Simple program using Pipes hangs

Question

I have the following program, which produces no output when run with runhaskell Toy.hs, and instead hangs indefinitely. By my understanding, the program should print "hi" and then exit. I would appreciate an answer and/or advice about how to debug such an issue. I'm using Pipes 4.0.0 off of github (github.com/Gabriel439/Haskell-Pipes-Library).

module Toy where

import Pipes
import Control.Monad.State

type Request = String
type Response = String

serveChoice :: Request -> Server Request Response IO ()
serveChoice = forever go
  where go req = do
        lift $ putStrLn req
        respond req

run :: Monad m => () -> Client Request Response (StateT Int m) ()
run () = do
    request "hi"
    return ()

main :: IO ()
main = evalStateT (runEffect $ hoist lift . serveChoice >-> run $ ()) 0

Answer 1

You need to use foreverK instead of forever, like this:

module Toy where

import Pipes
import Pipes.Prelude (foreverK)
import Control.Monad.State

type Request = String
type Response = String

serveChoice :: Request -> Server Request Response IO ()
serveChoice = foreverK go
  where go req = do
        lift $ putStrLn req
        respond req

run :: Monad m => () -> Client Request Response (StateT Int m) ()
run () = do
    request "hi"
    return ()

main :: IO ()
main = evalStateT (runEffect $ hoist lift . serveChoice >-> run $ ()) 0

The reason your original version hangs is that you used forever in the Reader monad (i.e. the ((->) a) monad) and not the pipe monad. Within this monad, forever is equivalent to ⊥:

-- i.e.        m b  ->     m c
forever :: (a -> b) -> (a -> c)
forever m = m >> forever m
          = m >>= \_ -> forever m
          = \a -> (\_ -> forever m) (m a) a
          = \a -> forever m a
          = forever m

foreverK is probably what you wanted, since it is the same idiom for Servers introduced in the pipes-3.3.0 tutorial.

This change fixes the program which now completes normally:

>>> main
hi
>>>