{-# LANGUAGE CPP #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE Trustworthy #-} {-| This module is the recommended entry point to the @pipes@ library. Read "Pipes.Tutorial" if you want a tutorial explaining how to use this library. -} module Pipes ( -- * The Proxy Monad Transformer Proxy , X , Effect , Effect' , runEffect -- ** Producers -- $producers , Producer , Producer' , yield , for , (~>) , (<~) -- ** Consumers -- $consumers , Consumer , Consumer' , await , (>~) , (~<) -- ** Pipes -- $pipes , Pipe , cat , (>->) , (<-<) -- * ListT , ListT(..) , runListT , Enumerable(..) -- * Utilities , next , each , every , discard -- * Re-exports -- $reexports , module Control.Monad , module Control.Monad.IO.Class , module Control.Monad.Trans.Class , module Control.Monad.Morph , Foldable ) where import Control.Monad (void, MonadPlus(mzero, mplus)) import Control.Monad.Catch (MonadThrow(..), MonadCatch(..)) import Control.Monad.Except (MonadError(..)) import Control.Monad.Fail (MonadFail(..)) import Control.Monad.IO.Class (MonadIO(liftIO)) import Control.Monad.Reader (MonadReader(..)) import Control.Monad.State (MonadState(..)) import Control.Monad.Trans.Class (MonadTrans(lift)) import Control.Monad.Trans.Except (ExceptT, runExceptT) import Control.Monad.Trans.Identity (IdentityT(runIdentityT)) import Control.Monad.Trans.Maybe (MaybeT(runMaybeT)) import Control.Monad.Writer (MonadWriter(..)) import Control.Monad.Zip (MonadZip(..)) import Pipes.Core import Pipes.Internal (Proxy(..)) import qualified Data.Foldable as F #if MIN_VERSION_base(4,8,0) import Control.Applicative (Alternative(..)) #else import Control.Applicative import Data.Foldable (Foldable) import Data.Traversable (Traversable(..)) #endif import Data.Semigroup -- Re-exports import Control.Monad.Morph (MFunctor(hoist), MMonad(embed)) infixl 4 <~ infixr 4 ~> infixl 5 ~< infixr 5 >~ infixl 7 >-> infixr 7 <-< {- $producers Use 'yield' to produce output and ('~>') \/ 'for' to substitute 'yield's. 'yield' and ('~>') obey the 'Control.Category.Category' laws: @ \-\- Substituting \'yield\' with \'f\' gives \'f\' 'yield' '~>' f = f \-\- Substituting every \'yield\' with another \'yield\' does nothing f '~>' 'yield' = f \-\- \'yield\' substitution is associative (f '~>' g) '~>' h = f '~>' (g '~>' h) @ These are equivalent to the following \"for loop laws\": @ \-\- Looping over a single yield simplifies to function application 'for' ('yield' x) f = f x \-\- Re-yielding every element of a stream returns the original stream 'for' s 'yield' = s \-\- Nested for loops can become a sequential 'for' loops if the inner loop \-\- body ignores the outer loop variable 'for' s (\\a -\> 'for' (f a) g) = 'for' ('for' s f) g = 'for' s (f '~>' g) @ -} {-| Produce a value @ 'yield' :: 'Monad' m => a -> 'Producer' a m () 'yield' :: 'Monad' m => a -> 'Pipe' x a m () @ -} yield :: Functor m => a -> Proxy x' x () a m () yield = respond {-# INLINABLE [1] yield #-} {-| @(for p body)@ loops over @p@ replacing each 'yield' with @body@. @ 'for' :: 'Functor' m => 'Producer' b m r -> (b -> 'Effect' m ()) -> 'Effect' m r 'for' :: 'Functor' m => 'Producer' b m r -> (b -> 'Producer' c m ()) -> 'Producer' c m r 'for' :: 'Functor' m => 'Pipe' x b m r -> (b -> 'Consumer' x m ()) -> 'Consumer' x m r 'for' :: 'Functor' m => 'Pipe' x b m r -> (b -> 'Pipe' x c m ()) -> 'Pipe' x c m r @ The following diagrams show the flow of information: @ .