[Haskell-cafe] Re: Implicit parameters and Arrows/Yampa?

[ChrisK]

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<----