1 files changed, 30 insertions, 0 deletions
diff --git a/src/Parallel.hs b/src/Parallel.hs
new file mode 100644
index 0000000..cc6eb22
--- /dev/null
+++ b/src/Parallel.hs
@@ -0,0 +1,30 @@
+module Parallel where
+
+import Control.Concurrent
+import Control.Monad (replicateM_)
+import Data.IORef
+
+
+-- | Does not return results in-order.
+parallelForM :: [a] -> (a -> IO b) -> IO [b]
+parallelForM inputList action = do
+  nthread <- getNumCapabilities
+
+  listref <- newIORef inputList
+  outref <- newIORef []
+  donechan <- newChan
+  replicateM_ nthread $ forkIO $
+    let loop = do
+          mitem <- atomicModifyIORef' listref (\case l@[] -> (l, Nothing)
+                                                     item : l -> (l, Just item))
+          case mitem of
+            Just item -> do
+              res <- action item
+              atomicModifyIORef' outref (\l -> (res : l, ()))
+              loop
+            Nothing -> do
+              writeChan donechan ()
+    in loop
+
+  replicateM_ nthread $ readChan donechan
+  reverse <$> readIORef outref