PipesMean.hs

{-# LANGUAGE ViewPatterns #-}

import           Control.Monad
import           Control.Monad.Trans.State.Strict
import           Data.Sequence
import           Pipes
import           Pipes.Lift
import qualified Pipes.Prelude                    as P
import           Prelude                          hiding (length)

mean :: Monad m => Int -> Pipe Double Double (StateT (Seq Double) m) ()
mean n = forever $ do
  r  <- await
  rs <- lift get
  let rs' = r <| (if length rs < n then rs else seqInit rs)
  lift  $ put rs'
  yield $ sum rs' / fromIntegral (length rs')

-- unsafe, ma per questo esempio non mi interessa
seqInit :: Seq a -> Seq a
seqInit (viewr -> rs :> r) = rs

exampleUsage :: IO ()
exampleUsage = runEffect $
  P.readLn
  >-> evalStateP (singleton 0) (mean 2)
  >-> P.map show
  >-> P.stdoutLn