---> b / | +-----------+ / +-----|-----+ +---------------+ | | / | v | | | | | / | | | | x ==> p ==> b ---' x ==> body ==> c = x ==> 'for' p body ==> c | | | | | | | | | | | | | | | +-----|-----+ +-----|-----+ +-------|-------+ v v v r () r @ For a more complete diagram including bidirectional flow, see "Pipes.Core#respond-diagram". -} for :: Functor m => Proxy x' x b' b m a' -- ^ -> (b -> Proxy x' x c' c m b') -- ^ -> Proxy x' x c' c m a' for = (//>) -- There are a number of useful rewrites which can be performed on various uses -- of this combinator; to ensure that they fire we defer inlining until quite -- late. {-# INLINABLE [0] for #-} {-# RULES "for (for p f) g" forall p f g . for (for p f) g = for p (\a -> for (f a) g) ; "for p yield" forall p . for p yield = p ; "for (yield x) f" forall x f . for (yield x) f = f x ; "for cat f" forall f . for cat f = let go = do x <- await f x go in go ; "f >~ (g >~ p)" forall f g p . f >~ (g >~ p) = (f >~ g) >~ p ; "await >~ p" forall p . await >~ p = p ; "p >~ await" forall p . p >~ await = p ; "m >~ cat" forall m . m >~ cat = let go = do x <- m yield x go in go ; "p1 >-> (p2 >-> p3)" forall p1 p2 p3 . p1 >-> (p2 >-> p3) = (p1 >-> p2) >-> p3 ; "p >-> cat" forall p . p >-> cat = p ; "cat >-> p" forall p . cat >-> p = p #-} {-| Compose loop bodies @ ('~>') :: 'Functor' m => (a -> 'Producer' b m r) -> (b -> 'Effect' m ()) -> (a -> 'Effect' m r) ('~>') :: 'Functor' m => (a -> 'Producer' b m r) -> (b -> 'Producer' c m ()) -> (a -> 'Producer' c m r) ('~>') :: 'Functor' m => (a -> 'Pipe' x b m r) -> (b -> 'Consumer' x m ()) -> (a -> 'Consumer' x m r) ('~>') :: 'Functor' m => (a -> 'Pipe' x b m r) -> (b -> 'Pipe' x c m ()) -> (a -> 'Pipe' x c m r) @ The following diagrams show the flow of information: @ a .---> b a | / | | +-----|-----+ / +-----|-----+ +------|------+ | v | / | v | | v | | | / | | | | x ==> f ==> b ---' x ==> g ==> c = x ==> f '~>' g ==> c | | | | | | | | | | | | | | | +-----|-----+ +-----|-----+ +------|------+ v v v r () r @ For a more complete diagram including bidirectional flow, see "Pipes.Core#respond-diagram". -} (~>) :: Functor m => (a -> Proxy x' x b' b m a') -- ^ -> (b -> Proxy x' x c' c m b') -- ^ -> (a -> Proxy x' x c' c m a') (~>) = (/>/) {-# INLINABLE (~>) #-} -- | ('~>') with the arguments flipped (<~) :: Functor m => (b -> Proxy x' x c' c m b') -- ^ -> (a -> Proxy x' x b' b m a') -- ^ -> (a -> Proxy x' x c' c m a') g <~ f = f ~> g {-# INLINABLE (<~) #-} {- $consumers Use 'await' to request input and ('>~') to substitute 'await's. 'await' and ('>~') obey the 'Control.Category.Category' laws: @ \-\- Substituting every \'await\' with another \'await\' does nothing 'await' '>~' f = f \-\- Substituting \'await\' with \'f\' gives \'f\' f '>~' 'await' = f \-\- \'await\' substitution is associative (f '>~' g) '>~' h = f '>~' (g '>~' h) @ -} {-| Consume a value @ 'await' :: 'Functor' m => 'Pipe' a y m a @ -} await :: Functor m => Consumer' a m a await = request () {-# INLINABLE [1] await #-} {-| @(draw >~ p)@ loops over @p@ replacing each 'await' with @draw@ @ ('>~') :: 'Functor' m => 'Effect' m b -> 'Consumer' b m c -> 'Effect' m c ('>~') :: 'Functor' m => 'Consumer' a m b -> 'Consumer' b m c -> 'Consumer' a m c ('>~') :: 'Functor' m => 'Producer' y m b -> 'Pipe' b y m c -> 'Producer' y m c ('>~') :: 'Functor' m => 'Pipe' a y m b -> 'Pipe' b y m c -> 'Pipe' a y m c @ The following diagrams show the flow of information: @ +-----------+ +-----------+ +-------------+ | | | | | | | | | | | | a ==> f ==> y .