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<----
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