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diff --git a/com/simatime/sema.hs b/com/simatime/sema.hs
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+++ b/com/simatime/sema.hs
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+module Com.Simatime.Sema
+    ( mapPool
+    )
+where
+
+import qualified Control.Concurrent.MSem as Sem
+
+-- | Simaphore-based throttled 'mapConcurrently'.
+mapPool :: Traversable t => Int -> (a -> IO b) -> t a -> IO (t b)
+mapPool n f xs = do
+    sima <- Sem.new n
+    mapConcurrently (Sem.with sima . f) xs