Monad.hs

{- |
  A design study about how to design signal processors
  that adapt to a common sample rate.
  I tried to simplify "Synthesizer.Inference.DesignStudy.Arrow" to this module which uses only Monads.
  However the module is now very weird and does not really represent,
  what I intended to do.
-}
module Synthesizer.Inference.DesignStudy.Monad where

import Control.Monad.Writer (Writer, execWriter, tell)
import Data.List (intersect)

data Rates = Rates [Int] | Any deriving Show
-- it is a combination of Reader and Writer monad with context processing
data Processor a = P Rates (Rates -> Writer Stream a)

-- test Stream
type Stream = String

intersectRates :: Rates -> Rates -> Rates
intersectRates Any y = y
intersectRates x Any = x
intersectRates (Rates xs) (Rates ys) = Rates $ intersect xs ys

instance Monad Processor where
   return x = P Any (\_ -> return x)
   -- maybe we should turn this into an Applicative instance
   (P r0 f0) >> (P r1 f1)  =
       P (intersectRates r0 r1) (\r -> f0 r >> f1 r)
   (P _ _) >>= _ = error "Is it possible to implement that?"

runProcessor :: Processor a -> Stream
runProcessor (P r f) = execWriter (f r)

-- test processors
process, processor1, processor2, processor3 :: Processor ()
processor1 = P (Rates [44100, 48000]) (tell . show)
processor2 = P Any                    (tell . show)
processor3 = P (Rates [47000])        (tell . show)

process = processor1 >> processor2 >> processor3

test :: Stream
test = runProcessor process