---> b ==> g ==> y = a ==> f '>~' g ==> y | | / | | | | | | | / | | | | | | +-----|-----+ / +-----|-----+ +------|------+ v / v v b ----' c c @ For a more complete diagram including bidirectional flow, see "Pipes.Core#request-diagram". -} (>~) :: Functor m => Proxy a' a y' y m b -- ^ -> Proxy () b y' y m c -- ^ -> Proxy a' a y' y m c p1 >~ p2 = (\() -> p1) >\\ p2 {-# INLINABLE [1] (>~) #-} -- | ('>~') with the arguments flipped (~<) :: Functor m => Proxy () b y' y m c -- ^ -> Proxy a' a y' y m b -- ^ -> Proxy a' a y' y m c p2 ~< p1 = p1 >~ p2 {-# INLINABLE (~<) #-} {- $pipes Use 'await' and 'yield' to build 'Pipe's and ('>->') to connect 'Pipe's. 'cat' and ('>->') obey the 'Control.Category.Category' laws: @ \-\- Useless use of cat 'cat' '>->' f = f \-\- Redirecting output to cat does nothing f '>->' 'cat' = f \-\- The pipe operator is associative (f '>->' g) '>->' h = f '>->' (g '>->' h) @ -} -- | The identity 'Pipe', analogous to the Unix @cat@ program cat :: Functor m => Pipe a a m r cat = pull () {-# INLINABLE [1] cat #-} {-| 'Pipe' composition, analogous to the Unix pipe operator @ ('>->') :: 'Functor' m => 'Producer' b m r -> 'Consumer' b m r -> 'Effect' m r ('>->') :: 'Functor' m => 'Producer' b m r -> 'Pipe' b c m r -> 'Producer' c m r ('>->') :: 'Functor' m => 'Pipe' a b m r -> 'Consumer' b m r -> 'Consumer' a m r ('>->') :: 'Functor' m => 'Pipe' a b m r -> 'Pipe' b c m r -> 'Pipe' a c m r @ The following diagrams show the flow of information: @ +-----------+ +-----------+ +-------------+ | | | | | | | | | | | | a ==> f ==> b ==> g ==> c = a ==> f '>->' g ==> c | | | | | | | | | | | | | | | +-----|-----+ +-----|-----+ +------|------+ v v v r r r @ For a more complete diagram including bidirectional flow, see "Pipes.Core#pull-diagram". -} (>->) :: Functor m => Proxy a' a () b m r -- ^ -> Proxy () b c' c m r -- ^ -> Proxy a' a c' c m r p1 >-> p2 = (\() -> p1) +>> p2 {-# INLINABLE [1] (>->) #-} {-| The list monad transformer, which extends a monad with non-determinism The type variables signify: * @m@ - The base monad * @a@ - The values that the computation 'yield's throughout its execution For basic construction and composition of 'ListT' computations, much can be accomplished using common typeclass methods. * 'return' corresponds to 'yield', yielding a single value. * ('>>=') corresponds to 'for', calling the second computation once for each time the first computation 'yield's. * 'mempty' neither 'yield's any values nor produces any effects in the base monad. * ('<>') sequences two computations, 'yield'ing all the values of the first followed by all the values of the second. * 'lift' converts an action in the base monad into a ListT computation which performs the action and 'yield's a single value. 'ListT' is a newtype wrapper for 'Producer'. You will likely need to use 'Select' and 'enumerate' to convert back and forth between these two types to take advantage of all the 'Producer'-related utilities that "Pipes.Prelude" has to offer. * To lift a plain list into a 'ListT' computation, first apply 'each' to turn the list into a 'Producer'. Then apply the 'Select' constructor to convert from 'Producer' to 'ListT'. * For other ways to construct 'ListT' computations, see the “Producers” section in "Pipes.Prelude" to build 'Producer's. These can then be converted to 'ListT' using 'Select'. * To aggregate the values from a 'ListT' computation (for example, to compute the sum of a 'ListT' of