Could I has one question? What is the purpose of the "stream" function in the ArrowLoop instance? Is it just to catch an unexpected [] at runtime?
----8<---- module Main where
import Control.Arrow import Control.Arrow.Operations import Control.Arrow.Transformer.Reader
-- -- Standard list/stream arrow. --
newtype SF b c = SF { runSF :: [b] -> [c] }
instance Arrow SF where arr f = SF (map f) SF f >>> SF g = SF (g . f) first (SF f) = SF (uncurry zip . (f *** id) . unzip) second (SF f) = SF (uncurry zip . (id *** f) . unzip)
instance ArrowLoop SF where loop (SF f) = SF $ \as -> let (bs,cs) = unzip (f (zip as (stream cs))) in bs where stream ~(x:xs) = x:stream xs
It looks like stream is (almost) an identity which would crash at runtime if it encountered a []. In particular it is equivalent to
where stream xs = head xs:stream (tail xs)
instance ArrowCircuit SF where delay x = SF (init . (x:))
-- -- Some state we want to pass around without manual plumbing. --
data AudioState = AudioState { sampleRate :: Double }
runAudio state graph = proc p -> (| runReader (graph -< p) |) state
-- -- Some unit generators for audio. --
wrap x = x - fromIntegral (floor x)
-- phasor needs the sample rate phasor phase0 = proc hz -> do sr <- pure sampleRate <<< readState -< () rec accum <- delay (wrap phase0) -< wrap (accum + hz / sr) returnA -< accum
-- osc doesn't need to know about sample rate osc phase0 = proc hz -> do phase <- phasor phase0 -< hz returnA -< cos (2 * pi * phase)
-- -- Test it out. --
main = print (runSF (runAudio (AudioState{sampleRate=1000}) (osc 0)) (replicate 10 100))
----8<----