Mutable buffered Iteratee-like things. Goal is to let me reuse the input "chunks" but to prevent users from hanging onto them and seeing data they shouldn't see. Does it work? You can leak the MVector reference in an existential but then you can't do anything with it, right?

Mutable.hs

{-# LANGUAGE RankNTypes, BangPatterns #-}
module Mutable where

import Control.Monad
import Control.Monad.ST
import Data.STRef
import Data.Vector.Mutable (MVector)
import qualified Data.Vector.Mutable as MV

-- A read-only view into an MVector or similar
data Chunk s a = Chunk { size :: Int, get :: Int -> ST s a }

data Iteratee i o
  = Done o
  | Moar (forall s. Chunk s i -> ST s (Iteratee i o))

foo :: Int -> Iteratee Int Double
foo n = Moar (f n 0)
  where
  f :: Int -> Int -> Chunk s Int -> ST s (Iteratee Int Double)
  f 0 v _ = return $ Done (fromIntegral v)
  f c v chunk = do
    let go n !c | n < 0 = return c
        go n !c = do 
          x <- get chunk n
          go (n - 1) (c + x)
    x <- go (size chunk - 1) 0
    return $ Moar (f (c - 1) (v + x))

bufSize = 16

run :: Iteratee i o -> [i] -> o
run i xs = runST $ do
  ref <- newSTRef xs
  buf <- MV.new bufSize
  let chunk = Chunk bufSize (MV.read buf)
      go (Done x) = return x
      go (Moar f) = do
        cur <- readSTRef ref
        modifySTRef ref (drop bufSize)
        forM_ (zip [0..bufSize - 1] cur) (\(i, x) -> MV.write buf i x)
        next <- f chunk
        go next
  go i

I worked out another scheme that might be nicer for this - basic idea is to have the driver maintain a weak reference to the Vector it returns. When it goes to read the next value, it checks whether the weak reference points to anything. If not, it can mutate the underlying buffer. If the weak reference is not null, that (conservatively) implies that something is still holding on to the vector, which is fine, it can create a fresh buffer. It's 'conservative', in the sense that the weak reference might not get nulled out in time, but that is perfectly okay here since it's just an optimization. I also suspect with generational GC that it would end up working quite well in practice.

I kind of prefer this to hitting it with the ST sledgehammer. Using ST totally restricts aliasing / sharing. With this approach, you are free to alias / share the vector you get back, it's just that if you keep it around, the driver will be forced to make a copy.