pipes. When you compose two pipes, the internal await and yield statements fuse and completely disappear. 'runFreeT' is ideal for more advanced users who wish to write their own 'Pipe' functions while waiting for me to find more elegant solutions. -} {-| Run the 'Pipe' monad transformer, converting it back into the base monad. 'runPipe' imposes two conditions: * The pipe's input, if any, is trivially satisfiable (i.e. @()@) * The pipe does not 'yield' any output The latter restriction makes 'runPipe' less polymorphic than it could be, and I settled on the restriction for three reasons: * It prevents against accidental data loss. * It prevents wastefully draining a scarce resource by gratuitously demanding values from it. * It encourages an idiomatic pipe programming style where input is consumed in a structured way using a 'Consumer'. If you believe that discarding output is the appropriate behavior, you can specify this by explicitly feeding your output to a pipe that gratuitously discards it: > runPipe $ forever await <+< p -} runPipe :: (Monad m) => Pipeline m r -> m r runPipe p = do e <- runFreeT p case e of Pure r -> return r Free (Await f) -> runPipe $ f () Free (Yield y) -> runPipe $ snd y