Knitting.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Control.Monad.Trans.RWS.Strict
import Control.Monad

newtype Stitch = Stitch Int
  deriving (Eq, Ord, Show, Num)

type Knitting a = RWS () [Stitch] Stitch a

emitRow :: Knitting ()
emitRow = tell . pure =<< get

increase :: Stitch -> Knitting ()
increase diff = modify (diff+)

decrease :: Stitch -> Knitting ()
decrease diff = modify (subtract diff)

start :: Stitch -> Knitting ()
start = put

pattern = do
  start 100
  increase 2
  emitRow
  increase 2
  now <- get
  unless (now == 104) $ error "Doesn't match."
  emitRow
  decrease 4
  emitRow

-- runRWS

main = do
  let (_, final, rows) = runRWS pattern () (Stitch 0)
  mapM_ print rows