numbers), first apply 'enumerate' to obtain a 'Producer'. Then see the “Folds” section in "Pipes.Prelude" to proceed. -} newtype ListT m a = Select { enumerate :: Producer a m () } instance Functor m => Functor (ListT m) where fmap f p = Select (for (enumerate p) (\a -> yield (f a))) {-# INLINE fmap #-} instance Functor m => Applicative (ListT m) where pure a = Select (yield a) {-# INLINE pure #-} mf <*> mx = Select ( for (enumerate mf) (\f -> for (enumerate mx) (\x -> yield (f x) ) ) ) instance Monad m => Monad (ListT m) where return = pure {-# INLINE return #-} m >>= f = Select (for (enumerate m) (\a -> enumerate (f a))) {-# INLINE (>>=) #-} #if !MIN_VERSION_base(4,13,0) fail _ = mzero {-# INLINE fail #-} #endif instance Monad m => MonadFail (ListT m) where fail _ = mzero {-# INLINE fail #-} instance Foldable m => Foldable (ListT m) where foldMap f = go . enumerate where go p = case p of Request v _ -> closed v Respond a fu -> f a `mappend` go (fu ()) M m -> F.foldMap go m Pure _ -> mempty {-# INLINE foldMap #-} instance (Functor m, Traversable m) => Traversable (ListT m) where traverse k (Select p) = fmap Select (traverse_ p) where traverse_ (Request v _ ) = closed v traverse_ (Respond a fu) = _Respond <$> k a <*> traverse_ (fu ()) where _Respond a_ a' = Respond a_ (\_ -> a') traverse_ (M m ) = fmap M (traverse traverse_ m) traverse_ (Pure r ) = pure (Pure r) instance MonadTrans ListT where lift m = Select (do a <- lift m yield a ) instance (MonadIO m) => MonadIO (ListT m) where liftIO m = lift (liftIO m) {-# INLINE liftIO #-} instance (Functor m) => Alternative (ListT m) where empty = Select (return ()) {-# INLINE empty #-} p1 <|> p2 = Select (do enumerate p1 enumerate p2 ) instance (Monad m) => MonadPlus (ListT m) where mzero = empty {-# INLINE mzero #-} mplus = (<|>) {-# INLINE mplus #-} instance MFunctor ListT where hoist morph = Select . hoist morph . enumerate {-# INLINE hoist #-} instance MMonad ListT where embed f (Select p0) = Select (loop p0) where loop (Request a' fa ) = Request a' (\a -> loop (fa a )) loop (Respond b fb') = Respond b (\b' -> loop (fb' b')) loop (M m ) = for (enumerate (fmap loop (f m))) id loop (Pure r ) = Pure r {-# INLINE embed #-} instance (Functor m) => Semigroup (ListT m a) where (<>) = (<|>) {-# INLINE (<>) #-} instance (Functor m) => Monoid (ListT m a) where mempty = empty {-# INLINE mempty #-} #if !(MIN_VERSION_base(4,11,0)) mappend = (<|>) {-# INLINE mappend #-} #endif instance (MonadState s m) => MonadState s (ListT m) where get = lift get {-# INLINE get #-} put s = lift (put s) {-# INLINE put #-} state f = lift (state f) {-# INLINE state #-} instance (MonadWriter w m) => MonadWriter w (ListT m) where writer = lift . writer {-# INLINE writer #-} tell w = lift (tell w) {-# INLINE tell #-} listen l = Select (go (enumerate l) mempty) where go p w = case p of Request a' fa -> Request a' (\a -> go (fa a ) w) Respond b fb' -> Respond (b, w) (\b' -> go (fb' b') w) M m -> M (do (p', w') <- listen m return (go p' $! mappend w w') ) Pure r -> Pure r pass l = Select (go (enumerate l) mempty) where go p w = case p of Request a' fa -> Request a' (\a -> go (fa a ) w) Respond (b, f) fb' -> M (pass (return (Respond b (\b' -> go (fb' b') (f w)), \_ -> f w) )) M m -> M (do (p', w') <- listen m return (go p' $! mappend w w') ) Pure r -> Pure r instance (MonadReader i m) => MonadReader i (ListT m) where ask = lift ask {-# INLINE ask #-} local f l = Select (local f (enumerate