gistfile1.txt

λ: let a = stagger [] (arr (1:)) (arr (0:)) in mapM_ print $ scan a (Prelude.replicate 5 ())
([1],[0,1])
([1,0,1],[0,1,0,1])
([1,0,1,0,1],[0,1,0,1,0,1])
([1,0,1,0,1,0,1],[0,1,0,1,0,1,0,1])
([1,0,1,0,1,0,1,0,1],[0,1,0,1,0,1,0,1,0,1])

stagger.hs

stagger :: b -> (b -> a) -> (a -> b) -> Scan () (a, b)
stagger start consume determine = Scan (state . step) initial
  where
    initial = start
    step () b =
      let a' = consume b
          b' = determine a'
      in ((a', b'), b')

staggerScan :: b -> (Scan (x, b) a) -> (Scan (y, a) b) -> Scan (x, y) (a, b)
staggerScan start (Scan s sInitial) (Scan t tIinitial) =
  Scan (state . step) initial
  where
    initial = (start, sInitial, tIinitial)
    step (x, y) (b, i, j) = let
      (a', i') = runState (s (x, b)) i
      (b', j') = runState (t (y, a')) j
      in ((a', b'), (b', i', j'))