{-# OPTIONS_GHC -fglasgow-exts #-} ----------------------------------------------------------------------------- -- | -- Module : Control.Arrow.Transformer.Stream -- Copyright : (c) Ross Paterson 2003 -- License : BSD-style (see the LICENSE file in the distribution) -- -- Maintainer : ross@soi.city.ac.uk -- Stability : experimental -- Portability : non-portable (multi-parameter type classes) -- -- Arrow transformer lifting an arrow to streams. module Control.Arrow.Transformer.Stream( StreamArrow, runStream, StreamMap, StreamMapST, runStreamST, ArrowAddStream(..), ) where import Control.Monad.ST import Control.Arrow import Control.Arrow.Internals import Control.Arrow.Operations import Control.Arrow.Transformer import Data.Stream (Stream(..)) import qualified Data.Stream as Stream -- | Arrows between streams. -- -- /Note/: 'lift' is only a functor if '***' in the underlying arrow is. newtype StreamArrow a b c = Str (a (Stream b) (Stream c)) instance Arrow a => Arrow (StreamArrow a) where arr f = Str (arr (fmap f)) Str f >>> Str g = Str (f >>> g) first (Str f) = Str (arr Stream.unzip >>> first f >>> arr (uncurry Stream.zip)) genmap :: Arrow a => a b c -> a (Stream b) (Stream c) genmap f = arr (\xs -> (Stream.head xs, Stream.tail xs)) >>> f *** genmap f >>> arr (uncurry (Stream.Cons)) -- Caution: genmap is only a functor if *** for the base arrow is. -- (For Kleisli arrows, that would mean a commutative monad.) -- The same goes for the equivalent lift: it can be used to lift arrows, -- but won't preserve composition unless *** does. instance Arrow a => ArrowTransformer (StreamArrow) a where lift f = Str (genmap f) -- The following promotions follow directly from the arrow transformer. instance ArrowZero a => ArrowZero (StreamArrow a) where zeroArrow = lift zeroArrow instance ArrowState s a => ArrowState s (StreamArrow a) where fetch = lift fetch store = lift store instance ArrowWriter w a => ArrowWriter w (StreamArrow a) where write = lift write newWriter (Str f) = Str (newWriter f >>> arr strength) where strength :: Functor w' => (w' a',b) -> w' (a',b) strength (v, y) = fmap (\x -> (x, y)) v -- liftings of standard classes instance Arrow a => ArrowChoice (StreamArrow a) where left (Str f) = Str ((arr getLeft >>> f) &&& arr id >>> arr replace) where getLeft (Cons (Left x) xs) = Cons x (getLeft xs) getLeft (Cons (Right _) xs) = getLeft xs replace (~(Cons x xs), Cons (Left _) ys) = Cons (Left x) (replace (xs, ys)) replace (xs, Cons (Right y) ys) = Cons (Right y) (replace (xs, ys)) instance ArrowLoop a => ArrowLoop (StreamArrow a) where loop (Str f) = Str (loop (arr (uncurry Stream.zip) >>> f >>> arr Stream.unzip)) instance ArrowPlus a => ArrowPlus (StreamArrow a) where Str f <+> Str g = Str (f <+> g) -- I don't know of any other useful promotions. -- (elimWriter can be promoted, but doesn't seem useful.) -- Circuits instance ArrowLoop a => ArrowCircuit (StreamArrow a) where delay x = Str (arr (Cons x)) -- | Run a stream processor on a stream of inputs, obtaining a stream -- of outputs. -- -- Typical usage in arrow notation: -- -- > proc p -> do -- > ... -- > ys <- (|runStream (\x -> ...)|) xs -- -- Here @xs@ refers to the input stream and @x@ to individual -- elements of that stream. @ys@ is bound to the output stream. runStream :: ArrowLoop a => StreamArrow a (e,b) c -> a (e,Stream b) (Stream c) runStream (Str f) = arr (\(e, xs) -> fmap (\x -> (e, x)) xs) >>> f instance ArrowLoop a => ArrowAddStream (StreamArrow a) a where liftStream = lift elimStream = runStream -- | Mappings of streams type StreamMap = StreamArrow (->) -- | In-place state updates. -- -- /Note/: this is an arrow type, and 'lift' can be used to promote arrows -- from @'Kleisli' ('ST' s)@: the resulting arrow updates the state for -- each stream element in turn, and as long as the final state in not -- required all is well. However, 'lift' does not preserve composition, -- because this monad isn't commutative. In particular, a composition -- of 'lift's of state transformers will not work, as the second will -- require the final state of the first. type StreamMapST s = StreamArrow (Kleisli (ST s)) -- | Encapsulate a local state. runStreamST :: (forall s. StreamMapST s e c) -> StreamMap e c runStreamST cf = Str $ \ input -> runST (let Str (Kleisli f) = cf in f input)