l)) {-# INLINE local #-} reader f = lift (reader f) {-# INLINE reader #-} instance (MonadError e m) => MonadError e (ListT m) where throwError e = lift (throwError e) {-# INLINE throwError #-} catchError l k = Select (catchError (enumerate l) (\e -> enumerate (k e))) {-# INLINE catchError #-} instance MonadThrow m => MonadThrow (ListT m) where throwM = Select . throwM {-# INLINE throwM #-} instance MonadCatch m => MonadCatch (ListT m) where catch l k = Select (Control.Monad.Catch.catch (enumerate l) (\e -> enumerate (k e))) {-# INLINE catch #-} instance Monad m => MonadZip (ListT m) where mzipWith f = go where go xs ys = Select $ do xres <- lift $ next (enumerate xs) case xres of Left r -> return r Right (x, xnext) -> do yres <- lift $ next (enumerate ys) case yres of Left r -> return r Right (y, ynext) -> do yield (f x y) enumerate (go (Select xnext) (Select ynext)) -- | Run a self-contained `ListT` computation runListT :: Monad m => ListT m a -> m () runListT l = runEffect (enumerate (l >> mzero)) {-# INLINABLE runListT #-} {-| 'Enumerable' generalizes 'Data.Foldable.Foldable', converting effectful containers to 'ListT's. Instances of 'Enumerable' must satisfy these two laws: > toListT (return r) = return r > > toListT $ do x <- m = do x <- toListT m > f x toListT (f x) In other words, 'toListT' is monad morphism. -} class Enumerable t where toListT :: Monad m => t m a -> ListT m a instance Enumerable ListT where toListT = id instance Enumerable IdentityT where toListT m = Select $ do a <- lift $ runIdentityT m yield a instance Enumerable MaybeT where toListT m = Select $ do x <- lift $ runMaybeT m case x of Nothing -> return () Just a -> yield a instance Enumerable (ExceptT e) where toListT m = Select $ do x <- lift $ runExceptT m case x of Left _ -> return () Right a -> yield a {-| Consume the first value from a 'Producer' 'next' either fails with a 'Left' if the 'Producer' terminates or succeeds with a 'Right' providing the next value and the remainder of the 'Producer'. -} next :: Monad m => Producer a m r -> m (Either r (a, Producer a m r)) next = go where go p = case p of Request v _ -> closed v Respond a fu -> return (Right (a, fu ())) M m -> m >>= go Pure r -> return (Left r) {-# INLINABLE next #-} {-| Convert a 'F.Foldable' to a 'Producer' @ 'each' :: ('Functor' m, 'Foldable' f) => f a -> 'Producer' a m () @ -} each :: (Functor m, Foldable f) => f a -> Proxy x' x () a m () each = F.foldr (\a p -> yield a >> p) (return ()) {-# INLINABLE each #-} {- The above code is the same as: > each = Data.Foldable.mapM_ yield ... except writing it directly in terms of `Data.Foldable.foldr` improves build/foldr fusion -} {-| Convert an 'Enumerable' to a 'Producer' @ 'every' :: ('Monad' m, 'Enumerable' t) => t m a -> 'Producer' a m () @ -} every :: (Monad m, Enumerable t) => t m a -> Proxy x' x () a m () every it = discard >\\ enumerate (toListT it) {-# INLINABLE every #-} -- | Discards a value discard :: Monad m => a -> m () discard _ = return () {-# INLINABLE discard #-} -- | ('>->') with the arguments flipped (<-<) :: Functor m => Proxy () b c' c m r -- ^ -> Proxy a' a () b m r -- ^ -> Proxy a' a c' c m r p2 <-< p1 = p1 >-> p2 {-# INLINABLE (<-<) #-} {- $reexports "Control.Monad" re-exports 'void' "Control.Monad.IO.Class" re-exports 'MonadIO'. "Control.Monad.Trans.Class" re-exports 'MonadTrans'. "Control.Monad.Morph" re-exports 'MFunctor'. "Data.Foldable" re-exports 'Foldable' (the class name only). -}