How to detect last chunk in a Haskell Pipe?

I have a small Haskell Pipe that prints out how many times it has run:

counterPipe :: Pipe String String IO r
counterPipe = go 0
  where
    go n = do
      await >>= yield
      let n' = succ n
      liftIO $ putStrLn $ "Chunk " ++ show n'
      go n'

I'd like to be able to print out a message, and potentially perform other tasks, once it has processed the last chunk. How do I go about doing this?

Solution

I was able to get this to work by changing counterPipe's input type to Maybe String and injecting an extra Nothing after the upstream pipe finishes:

import Pipes
import Pipes.Core (respond)
import Control.Applicative ((<*))

withEOF :: (Monad m) => Proxy a' a b' b m r -> Proxy a' a b' (Maybe b) m r
withEOF p = for p (respond . Just) <* respond Nothing

counterPipe :: Pipe (Maybe String) String IO Int
counterPipe = go 0
  where
    go n = do
        mx <- await

        case mx of
            Just x -> do
                yield x
                let n' = succ n
                liftIO $ putStrLn $ "Chunk " ++ show n'
                go n'
            Nothing -> do
                return n

finishCounter :: Int -> Pipe a b IO ()
finishCounter n = liftIO $ putStrLn $ unwords ["Finished after", show n, "chunks"]

Example driver:

import qualified Pipes.Prelude as P
main = runEffect $ withEOF P.stdinLn >-> (counterPipe >>= finishCounter) >-> P.stdoutLn

I think this pattern should be abstractable into something like

whileJust :: (Monad m) => Proxy a' a b' b m r -> Proxy a' (Maybe a) b' b m (Maybe r)

so you could write

withEOF P.stdinLn >-> (whileJust counterPipe >>= maybe (return ()) finishCounter) >-> P.stdoutLn

without having to change your original counterPipe definition; but I've never used Pipes before (the above solution was figured out by just looking at the types and playing type-domino) and so I haven't managed to write whileJust (the signature is probably too generic in ways that I cannot figure out).