A first attempt at making an enumeratee equivalent to the filter function for lists.

enumerator-filter.hs

import           Prelude hiding (sequence, map, head, filter)
import           Control.Monad.Trans (lift, MonadIO, liftIO)
import           Data.Enumerator


-- | Enumerator analogue of 'Data.List.filter'.
filter :: Monad m => (a -> Bool) -> Enumeratee a a m b
filter pred (Continue k) = do
  x <- head
  case x of
      Nothing -> return $ Continue k
      Just y -> do
        newStep <- if (pred y)
          then lift $ runIteratee $ k $ Chunks [y]
          else lift $ runIteratee $ k $ Chunks []
        filter pred newStep
filter pred step = return step


-- | Enumerator analogue of 'Control.Monad.filterM'.
filterM :: Monad m => (a -> (m Bool)) -> Enumeratee a a m b
filterM pred (Continue k) = do
  x <- head
  case x of
      Nothing -> return $ Continue k
      Just y -> do
        valid <- lift $ pred y
        newStep <- if valid
          then lift $ runIteratee $ k $ Chunks [y]
          else lift $ runIteratee $ k $ Chunks []
        filterM pred newStep
filterM pred step = return step