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diff --git a/src/Flag.hs b/src/Flag.hs
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+-- Copyright © 2015 Benjamin Barenblat
+--
+-- Licensed under the Apache License, Version 2.0 (the "License"); you may not
+-- use this file except in compliance with the License.  You may obtain a copy
+-- of the License at
+--
+--     http://www.apache.org/licenses/LICENSE-2.0
+--
+-- Unless required by applicable law or agreed to in writing, software
+-- distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+-- WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+-- License for the specific language governing permissions and limitations under
+-- the License.
+
+module Flag ( Flag
+            , new
+            , wave
+            , wait
+            ) where
+
+import Control.Concurrent.MSampleVar
+
+-- | A flag that a thread can wave at another thread.
+newtype Flag = Flag (MSampleVar ())
+
+-- | Creates a new flag.
+new :: IO Flag
+new = fmap Flag newEmptySV
+
+-- | Waves the flag, allowing a thread waiting on the flag to wake up.
+wave :: Flag -> IO ()
+wave (Flag v) = writeSV v ()
+
+-- | Waits until the flag is waved.
+wait :: Flag -> IO ()
+wait (Flag v) = readSV v