track a streaming filter activity

TrackFilter.hs

{-# LANGUAGE BlockArguments #-}
{-# LANGUAGE Rank2Types #-}
{-# LANGUAGE NoMonomorphismRestriction #-}



import Control.Monad.Fix (fix)
import Data.List (tails)
import Streaming (Of ((:>)), Stream)
import qualified Streaming.Prelude as S
import Test.Hspec (shouldBe)


type Filter a =
  forall n x.
  Monad n
  => Stream (Of a) n x
  -> Stream (Of a) n x

trackFilter
  :: Monad m
  => Filter a
  -> Stream (Of a) m r
  -> Stream (Of a) (Stream (Of Int) m) r
trackFilter g =
   -- attach a substream of (number of elements discarded by filter g + 1)
   error "program me"

-- did you know all multiple of 13 in fibonacci happens every 7 elements ?

main :: IO ()
main = do
  l1 :> l2 <-
    S.toList
      . S.toList_
      . S.take 10
      $ trackFilter 
          do selectMultiples 13
          do fib 
  (l1, l2)
    `shouldBe` ( [7, 7, 7, 7, 7, 7, 7, 7, 7, 7]
               , [ 13
                 , 377
                 , 10946
                 , 317811
                 , 9227465
                 , 267914296
                 , 7778742049
                 , 225851433717
                 , 6557470319842
                 , 190392490709135
                 ]
               )

fib :: Monad m => Stream (Of Integer) m ()
fib = S.each do fix $ \xs -> 1 : 1 : (sum . take 2 <$> tails xs)

selectMultiples :: Integer -> Filter Integer
selectMultiples  q s = S.for
  do s
  do
    \n -> case mod n q of
      0 -> S.yield n
      _ -> pure ()