[Haskell-cafe] ANN: Updates in the monadic regions family

Bas van Dijk v.dijk.bas at gmail.com
Thu Mar 10 14:03:03 CET 2011

In regions-0.9 I removed support for forking threads because it
allowed you to use a closed handle in a forked thread. Unfortunately I
just realized that it's still possible to fork threads in a region.
The reason is that I've derived a MonadControlIO instance for RegionT
which enables you to use forkIO as demonstrated by the following

{-# LANGUAGE UnicodeSyntax, NoImplicitPrelude, KindSignatures #-}

module Main where

-- from base:
import Data.Function      ( ($) )
import Control.Concurrent ( ThreadId, forkIO, threadDelay )
import Control.Monad      ( (>>=), liftM, void )
import System.IO          ( IO )

-- from transformers:
import Control.Monad.IO.Class ( liftIO )

-- from regions:
import Control.Monad.Trans.Region ( RegionT, runRegionT )

-- from safer-file-handles:
import System.IO.SaferFileHandles ( openFile
                                  , IOMode(ReadMode)
                                  , hGetContents
                                  , putStrLn

-- from pathtype:
import System.Path.Posix ( asAbsFile )

-- from monad-control:
import Control.Exception.Control ( mask_ )
import Control.Monad.IO.Control  ( MonadControlIO, liftControlIO )

main ∷ IO ()
main = do runRegionT region
          threadDelay 1500000

region ∷ MonadControlIO pr ⇒ RegionT s pr ()
region = do
  putStrLn "Running region"

  h ← openFile (asAbsFile "/etc/passwd") ReadMode

  _ ← liftForkIO $ do
           putStrLn "Forked region"
           liftIO $ threadDelay 1000000
           hGetContents h >>= putStrLn

  liftIO $ threadDelay 500000
  putStrLn "Exiting region"

liftForkIO ∷ MonadControlIO m ⇒ m α → m ThreadId
liftForkIO m = liftControlIO $ \runInIO →
                 forkIO $ void $ runInIO m

Executing main yields the following error:

> main
Running region
Forked region
Exiting region
<interactive>: /etc/passwd: hGetContents: illegal operation (handle is closed)

I think the only solution is to drop the derived MonadControlIO and
MonadTransControl instances. Unfortunately the packages that use
regions require this instance because they need to use mask_ when
opening resources. Here an example from safer-file-handles:

openFile ∷ (MonadControlIO pr, AbsRelClass ar)
         ⇒ FilePath ar
         → IOMode ioMode
         → RegionT s pr
             (RegionalFileHandle ioMode (RegionT s pr))
openFile = openNormal E.openFile

openNormal open = \filePath ioMode → mask_ $ do
  h ← liftIO $ open (getPathString filePath) ioMode
  ch ← onExit $ sanitizeIOError $ hClose h
  return $ RegionalFileHandle h ch

I guess I have to solve this by providing a custom mask_ function or
using MonadCatchIO-transformers as I did before.

I'm going to think about the best solution. In the mean time just
don't use something like liftForkIO.



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